Surface modification of high-performance aramid and polyethylene fibres for improved adhesive bonding to epoxy resins

Citation for published version (APA): Mercx, F. P. M. (1996). Surface modification of high-performance aramid and polyethylene fibres for improved adhesive bonding to epoxy resins. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR455550

DOI: 10.6100/IR455550

Document status and date: Published: 01/01/1996

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SURFACE MODIFICATION OF HIGH-PERFORMANCE ARAMlD AND POLYETHYLENE FIBRES FOR IMPROVED ADHESIVE BONDING TO EPOXY RESINS Cover: Typical surface structure of air-plasma-treated PE tapes, showing many small pits (see chapter 5)

Omslag: Karakteristieke oppervlaktestructuur van een met lucht-plasma behandelde PE film (zie hoofdstuk 5) SURFACE MODIFICATION OF HIGH-PERFORMANCE ARAMlD AND POL YETHYLENE FIBRES FOR IMPROVED ADHESIVE BONDING TO EPOXY RESINS

Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof. dr. J.H. van Lint, voor een commissie aangewezen door het College van Dekanen in het openbaar te verdedigen op donderdag 7 maart 1996 om 16. 00 uur

door

Franciscus Petros Maria Mercx

Geboren te Halsteren Dit proefschrift is goedgekeurd door

de promotoren prof. dr. P.J. Lemstra prof. dr. ir. J. van Turnhout en de copromotor dr. ing. A.A.J.M. Peijs Contents

Contents

Chapter 1 Introduetion 1

1.1 Fibre-Reinforced Polymers 1 1.2 Adhesion 2 1.3 Developments in Aramid Fibre-Matrix and PE Fibre-Matrix Adhesion 4 1.3.1 Aramid Fibre-Matrix Adhesion 4 1.3.2 Polyethylene Fibre-Matrix Adhesion 7 1.4 Objective of the Present Investigation 8 1.5 Outline of the Thesis 9 1.6 References 10

Part A: Aramid Fibres

Chapter 2 The Selective Introduetion of Specific Organic 15 Groups at the Surface of Aramid Fibres: A Model Compound Study

2.1 Introduetion 15 2.2 Experimental 15 2.2.1 Materials 15 2.2.2 Reactions 16 2.2.3 Characterization Methods 17 2.3 Results and Discussion 17 2.3.1 Chemica! Structure 17 2.3.2 Higher Homologues 23 2.3.3 Thermal Stability 24 2.3.4 Condusion 24 2.4 References 26 i i Contents

Chapter 3 Surface Modification of Aramid Fibres 27

3.1 Introduetion 27 3.2 Experimental 27 3.2.1 Reactions 27 3.2.2 X -ray Photoelectron Spectroscopy 28 3.2.3 Scanning Electron Microscopy 28 3.2.4 Determination of Acthesion 28 3.2.5 Determination of Mechanical Properties 29 3.3 Results and Discussion 29 3.3.1 Chemical Structure 29 3.3.2 Acthesion and Mechanical Properties 33 3.4 References 35

Part B: Polyethylene Fibres

Chapter 4 Oxidative Acid Etching 39

4.1 Introduetion 39 4.2 Experimental 40 4.2.1 Prepararlon of Tapes 40 4.2.2 Acid Treatment 41 4.2.3 Determination of Acthesion 41 4.2.4 Determination of Mechanica} Properties 42 4.2.5 X-ray Photoelectron Spectroscopy 42 4.2.6 Infrared Spectroscopy 42 4.2.7 Scanning Electron Microscopy 42 4.3 Results 43 4.3.1 Acthesion versus Mechanical Properties 43 4.3.2 Scanning Electron Microscopy 45 4.3.3 Weight Loss 47 4.3.4 Infrared Spectroscopy 47 4.3.5 X-ray Photoelectron Spectroscopy 48 4.4 Discussion 50 4.5 References 52 Contents iii

Chapter 5 Air- and Ammonia-Plasma Treatment 55

5.1 Introduetion 55 5.2 Experimental 56 5.2.1 Polyethylene Tapes 56 5.2.2 Plasma Treatment 56 5.2.3 Adhesion, Mechanica! Properties and 57 Chemica! Characterization 5.2.4 Scanning Electron Microscopy 57 5.3 Influence of Process Parameters 58 5.4 Results and Discussion 59 5.4.1 Tape Charaterization 59 5.4.2 Acthesion and Failure Mode 65 5.4.3 Mechanism of Acthesion 67 5.5 Conclusions 71 5.6 References 72

Chapter 6 Corona Grafting of Acrylic Acid 75

6.1 Introduetion 75 6.2 Experimental 75 6.2.1 Polyethylene Tapes 75 6.2.2 Corona Grafting 76 6.2.3 Characterization 76 6.3 Results and Discussion 76 6.3.1 Tape Characterization 77 6.3.2 Acthesion and Mechanica! Properties 80 6.3.3 Surface Treatment and Shear Strength 80 6.4 References 81

Epilogue The Role of Fibre Anisotropy and Adhesion on 83 Composite Performance iiii Contents

Summary 88

Samenvatting 92

Curriculum Vitae 96

Dankwoord 97 Introduetion 1

Chapter 1 Introduetion

1.1 Fibre-Reinforced Polymers

The use of fibre-reinforced polymers has rapidly grown over the past few decades and there is every indication that this will continue. This growth has been achieved mainly by the reptacement of traditional construction materialsas metals, wood and concrete and was driven by the superior properties per unit weight (specific properties) of fibre-reinforced polymerie materials. The higher specific modulus and strength of fibre-reinforced polymers means that weight savings can be realized when constructing with these composite materials, which results in a greater efficiency and energy savings. Initially applied in military and aerospace applications, fibre-reinforced composites have now penetrated other segments of the market as well, including the automotive industry. Some examples of the various realized applications are given in table 1.1.

Table 1.1 Applications offibre-reinforced polymeri·5

Industry Examples

Aerospace Antennas, wings, radomes, helicopter blades, landing gears

Marine Hulls, decks, masts

Automobile Bumpers, drive shafts, seats, trailers

Sport Tennis and squash rackets, fishing rods, skis, canoes, golf clubs

Fumiture and equipment Chairs, tables, lamps, ladders

Chemica! Pressure vessels, pipes 2 Chapter 1

Partienlady the inexpensive glass-fibre-reinforced polymers contributed much to the growth of polymerie composites in the last decade. The more actvaneed composites, based on carbon and/or aramid fibres, are stilllimited intheir commercial use because of high material costs. However, they are widely applied in the aerospace industry to satisfy requirements for enhanced performance and reduced maintenance. Moreover, since the sports industry discovered these advanced polymerie composites, the number of applications and consequently 2 3 their commercial importance is growing • .s. The reptacement of traditional materials as metals by polymerie composites was not achieved easily. It was in fact preceded by elaborate research to optimize the (mechanical) properties of fibre reinforeed polymers. The development of new high-performance fibres with improved strengthand stiffness to weight ratios was but one important step. Decisive for the evolution of fibre reinforeed polymers to its present accepted status as competitive construction material were, however, the developments in the area of fibre-matrix adhesion.

1.2 Adhesion

The first applications of fibre reinforeed polymers can be traeed back to 1940s when glass fibres were first used as reinforcement in polyester resins. It soon became apparent that these polymerie composites may loose much of their strength in every day practice, resulting in 6 7 premature failures • • The in-depth investigations that foliowed traeed this back to the low initia! adhesion, that could not withstand the intrusion of water. Eventually this leads to the debonding of resin from the hydrophillic glass, causing the observed deterioration in properties. Following the recognition that the level of fibre-matrix adhesion was the key factor to composite performance, a search began for glass fibre sizings that could improve the adhesion between such dissimHar matenals as glass and polyester. To this end numerous compounds were evaluated. Not surprisingly, organofunctional silanes, which are hybrids of silica and organic matenals related to resins, were among the compounds tested. They proved to be highly effective in increasing both the dry- and wet-strengthof glass-fibre-reinforced 6 7 polyesters • • Moreover, by tailoring the organic part of these silanes, it proved to be relatively easy to optimize the adhesion of the glass fibres to other polymerie materials, 6 7 including epoxy resins, polyamides and even polyolefms • • It was these developments in the area of adhesion that increased the (long-term) performance and ensured the reliable use of glass-fibre-reinforced polymers in every day practice. Experiments conducted at the end of the fifties showed that carbonization of fibrous materials yielded a continuons carbon fibre with exceptional specific properties8 ( see fig 1.1). Analogous to glass fibres, these carbon fibres could be used to provide a reinforcemet;lt in various resin systems for the fabrication of structural composites. However, the initial carbon- Introduetion 3

fibre-reinforced polymers did not achieve the expected mechanica! properties derived from the properties of fibre and matrix separately. Similar to glass-fibre-reinforced polymers, this could be traeed backtoa lack of acthesion between the carbon fibres and the polymer matrix. Again, numerous surface treatments were developed to overcome the initia! weak bond strength of the as-made carbon fîbres. Of these, only the oxidative pretreatrnents gained 8 9 commercial importance • . Electrochemical oxidation is now the most widely used industrial technique and has replaced other wet methods as immersing the fibres in oxidizing agents such as nitric and chromic acid or dry methods as the oxidation in air or oxygen9• Owing to the increased adhesion, the full potential of the carbon fibres could finally be exploited, leading to the penetration of these polymerie composites in high-performance markets such 2 3 5 as the aerospace, military and sporting goods • • • The success of carbon fibres in these appealing markets inspired the development of new families of high-performance fibres. Research mainly focused on the orientation of linear polymers and was driven by the theoretically high mechanical properties of a fully aligned 10 11 polymer chain and the low density of polymerie materials in general · • Although these efforts resulted in a number of high-performance fibres, only two have gained commercial importance. These are, in chronological order of development, aramid fibres (1973) and polyethylene (PE) fibres (1980). The Dutch companies Akzo and DSM played a leading role in the development and commercialization of these fibres. Although several aramid fibres exist, the term aramid fibre will be used in this thesis to indicate poly(p-phenylene terephthalamide) (PPTA) fibres, the most important representative of this class of fibres. Figure 1.1 shows the specific properties of the high-performance polymerie fibres compared to glass and carbon fibres, various metals and some bulk polymers. Following the research on glass and carbon fibres, it was generally accepted that the level of fibre-matrix acthesion is the key factor for the translation of fibre properties to composite performance. However, the chemica! nature of the as-made aramid and PE fibres in combination with the smooth surface provides only a moderate acthesion at best. Therefore, the improverneut in the acthesion of these fibres was thought to be of major importance for the successful introduetion as reinforcement in polymerie composites. As a direct result, this PhD study, directedat improving the acthesion of aramid and PE fibres to epoxy resins, was started in 1986. The importance of fibre-matrix acthesion research both from a scientific and economie point of view, may also be illustrated by the fact that the advisory board of the Junovation Oriented Research Programmes (IOPs), an initiative of the Dutch Ministry of Economie affairs, ranked fibre-matrix bonding as one of the primary areas for research in the first phase of this prograrmne ( 1987-1991). The incentive of these programmes is to develop new technical-scientific background knowledge and expertise in areas which are valuable for the consolidation and growth of the Dutch industry. 4 Chapter 1

Figure 1.1 Specific strength vs. specific modulus of various high-performance fibres 1 3 (N/Tex=GPa.p- , p=density in g.cm- )

1.3 Developments in Aramid Fibre-Matrix and PE Fibre-Matrix Adhesion

The most important developments in the adhesion of aramid and PE fibres to polymerie matrices, prior to the investigations described in this thesis, are summarized below.

1.3.1 Aramid Fibre-Matrix Adbesion lnvestigations on the effect of surface treatments on the adhesion of aramid fibres started in the mid-seventies. Since then a lot of different methods have been developed. Rougbly, these methods can be divided into three groups, i.e. the use of coupling agents, surface roughening and the introduetion of functional groups. The majority of the investigations concentrared on epoxy resin as a matrix material and the results presented here are for epoxy resin composites unless stated otherwise. Introduetion 5

Initia! attempts to improve the adhesion of aramid fibres focused on the use of coupling 12 16 agents - _ Preferentially, low molecular weight organic compounds were applied. Generally, these compounds proved to be of limited use and increase the acthesion only marginally. This was attributed to the fact that although immobilized on the surface, most of these compounds 13 do not penetrate or react with the aramid fibre • Positive effects were only noted for highly 1 reactive coupling agents. Examples include the use of polyfunctional aziridines \ which more than double the interlaminar shear strength (ILSS) of polyester composites, and the actdition of diisocyanates15 in case of aramid reinforeed rubber. Martin et al. 16 synthesized blockcopolymers consisting of a rigid polybenzamide block and a flexible copolyamide 6/6.6 block which markedly improved the acthesion to polar thermoplastic resins. The proposed metbod bas, however, the drawback that sulphuric acid used to apply the blockcopolymer attacks and partially dissolves the surface of the aramid filaments. Although this ensures a strong acthesion between the blockcopolymer and the aramid, it has a detrimental effect on the tensite strength of the aramid fibres. Surface roughening of fibres will increase the mechanica! keying effect but it will also adversely affect the tensile strength. Consequently, only a few studies have been devoted to this subject. Roebreeks et aL 17 used sandpapers attached to a rotating drum for controlled fibrillation of strands and fabrics. Inherently related to the metbod employed, only the outermost filaments of a strand or a fabric are fibrillated. Although this gave a large effect on the lap-shear strength values measured, the effectiveness of this metbod for improving the in-plane shear strength of real composites must be doubted. An interesting metbod was developed by Breznick et al. 18 who first absorbs bromine in the outer surface layers foliowed by the neutralization with an ammonium salt solution. The gaseous nitrogen formed, and initially occluded under the fibre surface will pierce the surface, leading to the many small pores detected by scanning electron microscopy (SEM). This treatment produces a 20% improverneut in ILSS value but also invokes a drop in fibre tensile strength of 15%. Although both described approaches (coupling agents and surface roughening) have been successful to some extent, the absolute values of the acthesion strength as well as the scanning electron micrographs of the fracture surfaces indicate that for composites based on these fibres interfacial failure still dominates. In other words adhesion is still the limiting factor in the performance of these composites. A higher level of adhesion can generally be obtained through the introduetion of functional groups at the fibre surface by either physical or chemical methods. Especially amino, carboxylic acid and epoxy groups were found to be 19 21 19 effective. Amino groups can be introduced by either ammonia - , monomethyl amine or 21 22 23 nitrogenlhydrogen plasma treatment, a nitration-reduction cycle · or bromination followed 22 by aminolysis • The amino groups introduced in these ways are almost exclusively attached 19 22 19 22 20 23 2 24 to the phenyl rings - • Improved peel strength · , pull-out strength · and ILSS values L 20 21 up to 67-70 MPa we re attributed to improved physico-chemical interactions • and 6 Chapter 1

+ 2n Cf\-S-CHÏ No"' c-Q-c_HU\._Z-J11-11·~ 0 0 n 8

R R I I CHONo CHONo c-o-J~_r.i. 11 li~J t 0 0 n R'X

c-Q-cl'o!'l 11 11 - J t 0 0 n R'• ollyl or vinylbenzyl

Figure 1.2 Grafting of aramid polymer Introduetion 7

22 23 chemical • bonding. Hydroxyl, carbonyl and particularly carboxylic acid groups can be identified on the surface of oxygen and air plasma treated PPTA fibres, resulting from the 21 oxidation of the phenyl groups • Scanning electron micrographs of the fractured surfaces of ILSS samples indicate that the raise in ILSS value from 45-50 MPa for the untreated aramid fibres to 67-70 MPa for the air, oxygen, ammonia and nitrogen/hydrogen plasma treated fibres is accompanied by a change in failure mode from interfacial controlled to failure inside 21 the aramid fibre . A chemical methad for the selective introduetion of a variety of functional 25 groups was developed by Takayanagi • The method is schematically shown in figure 2 and camprises two successive stages. In the first step, PPTA is reacted with methylcarbanion in dimethylsulfonyloxide (DMSO) to yield a metalated PPTA. Subsequently, this intermediate is converted with alkyl halides or epoxies. Depending on the chemical nature of the epoxy or the alkyl halide used, different functional groups can be introduced, such as carboxylic 25 27 27 28 26 26 acid - , epoxy , allylic , acrylonitril and octadecyl groups. This approach allows the tailoring of PPTA fibres for the impravement in acthesion to a number of resins, such as epoxies, polyesters, phenolics and thermoplastic resins. Although the methad developed by Takayanagi is appealing in its versatility, the corrosive nature and the high costs of the chemieals used present a serious drawback for application. The reactivity of the amide group towards diacid chlorides such as oxalylchloride and its application for the surface modification and enhancement of adhesion, as described in this thesis, has not yet been investigated.

1.3.2 Polyethylene Fibre-Matrix Adhesion

Even though polyethylene is an apolar material that poorly bonds to most polymer matrices, surface modification via oxidation is relatively easy and has been employed with great success for improved metal-plating and printability of low and high density polyethylene in the past 30 33 30 years • • Consequently, oxidative pretreatments were among the first methods to be considered for improving the weak bond strength of high-performance PE fibres to polymerie matrices. Ladizesky and Ward34 investigated the effect of oxygen-plasma and chromic acid treatment on the acthesion of melt-spun polyethylene fibres to an epoxy matrix. Both treatrnents markedly improved the adhesion, although plasma treatment was far more effective as evidenced by a change in failure mode from interface failure to shear failure within the melt­ spun PE fibres. The higher effectiveness of plasma treatment was attributed to the resulting pitted surface which allowed penetration of the resin to produce a mechanical keying between fibre and matrix. Similar results following air- or oxygen-plasma treatrnents were reported 35 36 38 by Nguygen et al. , Nardin et al. , Kaplan et alY and Jacobs et al. . A marked increase 8 Chapter 1

in wettability and adhesion was observed after ammonia-plasma treatment, although scanning electron microscopy showed no changes in surface structure39• Plasma treatment affects the fibre tensile strength negatively, decreases up to 20% have been noted34•36• Corona discharge 41 resulted in approximately a two-fold increase in interlaminar shear strength40• • Postema et al. 42 reported a five-fold increase in the adhesion of gel-spun PE fibres to gypsum plaster after chlorosulfonation. According to the authors this improverneut could be related to surface roughening of the fibres. Based on evidence gathered on LDPE and HDPE, it is expected that the above described surface treatments will lead to oxidation or amination of the surface. Although some remarks concerning the introduetion of functional groups were made, no attempts were undertaken to monitor the changes in surface chernical composition, nor to reveal the nature of the chemical groups incorporated. In view of the large effects that functional groups can have on the adhesion, it seems premature to attribute the increased adhesion to surface roughening without the exact knowledge of the changes in surface chemica! composition and surface topography brought about by the different surface treatments.

1.4 Objective of the Present Investigation

The main objective of the research described in this thesis is to improve the adhesion of high­ performance aramid and PE fibres to epoxy resins via surface modification of the reinforcing fibres. An obvious requirement of any surface treatment procedure is that it should not affect the mechanica! properties of the reinforcing fibres, or at least not to a large extent {i.e. ::;; 10%). Consequently, the effect of the surface treatments on all relevant mechanica! properties of the high-performance fibres has been studied. Attention is focused on the relationship between surface chemistry, surface topography, adhesion and faiture mode. In this way the mechanisms responsible for the increased adhesion as well as the failure mode cao be assessed. The latter will indicate whether adhesion, the transverse or shear strengthof these high-performance fibres, or the (shear) strength of the matrix is the limiting factor in the performance of these composites. Introduetion 9

1.5 Outüne of the Thesis

The thesis is divided in two parts dealing with high performance aramid and (gel-spun) PE fibres, respectively.

Part A: Aramid Fibres

Chapter 2 describes the results of a model compound study, undertaken to evaluate the feasibility of a novel two step chemica] modification procedure for the selective introduetion of specific organic groups at the surface of aramid fibres.

Following the methodology developed in chapter 2, the selective introduetion of acid, ester, amine and epoxy groups at the surface of aramid fibres is reported on in chapter 3. Furthermore, the effect of these surface modifications on the adhesion to epoxy resin and the mechanica! properties of the aramid fibres is evaluated.

Part B: Polyethylene Fibres

Chapter 4 deals with the oxidative acid etching of high-performance PE fibres. Attention is given to the effect of the treatment on surface morphology, surface chemica] composition and mechanica! properties of the fibres and interfacial bond strength to epoxy resin.

In chapter 5, the influence of air and armnonia plasma treatment on surface chemical composition, surface morphology, mechanica! properties and interfacial bond strengthof high­ performance PE fibres is discussed.

Chapter 6 describes a novel method for the selective introduetion of carboxylic acid groups and the effect on the interfacial bond strength to epoxy re sin. Furthennore, some remarks are made with respect to the effect of oxidative processes on the shear strength of the outer PE surface layers.

In the epilogue, the role of fibre anisotropy and fibre-matrix acthesion on composite performance is commented upon. 10 Chapter 1

1.6 References

1. D. Huil, 'An Introduetion to Composite Materials', Cambridge University Press, Cambridge (1981) 2. I.C. Visconti, Polym. Plast Technol. Eng. 31, 1-59 (1992) 3. Composites, Engineered Materials Handbook-Vol. 1 (Eds. C.A. Dostal and M.S. Woods), ASM International, USA (1987) 4. J.D. Packer-Tursman, Adv. Comp. 2(2), 26-28 (1994) 5. C. Petersen, Adv. Comp. 2(2), 20-21 (1994) 6. E.P. Plueddemann, 'Silane Coupling Agents', Plenum Press, New York (1982) 7. G. Tesoro and Y. Wu, J. Adhesion Sci. Techno!. 2_, 771-784 (1991) 8. J.B. Donnet and R.P. Chandal, 'Carbon Fibres, International Fiber Science and Technology Series-Vol. 3' (Ed. M. Lewin), Marcel Dekker Inc., New York (1984) 9. J.D.H. Hughes, Comp. Sci. Technol41, 13-45 (1991) 10. P.J. Lemstra, R. Kirschbaum, T. Ohta and H. Yasuda in 'Developments in Oriented Polymers-2' (Ed. I.M. Ward), Elsevier, London (1987), p. 39-77 11. H. Jiang, W. W. Adams and R.K. Eby in 'Materials Science and Technology-Vol. 12 Structure and Properties of Polymers' (Ed. E.L. Thomas), VCH, Weinheim (1993), p.597-652 12. D.J. Vaughan, Polym. Eng. Sci. 18, 167-169 (1979) 13. L.S. Penn, F.A. Bystry and H.J. Marchionni, Polym. Comp. :!:. 26-31 (1983) 14. F.M. Lognllo and Y-T. Wu, United StatesPatent 4,418,164 (1983) 15. C. Hepburn and Y.B. Aziz, Int. J. Adhesion and Adhesives 2_, 153-159 (1985) 16. R. Martin, W. Götz and B. Vollmert, Angew. Makromol. Chem. 133, 121-140{1985) 17. G. Roebroeks and W.H.M. van Dreumel in 'Materials Science Monograhs: 35' (Eds. K. Brunsch, H-D. Gölden and C-M. Herkert), Elsevier, Amsterdam (1986), p.95-102 18. M. Breznick, J. Banbaji, H. Guttmann and G. Marom, Polym. Comm. 28, 55-56 (1987) 19. R.E. Allred, DSc Thesis, Massachusetts Institute of Technology (1983) 20. L.S. Penn and T.K. Liao, Comp. Technol. Rev. Q, 133-136 (1984} 21. E. Logtenberg and D. Deventer, Unpublished results TNO Delft 22. Y. Wu and G.C. Tesoro, J. Appl. Polym. Sci. 31, 1041-1059 (1986) 23. L.S. Penn, G.C. Tesoro and H.X. Zhou, Polym. Comp. 2. 184-191 (1988) 24. T.J.J.M. Koek and J.J.G. Smits, European Patent 0,006,275 (1982) 25. M. Takayanagi and T. Katayose, J. Polym. Sci., Polym. Chem. Ed. 19, 1133-1145 (1981) 26. M. Takayanagi, T. Kajiyarna and T. Katayose, J. Appl. Polym. Sci. 27, 3903-3917 (1982) Introduetion 11

27. M. Takayanagi, S. Ueta, W-Y. Lei and K.Koga, Polym. J. 19,467-474 (1987) 28. H. Ishizawa and Y. Hasuda, ACS 59, 362-366 (1988) 29. M. Takayanagi, S. Ueta and Y. Nishihara, Reports on Progress in Polym. Phys. in Japan 28, 343-346 (1985) 30. D.M. Brewis and D. Briggs, Polymer 22, 7-16 (1981) 31. S. Wu, 'Polymer Interface and Adhesion', Marcel Dekker, New York (1982), p.279 32. J.A Lanauze and D.L. Myers, J. Appl. Polym. Sci. 40, 595-611 (1990) 33. P. Gatenholm, C. Bonneropand E. Wallström, J. Acthesion Sci. Technol. :!, 817-827 (1990) 34. N.H. Ladizesky and I.M. Ward, J. Mater. Sci. 18, 533-544 (1983) 35. H.X. Nguygen, G. Riahi, G. Wood and A. Peursartip in 'Proceedings of 33th International SAMPE Symposium', Anaheim (1988), p.1721-1729 36. M. Nardin and I.M. Ward, Mater. Sci. Techno!. 38, 814-827 (1987) 37. S.L. Kaplan, P.W. Rose, H.X. Nguygen and H.W. Chang, SAMPE Q. 19(4), 55-59 (1988) 38. M.J.N. Jacobs and H.J.J. Rutten, Eur. Pat. Appl. EP 311197 A2, Dyneerna V.o.f. (1989) 39. S. Holmes and P. Schwartz, Comp. Sci. Technol. 38, 1-21 (1990) 40. R.J.H. Burlet, J.H.H. Raven and P.J. Lernstra, Eur. Pat. Appl. EP 144997 A2, DSM Stamicarbon (1985) 41. M.J.N. Jacobs and H.J.J. Rutten, Eur. Pat. Appl. EP 311198 A2, Dyneerna V.o.f. (1989) 42. A.R. Postema, A.T. Doornkamp, J.G. Meijer and H.D. Vlekkert, Polym. Bull. 1-6 (1986) 12 13

PART A: ARAMlD FIBRES 14 The selective introduetion of .. 15

Chapter 2 The Selective Introduetion of Specific Organic Groups at the Surface of Aramid Fibres: A Model Compound Study

2.1 Introduetion

In 1980, Vekemans and Hoornaert1 reported on a new synthetic route to isoquinolinetriones starting from benzamides. Basically, benzamides were reacted with oxalyl chloride to yield 2 N-aroyloxamoyl chlorides , which were subsequently converted to isoquinolinetriones (cyclization) by raising the temperature. Of particular interest are the N-aroyloxamoyl chloride intermediates, which still contain a reactive acid chloride group. If aramids react in a similar way, the acid chloride group can be used for various derivatizations enabling the introduetion of specific organic groups. With this consideration in mind, an extensive model compound study was undertaken to evaluate the feasibility of such an approach, the results of which are reported herein. The majority of the investigations was conducted on benzanilide as model compound for aramid, butsome control experiments on higher homologues were also performed.

2.2 Experimental

2.2.1 Materials

With the exception of diethyl ether and dichloromethane, all materials used were of reagent grade and were used without further purification. Diethyl ether was dried and stored over sodium, whereas dichloromethane was distilled and stored over 3 and 4 A molsieves. 16 Chapter 2

2.2.2 Reactions

Key intennediate: N-benzoyl-N-phenyloxamoyl chloride 1 A solution of 7.3 g (57 mmol) oxalyl chloride in 30 mi of carbon tetrachloride was added to 1 g (5 mmol) benzanilide and heated at 40 oe for 1 h. The benzanilide slowly dissolved after which the excess oxalyl chloride was removed by vacuum distillation. This solution was used for the subsequent reactions described below.

Reaction of 1 with water: N-benzoyl-N-phenyloxamic acid 2 Upon actdition of water, a white solid precipitated. The product was fittered off, dissolved in dichloromethane, dried over magnesium sulphate and filtered. Evaporating of the solvent afforded white needle-like crystals which were driedunder vacuum at 40 oe and stored in a desiccator. Yield: 91%. Anal. calc. for e, 5H110 4N: e 79.17; H 5.62; N 7.10. Found: C 79.11; H 5.67; N 7.02.

Reaction of 1 with methanol: methyl N-benzoyl-N-phenyloxamate 3 The white precipitate fonned after actdition of methanol was collected, rinsed with methanol, dried at 40 oe under vacuum and stored in a desiccator. Yield: 87%. Anal. calc. for e,JI130 4N: e 67.84; H 4.63; N 4.94. Found e: 67.78; H 4.64; N 4.86.

Reaction of 1 with glycidol: 2.3-epoxypropyl N-benzoyl-N-phenyloxamate 4 Prior to the actdition of an equimolar amount of glycidol, dissolved in a small amount of carbon tetrachloride, triethylamine was added to neutralize the hydrochloric acid formed during the course of the reaction. This may otherwise cause ring opening and polymerization of the epoxy groups of glycidol. All volatile substances were then removed under reduced pressure with a rotary evaporator. The remaining residue was taken up in acetone. Piltration of this suspension foliowed by evaporation of acetone yielded a sticky solid which, after drying, was stored in a desiccator. Yield: 64%. Anal. calc. for e, 8H150 5N: e 66.46; H 4.65; N 4.31. Found: e 66.16; H 5.01; N 4.53. The selective introduetion of .. 17

2.2.3 Characterization Methods

The infrared (IR) spectra were recorded on a Perkin Elmer 297 spectrophotometer applying either K.Br disks or NaCl mounted liquid cells. 1H-NMR spectra were recorded with a 200 MHz Broker AC-200 spectrometer using deuterated (D7) dimethylformamide as solvent. The signal of the deuterated methyl groups was used as internat standard. When solutions in carbon tetrachloride or sulfolane were measured, deuterated chloroform was added as internat standard and locking agent. The spectra had a speetral width of 2400 Hz and were generally obtained after accumulating 64 scans. The digital resolution amounted to 0.15 Hz, corresponding toa datalengthof 16K. The 50 MHz 13C-NMR spectra were also measured on the Broker AC-200 spectrometer with a pulse delay of 10 sec. Thermal gravimetrie analysis (TGA) was carried out on a Du Pont 951 Thermal Gravimetrie Analyzer with a heating rate of 10 °C/min in a nitrogen atmosphere. The temperature of 1 % weight loss was taken as the onset of decomposition.

2.3 Results and Discussion

2.3.1 Chemical Structure

The first step in the modification procedure is the reaction between benzanilide, used as model compound for aramid, and oxalylchloride giving 1 (scheme 2.1}. Figure 2.1 shows the IR spectra of the starting compound and the reaction product 1. The stretching vibrations of the N-H group at 3340 cm· 1 and of the amict 11 group at 1530 cm·1 present in the spectrum of benzanilide are absent in the spectrum of 1. This points towards N-substitution. Furthermore, two strong absorption bands located at 1830 cm· 1 and 1750 cm·1 appear in the infrared spectrum of 1. These bands were, referring to the Sadtler standard spectra of oxalylchloride and derivates, ascribed to C=O stretching vibrations ofthe O=C-C=O group. The amid I band (C=O) located at 1660 cm·1 in benzanilide is shifted 40 cm· 1 to higher field as a result of this electron-withdrawing N-substitution. Variabie temperature measurements did notchange the 1H-NMR spectrum of 1 as shown in figure 2.2. This rul es out the possibility that the position of the hydrogen atom of the N-H group (o=10.22 ppm in benzanilide), wbich depends on temperature, concentration and type of solvent, is located underneath the aryl-H bands in the 1H-NMR spectrum of 1. The absence of the N-H peak substantiates tbe IR results. 18 Chapter 2

Substitution with a strongly electron withdrawing group generally results in a deshielding of neighbouring protons. Contrary hereto a shielding effect is observed for the aromatic protons in benzanilide (compare fig. 2.2 a and b). This effect is thought to arise from the loss 3 in coplanarity upon N-substitution • In a coplanar structure, the carbonyl group exerts a de­ shielding effect". However, in non-coplanar structures a shielding effect ofthe carbonyl group 4 is observed • The opposed effect of the carbonyl group in compound 1 and benzanilide dominates over the deshielding effect exerted by the electron withdrawing group and explains the overallshielding effect observed.

0 0 0 11 11 Cl-C-C-Cl ..... ~-g-o + HCI o- c=o I C=O I Cl

0 0 o-~-ë-o ~-ë-o~ C=O o- c=o I I C=O C=O I I OH OCH3 2 3

4

Scheme 2.1

The reaction most likely proceeds through the 71"-electron system of the amide group to produce an 0-acylated product, which by intramolecular rearrangement gives the N-acylated product'. This is interesting in view of the apparent difficulty of a direct chemica} attack at the amide group of aramid due to the sterical bindrance of the neighbouring phenyl groups 5 lying in the same plane as the amide group • The selective introduetion of .. 19

b

4000 3000 2000 1800 1600 1400 1200 1000 800 600 cm·1

Figure 2.1 lnfrared spectra of (a) benzanilide and (b) its reaction product with oxalyl chloride 1 20 Chapter 2

a

12 10 8 6 4 2 0 PPM ,...--,a a a 0 a a aQ-~-~-oa C=O I C=C I Cl

b

12 10 8 6 4 2 0 PPM

Figure 2.2 1H-NMR spectra of (a) benzanilide and (b) its reaction product with oxalyl chloride I (* == solvent peaks)

In the secoud step, the highly reactive acid-chloride group is converted witheither water, methanol or glycidol to introduce acid, ester and epoxy groups. As expected, no absorption bands attributable to the N-H group and amid 11 are present in the IR spectra of these products (fig. 2.3). In addition, all spectra show more than ohe absorption attributed to C=O 1 stretching vibrations (1650-1850 cm· ). A broad absorption band ranging from 3350 to 2500 cm·1 is seen in the spectrum of 2. Absorptions showing this characteristic are distinctive for 6 carboxylic acids • The broadening is thought to be related to internal hydrogen bonding. H­ 1 CH stretching vibrations at 2920 and 2860 cm· , present in the spectra of 2 and 3, indicate the presence of alkyl groups. The assignment of the bands in the fingerprint region is hampered by the large number of bands present and was therefore not tried. The selective introduetion of .. 21

a

b

c

4000 2000 1800 1600 1400 1200 1000 800 600 cm-1 Figure 2.3 Infrared spectra of (a) 2, (b) 3 and (c) 4

Conclusive evidence for the structure of the reaction products could be derived from 1H­ NMR and 13C-NMR spectroscopy (fig. 2.4 and 2.5). The assignment is basedon the 1H- and 13 7 C-NMR spectra of the starting compounds and on tabulated increments . Of special interest is the 13C-NMR spectrum of 3 (fig. 2.5) which shows 3 carbonyl resonances, as expected. 22 Chopter 2

a a 0 a a ,...... ,a ao-~-~-oa C=O I c=o I OHb a b b J lJ J 12 10 8 6 2 0 PPM a a 0 aa ao-~-~-oa ,...... ,a C=O I c=o I OCHsb

b .I.

12 10 8 6 4 2 0 PPM aa 0 a a ao-~-~-oa ,.....,..a C=O I c=o I 0 I cH-CHb I dH-C, I 0 b c c 6H-C/ I H,

12 10 8 6 4 2 0 PPM

Figure 2.4 1H-NMR spectra of (a) 2 (b) 3 and (c) 4 (* =solvent peaks) The selective introduetion of .. 23

de f!!llÎ' k

b c

~~~~~.._;..l\IIIL~lloof"!-ni.._~~~~~~""""""",_,.,.~--~.~ÜU1 DMF DMF

200 175 150 125 100 75 50 PPM

Figure 2.5 13C-NMR spectrum of 3

All features of the NMR and IR spectra are consistent with the conversion of the remairring acid chloride group of 1, following well known chemica! reactions, thereby introducing carboxylic acid, ester and epoxy groups onto benzanilide (scheme 2.1). Additional support for the structure of these compounds comes from elemental analysis, which shows that the calculated and found weight percentages C, H and N are within experimental error identical (see experimental).

2.3.2 Higher Homolognes

To perform the above reactions on aramid fibres, the reaetauts have to be brought into close proximity of the surface of the fibres. This requires the swelling of the surface by a suitable solvent, which is a difficult task given the chemica! inertness and high crystallinity of aramid 24 Chapter 2

fibres. Sulfolane is one of the very few solvents capable of swelling aramid fibres. Moreover, it is chemically inert to oxalyl chloride, which explains the choice for this solvent when performing the experiments described below. Benzanilide is the simplest model compound for aramid. To check the validity of the above reaction sequence for the modification of aramid fibres, we performed some of the reactions on higher homologues, ha ving more than 1 amide group in para position and hence even more reminiscent of aramid. For these higher homologues, an oxalylchloride in sulfolane solution and temperatures of 80 "C were used to carry out the first step of the reaction sequence. The first step is the most crucial one in the proposed reaction sequence. The conversion of the remaining acid chloride group in the secoud step follows classica! organic chemistry. Basically the results were identical to those obtained for benzanilide as model compound. Figure 2. 6, showing the 1H-NMR spectra of di(1 ,4-methylbenzene)terephthalamide before and after the reaction with oxalylchloride, may serve as an example of this. The absence of the resonance of the amid proton (ó = 10.06 ppm in di(1 ,4-methylbenzene)terephthalamide) in the spectrum of the reaction product suggests N-substitution similar to the reaction product of benzanilide and oxalyl chloride.

2.3.3 Thermal Stability

A prime requirement of any modification procedure that aims at improving the adhesion is that the modification should be able to withstand the processing temperatures of the reinforeed composites. We therefore investigated the thermal stability of the model compounds 2, 3 and 4 by TGA. The onset of decomposition (temperature of 1% wt loss taken) under a nitrogen atmosphere starts at 140 oe for 3, 143 oe for 4 and 146"C for 2. Initially, the degradation proceeds slowly but increases progressively when heated above 150 "C.

2.3.4 Condusion

In conclusion, this model compound study shows, that the chemical procedure outlined is a versatile metbod for the selective introduetion of a variety of organic groups onto benzanilide, used as model compound for aramid, among them carboxylic acid, ester and epoxy groups. Tlie procedure is not limited to these examples. Due to the limited thermal stability, the applicability is, however confmed to those areas where the processing and/or the use temperatures will not exceed the 140 "C. Still, this temperature is high enough for the enhancement of the acthesion in the majority of the aramid-fibre-reinforced epoxy and unsaturated polyester composites. The selective introduetion of .. 25

eb OaaO bc dHsC -o- ~ -~-o- ~ -~-o- CHsd He He a

d

b a c

11 10 9 8 7 6 5 4 3 2 0 PPM

aa OaaO aa H3c-Q- ~ - ~-o- ~-~-o-CHs C=O C=O I I C=O a C=O I ,------, I Cl Cl

b

11 10 9 8 7 6 5 4 3 2 0 PPM

Figure 2.6 1H-NMR spectra of (a) N,N'-bis(4-methylphenyl)terephthalamide and (b) its reaction product with oxalyl chloride (* =solvent peaks) 26 Chapter 2

2.4 References

1. J. Vekemans and G. Hoornaert, Tetrabedrou 36, 943-950 (1980) 2. A.I. Speziale and L.R. Smith, J. Org. Chem. 28, 1805-1811 (1963} 3. V.N. Tsvetkov, M.M. Koton, I.N. Shtennikova, P.N. Lavrenko, T.V. peker, O.V. Okatava, V.B. Novakowski and G.l. Nosova, Polymer Sci. U.S.S.R. 1883-1893 (1980) 4. Private communications J. Vekemans 5. E.G. Chatzi, M.W. Urban, H.lshida and J.L. Koenig, Polymer 27, 1850-1854 (1986) 6. D.H. Williams and I. Fleming, 'Spektroskopische Methoden zur Strukturaufklärung', George-Thieme Verlag, Stuttgart, 1979, p.40-79 7. Idem, p.80-161 Surface modification of aramid fibres 27

Chapter 3* Surface Modification of Aramid Fibres

3.1 Introduetion

A novel two-step chemica! procedure for the selective introduetion of various functional 1 groups onto the surface of aramid fibres was proposed in the previous chapter . lts feasibility was demonstrated using benzanilide and some higher homologues as model compounds for PPTA. In this chapter, the actual surface modification of aramid fibres according to this novel two-step chemica! procedure will be discussed. Attention will focuss on the characterization of the modified aramid fibres in terms of the effect of the different functional groups on the acthesion to epoxy resin and the effect of the surface modification procedure on the mechanical properties of the aramid fibres.

3.2 Experimental

3.2.1 Reactions

The sizing of the aramid fibres used throughout this study (Twaron D1000) was removed by Soxhlet extraction in dichloromethane, prior to all experiments. The surface treatment procedure comprised two successive stages. At first the aramid fibres, loosely wound around a glass cagelike support, were immersed in a hot (50-60 °C) salution of sulpholane/ oxalylchloride (9: 1 vol/vol) for 1 hour. Next, the fibres we re reacted with water, methanol, ethylenediamine and glycidol, respectively. For the reaction with water and methanol, the oxalylchloride-treated fibres were simply immersed inthereagent soulutions, distilled water or methanol, followed by rinsing with distilled water or methanol. A slightly different procedure was used in the other cases. Before the oxalylchloride-treated fibres were

Reproduced in part from: F.P.M. Mercx and P.J. Lemstra, Polymer Commun. 31, 252- 255 (1990) 28 Chapter 3

immersed in ethylenediamine, the fibres were rinsed with dry dichloromethane to remove excess oxalylchloride adhering to the fibre surface, which otherwise would give rise to (homo )polymer formation. Por the same reason, rinsing with dry diethyl ether was performed prior to the immersion of the oxalylchloride-treated fibres in a glycidolldiethyl ether solution. Excess glycidol was removed by subsequent rinsing with diethyl ether. All the surface modified fibres were dried in vacuo and stored in a desiccator.

3.2.2 X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) was performed on a Physical Electronics 550 XPS/ ABS spectrometer equipped with a magnesium X-ray souree and a double pass cylindrical analyser. Spectra were recorded in steps of 0.05 eV. The pressure did not exceed 6.7xl0-6 Pa, and the eperating temperature was approximately 293 K. Operating conditions of the X-ray souree were 10 kV and 40 mA. A sweeptime of 10 min was used for complete speetral scans, while for detailed recordings a sweeptime of 20 min per element was used. The sample was placed at an angle of 50° to the analyzer, giving a probing depthof about

4 nm for the electrous of the C1, XPS line.

3.2.3 Scanning Electron Microscopy

Scanning electron microscopy (SEM) was performed using a Cambridge Stereoscan 200 microscope, eperating at a voltage of 25 kV. The aramid fibres were coated with a gold/palladium layer approximately 20 nm thick. The gold/palladium coated samples were pressed in silverpaint to ensure good conductivity.

3.2.4 Determination of Adhesion

The effect of the surface treatment, described above, on the fibre-matrix bonding was measured using a multifilament pull-out testl. Specimen preparation consisted of taking two strands of aramid fibres, which were subsequently twisted by 1 turn/cm and embedded in a disk of epoxy resin (1.5-2 mm thick). A medium-viscosity resin, Ciba Geigy LY 556, together with an amine hardener, Ciba Geigy HT 972, were used througbout this study. The following heating cycle was used to cure the resin: (1) heating from room temperature to 80 oe with 2 °C/min, (2) 2 hours at 80 oe, (3) raising the temperature with 4/3 °C/min to 120 oe, (4) 2 hours at 120 oe and (5) cooling toroom temperature by 6,67 °C/min. After curing Sulface modification of aramid fibres 29

and prior to testing, the samples were stored in a conditioned room (23 oe, 50% relative humidity). Tests were run on an lnstron tensile testing machine. The epoxy disc was tïxed on a specially designed grip by applying a slight (pre )strain. The crosshead speed was 10 mm/min. To compare the different results, the bundie pull-out shear strength (BPS) was calculated. The BPS is defined as:

p BPS n dl

where P is the maximum force measured during pull-out (N), d the fibre bundie diameter (mm) and I the embedded length of the fibre bundie (mm). At least six measurements were made for each average value of the BPS.

3.2.5 Determination of Mechanical Properties

The aramid fibres used for the determination of the mechanica! properties were twisted by 1 turn/cm. Tensile tests were performed on a Zwick Rel tensile testing machine. Closed loop operation made accurate constant strain rate experiments possible. The aramid fibres were tested at a strain rate of 10%/min in accordance with ASTM D-76. Initia! cross-sectional areas, used for the calculation of Young's modulus and tensile strengthwere obtained from 3 the mass and the length of the fibres, assuming a crystal density of 1440 kg/m • The values given are the average of at least six experiments.

3.3 Results and Discussion

3.3.1 Chemical Structnre

Evidence gathered on benzanilide as model compound for PPT A indicate that the following 1 reactions will proceed on the surface of the aramid fibres , following the experiments described above, see scheme 3.1. The formation of intermediate I (scheme 3.la) is the key­ step in the reaction sequence in that it provides a highly reactive intermediate. In a subsequent reaction step the 1-surface modified fibres are substituted with different functional groups by reaction with water (scheme 3.lb,c), methanol (scheme 3.lc), ethylene diamine (scheme 3.ld) or glycidol (scheme 3.1e). The reaction of intermediate I with water may be foliowed by a decarboxylation, yielding an aldehyde-modified aramid surface (scheme 3 .lc). 30 Chopter 3

0 0 11 H ~-o-~-~-o-r.l + Cl-C-C-Cl _.., +HCI H HO c~c=o~ oJ. ~-0-~J--0--~JI I n c c=o f I I Cl Cl t (o) n I

and/or ~-0-~J-0-~jC~C:O~ c~-o-~-g c=o~ 0--~j I I I I c c=o H H I l OH OH (b) n r (c) n

~-0-~J-0-~jc-~c=o~ cI c:oI I I 30 OCH3 t (d) n

0 0 ~-o-~-H-Q-a o=c c=o I l O:C C=O HN' NH ' ~' ?i> ' H.9 ?i> H,N NH, n (e)

0 I\ HO-CH,-C-C

(f) Scheme 3.1 Surface modijication of aramid fibres 31

3 4 XPS is a highly sensitive technique for surface analysis • . With a sampling depth of 4 nm, the results presented in tab ie 3. 1 represent surface plus some subsurface materiaL XPS does not analyze for hydrogen. eonsequently, the atom percentages are computed only on the basis of the analyzed elements. Tab ie 3.1 shows the surface composition of the aramid fibres and surface-modified aramid fibres as measured with XPS and expressed as the carbon to nitrogen to oxygen ratio, together with the calculated values according to reaction scheme 3 .1. The experimental error depends strongly on the absolute atom percentages of the elements present. When a particular element constitutes less than 10 atm% of the material the experimental error in the value given is about 15%. For elements which constitute about 20 and 80 atm% of the compound, the experimental error in the value given is about 10% and 5%, respectively. Note that the stoichiometry of the untreated sample points towards an oxidized surface. Similar findings were reported by Penn and Larsen3 and Allred4 and seems typical for all commercial aramid fibres. The surface composition of the treated aramid fibres is within experimental error identical to the calulated theoretica! values according to reaction scheme 3.1.

Table 3.1 Effect of the various treatments on surface composition of Twaron D 1000 aramid fibres

%C %N %0

Treatment meas. cal cd. meas. cal cd. meas. calcd.

None 77 77.8 8 11.1 15 11.1 Oxalylchloride-water 70 64.3./72.7b 5 7.1./9.1b 25 28.6./18.2b Oxalylchloride-methanol 67 66.7 5 6.6 28 26.7 Oxalylchloride-ethylenediamine 65 64.7 17 17.6 18 17.6 Oxalylchloride-glycidol 70 66.7 6 5.6 24 27.7

•calculated according to reaction scheme 3 .1 b bCalculated according to reaction scheme 3 .I c

Detailed information about the nature of the incorporated groups can be obtained from high resolution els• 0 1s and N1s spectra. The els spectra, shown in figure 3.1, are the most informative. The binding energy of carbon (1s) in hydrocarbons is 285 eV. Introduetion of oxygen induces a chemica! shift to higher binding energies for those carbon atoms chemically bonded to oxygen. These shifts relative to els (hydrocarbon) are 1.5 eV for ether/epoxy, 3 3 4 eV for carbonyllaldehyde and 4.5 eV for carboxylic acid/ester groups • • The chemica! shift 32 Chapter 3

3 of carbon in amide groups O=Ç-NH amounts to 3.5 eV • Note that the observed differences between the C1, spectra of surface-modified aramid fibres and untreated aramid fibres, when viewed in terms of the introduetion of the afore-mentioned carbon-oxygen groups are consistent with the reaction schemes outlined above. From line-shape analysis of the C1, spectrum of oxalyl chloride-water-treated aramid fibres and the XPS determined surface composition, it can be concluded that reaction scheme 3.1 b prevails, yielding mainly carboxylic acid-modified aramid fibres. The evidence presented by XPS thus verifies that the methodology developed is effective for the selective introduetion of carboxylic acid, ester, amine and epoxy groups.

c e

b d

a a

289 285 281 289 285 281 Binding energy f eV J Binding energy leV)

Figure 3.1 High resolution C1s spectra ofTwaron D 1000 aramidftbres: (a) untreated, (b) oxalylchloride-water treated, (c) oxalylchloride-methanol treated, (d) oxalylchloride-ethylenediamine treated, (e) oxalylchloride-glycidol treated Surface modification of aramid fibres 33

3.3.2 Adhesion and Mechanical Properties

The introduced amine, epoxy or carboxylic acid groups may or may not participate in subsequent covalent bonding with a curing epoxy resin network. Even if this is not the case, these groups as well as the ester group are capable of forming hydragen bonds with the hydroxyl groups of the resin network. Table 3.2 shows the effect of the various surface treatments on the acthesion to epoxy resin. The maximum impravement in acthesion relative to untreated aramid fibres is 70%. As evidenced by the extensive fibrillation of the epoxy­ modified aramid fibres subjected to the pull-out test, shear failure inside the aramid fibre occurs, indicating that the acthesion is no longer the limiting factor in these composites. Similar results with regard to the bundie pull-out test and failure mode were reported by Elkink et al. 2 for a non-disclosed modification procedure.

Table 3.2 Tensile strength and adhesion to epoxy resin for treated and untreated Twaron DJOOO aramid fibres

Treatment Tensile strength (GPa) Bundie pull-out shear strength (MPa)

None 2.2 (O.l)a 28.3 (2.1)" Oxalylchloride-water 2.2 (0. 1) 43.0 (1.6) Oxalylchloride-methanol 2.2 (0. 1) 38.6 (1.8) Oxalylchloride-ethylenediamine 2.1 (0.1) 38.3 (1.2) Oxalylchloride-glycidol 2.1 (0.1) 52.2 (2.1)

"Standard deviation given in parentheses

SEM micrographs show that the fibre surface remains just as smooth after the treatment as it was before (fig. 3.2). This is consistent with the improved acthesion being caused by the introduetion of the functional groups mentioned earlier. Of the different groups introduced, the epoxy groups are by far the most effective. Similar results were obtained by Takayanagi et aL 7 forT-peel tests performed on untreated, epoxy treated and carboxymethylated aramid fibres. They also noted that for the epoxy-modified aramid fibres, the skin layer was peeled during testing, representing the limit of acthesion at which the fibre itself can notendure the applied force. The amine and carboxylic acid groups, which can also form chemical bonds with the epoxy resin, give smaller improvements in adhesion. In fact, the results are roughly camparabie to the results obtained for the aramid fibers modified withester groups. These 34 Chapter 3

last groups are only capable of hydrogen bonding. This might suggest that the amine and carboxylic acid groups do not form covalent boncts with the epoxy resin. The rather low increase in acthesion following the introduetion of amine groups is rather surprising given the 6 8 excellent results that were previously reported for amine-modified aramid fibres · This could point towards (partial) internal cyclization of the amine group with the carbonyl groups, sim i lar to the cyclized structures found in y-aminopropyl silanes when coated onto glass fibres from solutions of pH= 1 and 79, as aresult of which the amine groups are not available for chemica! reaction with the curing epoxy resin. The tensile strength of the aramid fibres is not affected (table 3.2), suggesting that the procedure is limited to the outer surface layers.

Figure 3.2 Typical examples of scanning electron micrographs of (a) untreated and (b) treated aramid fibres

In conclusion, pull-out tests showed that this newly developed surface treatment procedure markediJ improves the acthesion to epoxy resins. Moreover, the improved acthesion is not achieved at the expense of a decrease in tensile strength of the aramid fibers. Surface modification of aramld fibres 35

3.4 References

1. Chapter 2 2. F. Elkink and J.H.M. Quaijtaal in 'Integration of Fundamental Polymer Science and Technology-3' (Eds. L.A. Kieintjens and P.J. Lemstra), Elsevier Applied Science Publishers, London (1989), p.228-234 3. C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder in 'Handbook of X-ray Photoelectron Spectroscopy' (Ed. G.E. Muilenberg, G.E.), Perkin-Elmer, U.S.A. (1979) 4. D. Briggs in 'Practical Surface Analysis' (Eds. D. Briggs and M.P. Seah), Wiley, Chichester (1983), p.359 5. L. Penn and F. Larsen, J. Appl. Pol. Sci. 23, 59-73 (1979) 6. R.E. Allred, 'Surface Chemica! Modifications of Polyaramid Filaments with Amine Plasmas', DSc Thesis, Massachusetts Institute of Technology (1983) 7. M. Takayanagi, S. Ueta, W-Y. Lei and K. Koga, Polym. J. 19, 467-474 (1987) 8. Y. Wu and G.C. Tesoro, J. Appl. Polym. Sci. 31, 1041-1059 (1986) 9. D.Wang and P.R. Jones, J. Mater. Sci. 28, 2481-2488 (1993) 36 37

PART B: POLYETHYLENE FIBRES 38 Oxidative acid 39

Chapter 4* Oxidative Acid Etching

4.1 Introduetion

Pretreatments are generally necessary to enable a polyethylene to be bonded, coated or printed upon. Oxidative acid etching is one of the most widely used commercial treatments and causes chemica! and physical changes in a thin surface layer. Hydroxyl, carbonyl, 1 4 carboxylic acid and sulphonic acid groups are found at the surfaces of chromic acid • , permanganate acid5 or potassiumchlorate acid5 treated polyethylene. The formation of carbonyl and carboxylic acid groups increases wîth increasîng oxidative power of the acid 2 3 5 1 5 solutîon used and with the time of exposure · • and is accompanîed by chain scission · • Eventually, chain scission will lead to the formation of small fragments that will go into solution. As the rate of oxidation is much faster for the amorphous than for the crystalline regions, oxidative acid etching preferentially removes the amorphous regions and increases 1 2 the surface roughness • . There has been a lively discussion in the literature on the importance of the introduetion of polar groups, surface roughening and the increased wettability that results from these 4 factors in improving the adhesion of oxidative acid treated PE • Although weak boundary layers, that may result from impurities or low molecular weight material, have often been mentioned as the major cause for the difficulty in bonding PE, evidence gathered in recent 6 years clearly shows that this is not the casé • There is a great difference in the surface morphology of the polyethylenes used in the studies mentioned above, which were mostly isotropie ftlms or films of low draw ratio, with the high-strength, high-modulus PE structures produced by gel-spînning. These differences are for instanee retlected in the extremely high crystallinity of the gel-spun PE structures exceeding 90%, compared to 20-60% for conventional LDPE-HDPE. Hence the question arises whether the above mentioned treatments are also effective in improving the adhesion

Reproduced in part from: F.P.M. Mercx, A. Benzina, A.D. van Langeveld and P.J. Lemstra, J. Mater. Sci. 753-759 (1993) 40 Chapter 4

of gel-spun PE structures. Ladizesky and W ard7 were the first to investigate the effect of chromic acid treatment on the adhesion of ultra-drawn PE structures to epoxy resin. Although the acthesion was markedly improved, the effect of acid treatment was less than that of oxygen plasma treatment. Surface roughening of the PE fibres following chlorosulphonic acid treatment was reported by Postema et al. 8 resulting in a five-fold increase in the acthesion to gypsum plaster. Recently, Hsieh et al. 9 attempt to improve the adhesion of gel-spun PE fibres to epoxy resin by pretteatment of the fibres with chromic acid and chromic trioxide solution. The wettability and the interfacial adhesion to the epoxy resin were both improved. The purpose of the present study is to explore whether oxidative acid treatment can improve the acthesion of gel-spun PE structures to epoxy resin and to relate this to the changes in surface chemical composition and surface topography.

4.2 Experimental

4.2.1 Preparation of PE Tapes

Oriented PE tapes were employed in this study as they offer better signal to noise ratios in the spectroscopie techniques used compared to fibres. The tapes were obtained by ultra­ 10 drawing cast films as described previously , except that decatin was replaced by xylene in the prepatation procedure. The cast films were drawnon hotshoes (T=125 °C) to À=60. The PE used was Hostalen Gur 412 with a weight average molar mass (Mw) of about 1.5xl03 kg/rooie. Stabilizer and remaining xylene were removed by subsequentextraction with hexane (15 hr) and methanol (5 hr). The tapes prepared possessed a Young's modulus of 140 GPa, and a tensile strength of 2.4 GPa at room temperature (measured at a strain rate of 10 %/min). It should be noted bere, that the tapes obtained by this batchwise process are identical to those obtained by gel-spinning, precluding that the concentration of the PE solution and the draw ratio are the same. Oxidative acid etching 41

4.2.2 Acid Treatment

PE tapes were irnmersed in chlorosulphonic acid, chromic acid i.e., K2er20iH20/H2S04

(7:12:150 by weight) or KMn0iH20/H2S04 (1:12:150 by weight) at room temperature for different exposure times. The chlorosulphonic acid-treated tapes were rinsed with concentrated sulphuric acid, whereas the KMnOiH20/H2S04-treated tapes were rinsed with concentrated Hel. Next, all tapes were rinsed with distilled water. Finally, the PE tapes were rinsed with acetone, dried and stored in a desiccator.

4.2.3 Determination of Adhesion

Pull-out tests were performed on specimens as illustrated in figure 4.1. A medium-viscosity resin, Europox 730, tagether with an aliphatic amine hardener XE-278 (both obtained from Schering) in the ratio 100115 wt/wt were used throughout this study. The resin was cured for 1 h at room temperature foliowed by heating to 80 oe at a rate of 2 oe/min and kept at this temperature for 1.5 h. After curing, and prior to testing, the samples were stored in a conditioned chamber (23 oe, 50% RH). Tests were run on an Instron tensile testing machine using specially designed grips. The crosshead speed was 10 mm/min. The adhesion was defined as the failure load divided by the interface area. At least 6 measurements were made for each average value of the adhesion strength.

PE- tape

010 mm

Resin cylinders

Figure 4.1 Pull-out specimen 42 Chapter 4

4.2.4 Determination of Mechanical Properties

Tensile tests were perfonned on a Zwick Rel tensile machine. Closed loop operation made accurate constant strain-rate experiments possible. The PE tapes were tested at a strain rate of 10%/min in accordance with ASTM D-76. Initial cross-sectionat areas, used for calculating Young's modulus and tensile strength, were obtained from the mass and the length of the 3 3 tapes assuming a crystal density of 10 kg/cm • The values given are the average of at least 6 experiments.

4.2.5 X-ray Photoelectron Spectroscopy

See § 3.2.2.

4.2.6 lnfrared Spectroscopy

Fourier transfonn reflection-infrared spectra were obtained using either a Perkin-Elmer 1750 equipped with a 1 GE-TRG attenuated total reflection (ATR) unit or a Nicolet 20 SXB equipped with a Specac ATR unit. A germanium crystal (45° face angle) was used at a nominal angle of incidence of 45°. Under these conditions the penetratien depth was about 400 nm at a wave length of 10 p.m.

4.2. 7 Scanning Electron Microscopy

Scanning electron micrographs (SEM) were taken with a Camscan 4-DV. A voltage of 20 kV was used, while the tapes were pressed in silver paint to ensure a good conductivity. The samples were first coated with carbon using an Emscope TB-500 Carbonstring coater. Secondly a gold/palladium (80/20 wtlwt) coating was applied in a Polaron E-5000 diode sputtercoater. The coating thus applied had a total thickness of about 50 nm. Oxidative acid etching 43

4.3 Results

4.3.1 Adhesion versus Mechanica! Properties

The etiect of exposure time to acids on the acthesion and tensite strength is shown in figures 4.2 and 4.3 and table 4.1. The time of exposure had no influence on the Young's modulus of 140 GPa. Chlorosulphonic acid and chromic acid only slightly affects the tensile strength of PE even after prolonged exposure. Postema et al. 8 reported a greater deercase in tensile strengthafter exposure of gel-spun PE fibres to chlorosulphonic acid. This difference can be explained by the more severe conditions, i.e. higher temperatures, used in these studies. The system K.Mn04/H20/H2S04 had a marked influence on the tensile strengthof the PE tapes.

Table 4.1 Adhesion, tensile strength and sulface composition of acid-etched oriented PE tapes

Treatment Time Pull-out Tensile Surface composition 0/C (min) strength strength atomie% atomie (MPa) (GPa) ratio 0 c s (xl02)

None 0.31 2.42 97.5 2.5 2.6

Chlorosulphonic 0.5 0.45 95.0 4.6 0.4 4.8 acid 0.54 92.9 6.8 0.3 7.3 5 0.65 93.2 6.4 0.4 6.9 30 1.00 2.33 90.3 8.7 1.0 9.6 240 1.07 1.98 91.3 7.4 1.3 8.1

Chromic acid 0.5 1.02 87.7 11.5 0.8 13.1 I 1.16 87.3 12.2 0.5 14.0 5 1.46 83.6 15.3 1.1 18.3 30 1.73 2.21 88.6 10.6 0.8 12.0 240 1.70 1.91 90.9 8.3 0.8 9.1

KMnO.fHp!H2S04 0.5 1.33 86.1 13.9 16.1 1.84 87.7 12.0 0.3 13.7 5 1.86 89.0 10.9 0.1 12.2 10 1.72 30 1.90 1.32 240 Tape broken 44 Chapter 4

25 ,...0.. CiS 20 0. ~v-.------x------.a Broken c è b .c 15 -C) ec: (/) - 10 a :I -0 • "3 0. 5

0 0 10 20 30 40 240 250 Time of treatment (min)

Figure 4. 2 Pull-out strength as a function of treatment time for (a) chlorosulphonic acid, (b)

chromic acid and (c) KMn0/H201H2S04

2.50

2.25 CiS 0.

SZ 2.00 D a .c 4. b Ë> e 1.75 '!ij .m 1.50 ïnc: t! x c 1.25

1.00 0 50 100 150 200 250 Time of treatment (min)

Figure 4.3 Tensile strength of the PE tapes as a function of treatment time for: (a) chlorosulphonic acid, (b) chromic acid and (c) KMnO/HPIH~04 Oxidative acid 45

Consequently, tensile failure rather than pull-out occurred for the PE sample exposed to

KMn04/H20/H2S04 for 240 min (fig. 4.2, table 4.1). Figure 4.2 also shows that the leveHing off acthesion value increases in the order chlorosulphonic acid, chromic acid, 5 11 KMn04/H20/H2S04, i.e., with the oxidation power ofthe acids applied • • The opposite order is found for the time needed to reach this levelling off value.

4.3.2 Scanning Electron Microscopy

The surface of an untreated PE tape, as shown in figure 4.4a, is rather smooth except for the typical microfibrillar structure caused by the hot-drawing process. No change in surface roughness was observed up to 10 min exposure to KMn0iH201H2S04 (fig. 4.4d). Upon further exposure a distinct texture developed, the result of degradation and dissolution of material (fig. 4.4e). Prolonged exposure (240 min) is accompanied by an extensive loss of material producing a highly irregular surface (fig. 4.41). In contrast, no evidence of an increase in surface roughness was found after prolonged exposure to chlorosulphonic or chromic acid (fig. 4.4b, 4.4c). The regionsof the PE tapes embedded in the epoxy resin and subjected to the pull-out test were also exarnined. Apparently, there was no difference in the appearance of the chlorosulphonic and chromic acid treated PE surfaces before and after pull-out, suggesting that failure occurred at the interface. A typical example of the groove in the epoxy matrix, left after pull-out of untreated, chromic or chlorosulphonic acid-treated PE tapes is seen in figure 4.5a. Note that even the typical microfibrillar structure present at the surfaces of these tapes are faithfully replicated in the matrix. It thus appears that even untreated PE tapes are completely wetted by the liquid resin. In the case of KMn04/H20/H2S04 (5 min)-treated PE the situation is quite different. Localized spots of drawn material are visible at the surface of the pulled tapes as well as on the surface of the groove left after pull-out (fig. 4.5b), suggesting the forcible removal of the top surface layer during pull-out. Further evidence for this is obtained by treating the groove left after pull-out of a KMn04/H20/H2S04 (5 min)­ treated PE tape with hot (120 °C) xylene, a known solvent for PE (fig. 4.5c). Clearly the adhering PE has dissolved, leaving a surface that shows a quite good resemblance with the original KMn04/H20/H2S04 (5 min)-treated PE surface. 46 Chapter 4

Figure 4.4 Scanning electron micrographs of acid-etched PE tapes: (a) untreated; (b) chlorosulphonic acid, 240 min; (c) chromic acid, 240 min; (d) KMnO/HPI

, , , H2S04 JO min; (e) KMn0/HPIH2S04 30 min; (f) KMnO/HPIH2S04 240 min Oxidative acid erehing 47

Figure 4.5 Typical examples of scanning electron micrographs of the grooves left ajter pull-out of (a) untreated, chromic or chlorosulphonic acid-treated PE tapes, (b)

KMnO/ HPIH2S04 (5 min)-treated PE tapes and (c) as (b) followed by treatment with hot (120 °C} xylene

4.3.3 Weight Loss

No weight toss was detectable for chromic or chlorosulphonic acid-treated PE samples, not even after prolonged exposure (18 h). Prolonged exposure to KMn04/H20 /H2S04, however, resulted in the partial degradation and dissalution of the PE tape (up to 67% weight toss).

4.3.4 lnfrared Spectroscopy

The ref1ection infrared spectra of treated and untreated PE tapes were identical in all cases, i.e. no oxidation products could be detected. Contrary to reflection infrared spectroscopy, 48 Chapter 4

XPS showed oxidation to have taken place (see § 4.3.5). The sensitivity of surface analysis is far better for XPS with shallow penetradon (4 nm) than for reflection infrared spectroscopy, because of its deep penetradon (400 nm). The fact that reflection infrared spectroscopy failed to detect any chemical changes, indicates that the etching/oxidation is conf'med to the outermost surface layers.

4.3.5. X-ray Photoelectron Spectroscopy

The amount of carbon, oxygen and sulphur as detected by XPS is shown in table 4.1. In the cases of chlorosulphonic acid treatment, traces of chlorine ( < < 1 %) were found. The amount of oxygen incorporated at the surface initially increases with time of treatment. It seems that the gradient of this initial increase is proportional to the oxidadon power of the acids applied. For longer treatment times the oxygen content reaches a maximum, after which it slowly levels off in all cases. The surface chemica! composition of three differently treated PE samples prior to, and after pull-out were determined. The samples investigated were chosen in such a way that a stepwise increase in adhesion level was obtained. No traces of nitrogen were detected, indicating that epoxy, i.e. amine hardener, was not present at the surface of the pulled samples.

Consequently, matrix failure during pull-out can be ruled out. The 0 1.:C1• peak intensity ratios forthese samples are displayed in table 4.2. Comparison ofthe two columns shows that the degree of oxidation before and after pull-out is within the experimental error (12%), for both the chlorosulphonic and chromic acid-treated samples. This suggests that failure occurs at the interface. The KMn04/H20/H2S04-treated sample, on the other hand, showed a significant decrease in oxygen content. This is attributed to faiture inside the PE tape, removing the highly oxidized surface layer, in agreement with the results obtained by SEM.

Table 4.2 The amount of oxygen relative to carbon present at the surface of the etched PE tapes before and ajter pull-out

2 Treatment Time Pull-out strength 0/C atomie ratio (xl0 )

(min) (MPa) before pull-out after pull-out

Chlorosulphonic acid 30 1.00 9.6 8.5 Chromic acid 5 1.46 18.3 16.2 KMn04/H20/H2S04 5 1.86 12.2 5.8 Oxidative acid etching 49

The binding energy of carbon (ls) in hydrocarbons is 285 eV. Introduetion of oxygen induces a chemical shift, only for those carbon atoms chemically bonded to oxygen, to higher binding energies. These shifts are 1.5 eV for hydroxyl, 3.0 eV for carbonyl and 4-4.5 eV for 12 3 carboxylic acid groups .1 • The preserree of sulphonic acid (S03H) groups was concluded 3 12 13 from the position of the S2P peak and tabulated data • • . The e 1s spectra of several treated

PE tapes as well as untreated PE tape are shown in figure 4.6. Note the tailing of the e1s peak on the high-energy side of the treated samples. In almost all cases shown this tail extends up to 5 eV, indicating the presence of hydroxyl, carbonyl and carboxylic acid groups.

The time of exposure had no influence on the general shape of the els spectra of chromic acid and KMn04/H20/H2S04-treated samples. Hence, it is concluded, that the same functional groups are present in all samples. This does not imply that the number of individual functional groups present are not subject to change. Differences were encountered when the els spectra of PE tapes exposed to chlorosulphonic acid for less than 30 minutes were examined; in this case the tailing is limited to 3.5 eV, indicating that carboxylic acid groups are not present.

c e

b d

a a

289 285 281 289 285 281 Binding energy (eV) Binding energy (eV)

Figure 4.6 High resolution C1s spectra of acid-etched PE-tapes: (a) untreated; (b) chlorosulphonic acid, 5 min; (c) chlorosulphonic acid, 30 min; (d) chromic acid,

5 min and (e) KMn0/HPIH2S04 , 5 min 50 Chapter 4

4.4 Discussion

As can be inferred from table 4.1 and tigure 4.2, adhesion of PE to epoxy resin is greatly enhanced by pretteatment of PE with oxidizing acids. The maximum increase in adhesion, as determined by pull-out, is 600% for KMnOiH20/H2S04 treatment. Chlorosulphonic and chromic acid treatment improves the adhesion by 300% and 550%, respectively. Of course, these values are only valid for the reaction conditions used. It is interesting to note that this improverneut in adhesion can be achieved without a severe loss in tensile strength and modulus. This is illustrated by table 4.3, which gives the etching time necessary to reach maximum adhesion as well as the corresponding tensile strength of the PE tapes. The remaining tensile strength was, regardless of type of treatment, 2.2-2.3 GPa, a drop of 10% or less compared to the initial value of 2.4 GPa. Prolonged exposure resulted in a further loss in tensile strength without further improverneut in adhesion. It is therefore peculiar that 14 15 16 Silverstein in bis studies on the wetting , adhesion and failnre of etched PE fibres uses a 4 hour chromic acid etch, well beyond the optimum conditions with respect to acthesion and mechanica! properties.

Table 4.3 The etching time required to reach maximum adhesion (fig. 4.2) and the corresponding tensile strength (fig. 4.3) of the PE tapes

Treatment Time (min) Tensile strength (GPa)

Chlorosulphonic acid 30 2.3 Chromic acid 30 2.2

KMnO.fH20/H2S04 1 2.2

The etching of polyolefins and model compounds by chromic acid is well 1 4 17 18 documented • • • • According to the literature, hydroxyl groups are the first species formed, and further oxidation causes chain scission to give carbonyUaldehyde or carboxylic acid groups. It is likely that the other two acids follow the same scheme, although no comprehensive information is available. The presence of carbonyl and carboxylic acid groups, at the surface of the treated tapes, as detected by XPS, indicates that chain scission bas taken place. Consequently, these broken chain ends act as flaws, initiating failure of the PE tape. This explains the observed decrease in tensile strength of the PE tapes upon exposure to chlorosulphonic acid, chromic acid, KMn04/H20/H2S04• Oxidative acid 51

The improved acthesion of polyolefins to epoxy resin after acid etching can, in general, be related to: 1) surface roughening; 2) an increase in the surface free energy, and consequently the wettability of the surface is improved and the interfacial energy increases; 3) the introduetion of specific functional groups, giving rise to an increase in the physico-chemical interactions at the interface; 2 6 19 or a combination of these · · . However, it should be noted here that these factors are mutually dependent. This makes it difficult to distinguish the influence of one specific factor from the others. SEM observations showed that chlorosulphonic and chromic acid treatment do notproduce significant changes in surface topography. Consequently, surface roughening can be ruled out as a reason for the improved adhesion. A somewhat different situation is encountered for the

KMn04/H20/H2S04-treated tapes. The surface of a 1 or 5 min etched tape is quite smooth and comparable to an untreated tape, whereas a 30 or 240 min treated tape is visibly etched. These differences are not reflected in the acthesion values which are equal within experimental error (fig. 4.2, table 4.1). Examinatien by SEM and XPS of the 5 min KMn04/H20/H2S04- treated tapes after pull-out, revealed that shear failure occurred inside the PE tape and not at the interface. Consequently, the increase in acthesion is not brought about by surface roughening. All the PE surfaces, treated as well as untreated, were completely wetted by the epoxy resin as shown by SEM examinatien of the groove left after pull-out. Hence, differences in wetting as an explanation can be ruled out too. The increase in acthesion is solely caused by the introduetion of functional groups. XPS showed these groups to be hydroxyl, carbonyl, carboxylic acid and sulphonic acid. They are the result from oxidation of the PE by the various acid treatments. As a first approximation we tried to relate the improved acthesion to the amount of oxygen, relative to carbon, i.e. the 0/C ratio. Figure 4. 7 shows, that a linear correlation exists during the initia! stages of oxidation. No such correlation is observed when the overall 0/C acthesion data are plotted in the same figure. This is not surprising. With increasing oxidation the type and number of functional groups are subjected to changes. Furthermore, the various groups differ in their efficiency to 19 improve the adhesion to epoxy resins • Consequently, an exact knowledge of the type and number of the different groups present at the surface is required to re late the differences in adhesion to time or type of treatment, rather than the amount of oxygen introduced. 52 Chapter4

20 .....0. a..CiS 15 A è x E A Cl c: C A ! 10 s [J 0 a 5 a "S a.. 0

0 0 5 10 15 20 0/C * 100

Figure 4.7 Pull-out strength as a function of the 0/C ratio; data taken from table I. o

control; D chlorosulphonic acid, 0-30 min; "' chromic acid, 0-5 min; x KMn04 IHPIH~04 , 0-0.5 min

In conclusion, SEM and XPS studies showed that the improved acthesion to epoxy resin after pretteatment with chlorosulphonic acid, chromic acid or KMnOiH20/H2S04 is brought about by the introduetion of functional groups. Furthermore, at the highest level of acthesion obtained (1.8-1.9 MPa), the limiting factor is no longer the adhesion, but the rather low shear strengthof the (treated) PB-tapes.

4.6 Relerences

1. S. Wu, 'Polymer Interface and Acthesion', Marcel Dekker, New York (1982), p.279- 336 2. P. Blais, D.J. Carlsson, G.W. Csullog and D.M. Wiles, J. Colloid and Interface Sci. 47' 636-649 (1974) 3. D. Briggs, D.M. Brewis and M.B. Konieczo, J. Mater Sci. ll , 1270-1277 (1976) 4. D.M. Brewis and D. Briggs, Polymer 22, 7-16 (1981) 5. C-G. Gölander, PhD-thesis, The Royal Institute of Technology, Stockholm (1986) 6. D.M. Brewis, Int. J. Acthesion and Acthesives 13, 251-256 (1993} 7. N.H. Ladizesky and I.M. Ward, J. Mater. Sci. 18, 533-544 (1983) Oxidative acid etching 53

8. A.R. Postema, A.T. Doornkamp, J.G. Meijer and H. v.d. Vlekkert, Polymer Bulletin 16, 1-6 (1986) 9. Y-L. Hsieh, S. Xu and M. Hartzell, J. Acthesion Sci. Technol. ~, 1023-1039 (1991) 10. P.J. Lemstra, N.A.J.M. van Aerle and C.W. Bastiaansen, Polym J. 19, 85-98 (1987) 11. Handhook of Chemistry and Physics, 66th ed. (Ed. R.C. Weast), CRC Press, Boca Raton (1985), p.Dl51-D158 12. D. Briggs in 'Practical Surface Analysis' (Eds. D. Briggs and M.P. Seah), John Wiley, Chichester (1983), p.359-396 13. C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder in 'Handbook of X-Ray Photoelectron Spectroscopy' (Ed. G.E. Muilenberg), Perkin Elmer, USA (1979) 14. M.S. Silverstein and 0. Breuer, Polymer 34, 3421-3427 (1993) 15. M.S. Silverstein and 0. Breuer, J. Mater. Sci. 28, 4718-4724 (1993) 16. M.S. Silverstein and 0. Breuer, J. Mater. Sci. 28, 4153-4158 (1993) 17. F. Holloway, M. Cohen and F.H. Westheimer, J. Am. Chem. Soc. 73 (1951) 65 18. K.B. Wiberg and R. Eisenthal, Tetrahedron 20 (1964) 1151 19. A. Chew, D.M. Brewis, D. Briggs and R.H. Dahm in 'Adhesion 8.' (Ed. K.W. Allen), Elsevier, London (1984), p.97-114 54 Air- and ammonia-plasma treatment 55

Chapter 5 Air- and Ammonia-Plasma Treatment

5.1 Introduetion

Plasma treatment is the most widely used technique both commercially and scientifically to improve the adhesion of high-modulus PE structures. In a plasma process, gas molecules are dissociated into ions, electrons, free radicals and neutral species. The interaction of these species with the surface of the PE causes chemica! and/or physical changes in a thin surface layer (1-100 nm). The type of plasma employed depends toa large extent on the chemistry 1 3 4 of the resin used. For PE-reinforced epoxy and polyester composites mainly air- • , oxygen- • 13 10, and ammonia-plasma9- treatments are used to increase the level of adhesion. 14 Air or oxygen plasma contains a mixture of active oxygen species, mainly atomie oxygen , and leads to oxidation of the PE. As a result, a variety of functional groups are introduced 1 3 7 9 onto the surface, including hydroxyl, carbonyl, ester and carboxylic acid groups · • • . Surface roughening of the PE fibres, after air- or oxygen-plasma treatment, was noted in some of the investigations1.4-8. Recently, Tissington et al. 7 reported the formation of a crosslinked skin, which was associated with the intense UV radiation of the oxygen plasma at higher input powers. Treatment of the high-modulus PE fibres with air or oxygen plasma markedly improved the acthesion to epoxy and polyester resins and resulted in a change in faiture mode 15 from interface controlled to internat shear within the PE fibre4-6.s-Jo. • There exists much controversy over the importance of the changes brought about by these treatments and, consequently, over the mechanism responsible for the increased adhesion. The discussion focuses on the importance of weak boundary layers, wetting/wettability, surface topography and specific interactions. 16 The composition of an ammonia plasma has been investigated by d'Agostino et al. . Several radicals exist in the plasma, including hydrogen, and primary and secondary amine radicals. Owing to the different reaction rates of the formation and disappearance of these

Reproduced in part form: F.P.M. Mercx, Polymer 35, 2098-2107 (1994) 56 Chapter 5

radicals, the concentration of primary amine radicals is much larger than that of the other radicals. From this observation d' Agostino concluded that the primary amine radical is the species most likely to react with surface radicals, resulting in the incorporation of primary amine groups on the substrate. Amine functional groups have iudeed been identified by dye exchange9-12 and/or XPS measurements9 on the surface of high-modulus PE fibres subjected to ammonia plasma. Scanning electron micrographs showed no change in the surface 11 13 texture • • Fracture surface analysis revealed different failure modes for composites made of ammonia-plasma-treated PE fibres. Fibrillation, which is indicative of shear failure within 21 the PE fibres, is reported by several authors 9- • Contrary to this, clean interface failure was 13 observed by Li et al. • Although no conclusive evidence was gathered, improved wetting and 9 12 covalent bonding • have been mentioned as the mechanisms responsible for the improverneut in acthesion of the ammonia-plasma-treated fibres to epoxy resins. The increase in adhesion 10 to vinylester resins has been attributed to non-covalent interactions • In this study, the effect of air- and ammonia-plasma treatment on gel-spun high-modulus PE structures will be investigated by surface analytica! techniques in actdition to mechanica! tests. Furthermore, the relationship between surface chemistry, surface topography and the adhesion is examined. The main purpose of the study is the assessment of the mechanisms responsible for the increased acthesion as a result of air- or ammonia-plasma treatment.

5.2 Experimental

5.2.1 Polyethylene Tapes

Oriented PE tapes were employed in this study. These tapes were obtained by ultradrawing cast films as described in § 4.2.1.

5.2.2 Plasma Treatment

Plasma treatment was performed using a plasma apparatus developed at the TNO laboratorles in Delft. It consists basically of a reaction chamber (Pyrex glass, 146 mm long, 25 mm diameter), a radiofrequency generatorand a vacuum system. The reaction chamber is fitted with a gas inlet, a pressure gauge, an externally placed inductive copper coil (10 turns, 90 mm long) and an exhaust valve. A 13.56 MHz radiofrequency generator (ENI ACG-3) coupled to a matching impedance network (Astech A TH-50) to minimize the reflected power, was employed for all the experiments. Pressure was measured continuously using a membrane Air- and ammonia-plasma treatment 57

pressure gauge (MKS Baratron 222B). Either dry air (Hoekloos, ;:::: 99,9 % pure) or ammonia (Hoekloos, ;::::99,5 %pure) was used as plasma gas.

The standard experimental procedure consistedof evacuation (P mm = 133.3 mPa) of the reactor for 5 min, foliowed by bleeding in the plasma gas (P 66.6 Pa) for 5 min before initiating the plasma. The flow rate (not measured) was adjusted in such a way that the pressure remained constant at the desired level. Following plasma treatment, the samples were kept in flowing gas for 5 min before the reactor was evacuated once again and air admitted to raise the pressure to environmental conditions. This was done to allow for the decay of residual free radicals. All treated samples were stored in a desiccator over P20 5•

5.2.3 Adhesion, Mechanica) Properties and Chemical Characterization

Pull-out specimens, similar to the ones used in§ 4.2.3, except that the diameter of the small resin cylinder was 3 insteadof 5 mm, were prepared and tested for interfacial shear strength evaluation. The mechanica} properties of the treated and untreated PE tapes were measured as described in § 4. 2.4. Both XPS and fourier reflection infrared spectroscopy were employed to characterize the surface of the treated and untreated PE tapes. For further details see § 3.2.2 and § 4.2.6, respectively.

5.2.4 Scanning Electron Microscopy

Scanning electron microscopy (SEM) was carried out with a Camscan 4-DV. SEM micrographs of the air-plasma-treated PE tapes were taken at an angle of 45 o to enhance the topological features. The samples were frrst coated with carbon using an Emscope TB-500 Carbonstring coater. Then a platina coating was applied in an Ion-Tech B50 ion-beam sputter coater. The coating thus applied had a total thickness of about 25 nm. A voltage of 20 kV or less was used, while the tapes were pressed in silver paint to ensure a good conductivity. Direct observation of the groove resulting from pull-out was in some cases obstructed by the slow degradation of the epoxy by the electron beam. Therefore negative replicas were studied, made by the following procedure. First a layer of silver was applied on top of the epoxy, using a Leybold Hereaus EPA-100 sputter unit. The mechanica! strength of the silver layer was increased by electrochemical treatment with a CuS04/H2S04 solution (250 g CuS04 , 2 40 ml conc. H2S04, 960 ml H20, current density 0.06 A/cm ). The resulting negative replica couid then be lifted easily from the epoxy. These replicas were coated in the same way as 58 Chapter 5

described above for direct comparison with the corresponding PB tapes.

5.3 lnfluence of Process Parameters

The plasma parameters that could be changed during the course of the experiments were residence time, pressure and input power. To study the influence of these variables, a wide range of air-plasma experiments were performed. Residence time was varled from 5 s up to 5 min, with pressures ranging from 20.0 to 66.6 Pa. Input power could not be changed substantially. To initiate the plasma, a minimum input power of about 9 Watt (W) was required. In practice, we therefore used a lower limit of 10 W. On the other hand, an excessive input power (;;:::: 30 W) instantly melted the PB tape. This melting was not solely governed by power, but by residence time as welt. At an input power of 25 W the residence time had to be kept below 120 s, and below 300 s for an input power of 10 W, in order to prevent melting. In contrast to this, no practicallimits were found for the ammonia-plasma treatment. All the air-plasma treatments markedly enhanced the acthesion to epoxy resin. Input power proved to be the only significant variable. With an increase in input power, the shear strength increased and reached a maximum at 25 W. Variations in residence time or pressure had no effect on the acthesion values obtained. SEM and XPS examination of the treated and untreated PB tapes revealed the following. Air-plasma treatment produced a dramatic change in the appearance of the PB surface. Whereas the untreated PB tapes were relatively smooth, air-plasma-treated tapes showed a pitted surface. The size and the shape of these pits appeared to be insensitive to changes in the process variables. A range of oxidation products were found at the surface of the PE tapes after air-plasma treatment. These included hydroxyl, carbonyl and carboxylic acid groups. The degree of oxidation was controlled by the input power and was independent of residence time and pressure. This indicates that oxidation of the surface was already complete within a few seconds, a feature reported by many other 3 7 investigators • • Prolonged exposure only resulted in an increase in depth of oxidation or alternatively a mechanism of simultaneous oxidation and ablation was operative. No attempt was tindertaken to investigate in detail the effect of the process variables on the acthesion of ammonia-plasma-treated PB tapes. As a result of the above experiments, input power was the only variabie taken into account in the main experiments, the results of which are reported below. A fixed residence time of 30 s was employed. The pressure was kept constant at 66.6 Pa. Air- and ammonia-plasma treatment 59

5.4 Results and discussion

5.4.1 Tape Characterization

XPS, IR spectroscopy, SEM and mechanica! tests were used to characterize the PE tapes befare and after plasma treatment. Table 5.1 shows the surface chemical composition of the untreated and plasma-treated PE tapes, as measured with XPS. The amount of oxidation is evident in the oxygen!carbon ratios and the amount of amination in the nitrogen!carbon ratios.

Tahle 5.1 Sulface composition of plasma-treated PE tapes

Plasma Power (W) Surface compositîon (atomie %) Atomie ratio

c 0 N 0/C N!C

None 98.2 1.8 0.02

Air 10 81.5 18.5 0.23 Air 25 73.1 26.9 0.37

Ammonia 10 88.9 2.9 8.2 0.03 0.09 Ammonia 25 88.9 3.0 8.1 0.03 0.09

Detailed information about the nature of the incorporated groups can be derived from the high resolution C1" 0 1, and N1, spectra taken, of which the C18 is the most informative. The binding energy of carbon (ls) in hydracarbon is 285 eV. Introduetion of oxygen induces a chemica! shift to higher binding energies for those carbon atoms chemically bonded to oxygen. These shifts are 1.5 eV for hydroxyl/ether, 3.0 eV for carbonyl/aldehyde and 4-4.5 17 19 eV for carboxylic acid/ester groups " • The chemica! shift of carbon bonded to nitrogen depends on the nature ofthe substituents and amounts to 0.6, 1.8 and 1.8 eV for -NH2, -NCO 17 18 and -N02 , respectively • • Oxygen spectra are less informative, because a similar binding 17 19 energy is aften observed for oxygen atorns in different chemica! environments " • The binding energy of oxygen in carbon-oxygen functionalities is usually about 532 eV, with the exception of O=C-Q-. When carboxylic acid or ester groups are detected in carbon spectra, a second contribution is also seen in the oxygen spectra, some 1.8-2.0 eV higher in binding energy. The binding energy of nitrogen in carbon-nitragen functionalities, i.e. amines, 17 18 nitriles, is 399-400 eV • • Neighbouring electron withdrawing groups, for instanee as found in amides and isocyanates (C=O), cause a small shift of0.5-1.0 eV to higher binding energy. 60 Chapter 5

17 18 A more distinctive shift (3-4 eV) is observed in oxidized nitrogen species (NO, N02) • •

XPS, air-plasma

The C1, and 0 1, spectra of untreated and air-plasma-treated PE are shown in figure 5 .1. The

sharp (1.5 eV at half height) and symmetrical C1s peak for untreated PE indicates a single predominant species of carbon, as expected. Although no high binding energy shoulder is

visually discemable in the e 1s peak, some minor oxidation is evidenced by the appearance of a small O~s peak at 532,4 eV. Compared to untreated PE, tailing on the high energy side of

the e1, spectrum is observed upon 10 W air-plasma treatment. This tail extends up to 6 eV, indicating the presence of hydroxyl, carbonyl and carboxylic acid groups. The corresponding oxygen peak is located at 532,4 eV, which is consistent with carbonyl and/or hydroxyl

groups. Closer examination shows that the 0 1, curve is slightly asymmetrie, with some tailing on the high energy side. Apparently a minor component with higher binding energy, assessed to carboxylic acid groups, is present. The amount of oxygen introduced increases with increasing power (table 5.1). This is not surprising, since at a given pressure and frequency, an increase in field strength (i.e. a rise in input power) causes a correspondingly higher concentration of active species owing to the increased number of electron collisions. The resulting increase in flux of active species at the PE surface may enhance the rates of the

oxidation reaction. A comparison of the e1, spectra of 10 and 25 W air-plasma-treated PE tapes reveals that the extent of tailing is identical for both spectra. Hence the same type of functional groups are present at the surface, although the concentration of the different groups changes with input power. Lineshape analysis indicates an increased concentration of carbonyl and, especially, carboxylic acid groups at the surface of the 25 W treated sample. The corresponding oxygen curve is located at 533.1 eV, intermediate between the binding energy of oxygen in carbonyl (532.4 eV) and carboxyl (O=e-Q, 533.9 eV) groups. The shift of0.7 e V, compared to the 0 1, spectrum of a 10 W treated PE sample, tends to conf'rrm the introduetion of a substantial amount of carboxylic acid groups. Air- and ammonia-plasma treatment 61

c

b

a

293 289 281 533 529 Binding energy (eV) Binding energy (eV)

Figure 5.1 High-resalution and 0 1, spectra of (a) untreated, (b) JO W air-plasma-treated and (c) 25 W air-plasma-treated PE tapes (spectra are scaled to have the same peak height for comparison)

XPS, ammonia-plasma The nitrogen species observed after treatment in ammonia plasma have a binding energy of 399.9 eV, regardless of the input power used (fig. 5.2). Furthermore, as judged from the symmetry and sharpness of the N 1, peak, only one nitrogen functionality is present, which is 17 19 most likely amine in nature - . The degree of amine attachment is independent of the input 11 power (table 5.1), as was also observed by Holmes and Schwartz • Although amination prevails, some oxidation occurs, as evident from the smal! increase in oxygen concentration. The binding energy of 532,4 eV for oxygen, indicates that oxygen is present in the form of hydroxyl and carbonyl functionalities. A question remains regarding the souree of oxygen giving rise to the observed oxidation. lt may stem from system contamination by oxygen or oxygen-containing species. Another possibility is the reaction of surface free radicals with atrnospheric oxygen. 62 Chapter 5

c c c

b b b

a

281 536 532 Binding energy (eV) Binding energy {eV) Binding energy (eV)

Figure 5.2 High-resalution C1s , 0 1s and N1, spectra of (a) untreated, (b) JO W ammonia­ plasma-treated and (c) 25 W ammonia-plasma-treated PE tapes (spectra are scaled to have the same peak height for comparison)

lnfrared spectroscopy In contrast to XPS, no traces of oxidation or amination products could be detected by reflection infrared spectroscopy. The sensitivity of surface analysis is far better for XPS with shallow penetration (4 nm) than for reflection infrared spectroscopy (400 nm). The fact that reflection infrared spectroscopy failed to detect any chemical changes indicates that oxidation and amination are confined to the outermost surface layers.

SEM The surface of an untreated PE tape, as shown in figure 5.3a, is rather smooth except for the typical microfibrillar structure caused by the hot-drawing process. Air-plasma treatment resulted in severe etching of the tapes (fig. 5.3b,c), giving a pattem of short cracks on the surface, perpendicular to the drawing direction, which will be referred to as pits. This pattem is not exclusive to plasma treatment: similar, although larger, pitted structures are reported Air- and ammonia-plasma treatment 63

Figure 5.3 Scanning electron micrographs of (a) untreated, (b) JO W air-plasma-treated, (c) 25 W air-plasma-treated, (d) JO W ammonia-plasma-treated and (e) 25 W ammonia-plasma-treated PE tapes (drawing axis vertical for air-plasma-treated PE tapes, otherwise horizontal) 64 Chapter 5

for shrinkable synthetic fibres (wool, polyamide 6.6, poly(ethylene terephthalate)(PET)) 20 21 exposed to laser excimer radiation • • A synergetic model based on thermal contributions and shrinkage behaviour was recently presented21 to explain the development of these pitted structures. Although no conclusive evidence was gathered, our observations indicate that the conditions as found in air-plasma treatment of PE tapes and laser excimer radiation of wool, polyamide 6.6 or PET fibres are more or less identical. We therefore assume that a similar mechanism to that described by Balmers and Scholmeyer!t is operative. In contrast to the above, no evidence of an increase in surface roughness was found after ammonia plasma treatment (fig. 5.3d,e). Differences in topography as a function of plasma gas were also 5 reported by Nardin and Ward . They observed that upon exposure of melt-spun PE filaments to a water-vapour plasma, smaller pits were produced i)l comparison to exposure of simHar ftlaments to an oxygen plasma, keeping the other conditions constant.

Crosslinking 19 22 Plasma treatment may produce crosslinking within the top surface layer1· • • To check this, we exposed the untreated and plasma-treated PE tapes to boiling xylene. For the 25 W air­ plasma-treated PE tapes only, a small, initially insoluble, fraction remained ({0.1 %), which finally dissolved after prolonged exposure (± 1 h) to boiling xylene. This indicates that some crosslinking takes place, although the extent is not enough to produce an insoluble network. All other PE tapes instantly dissolved in the boiling xylene. The crosslinking is thought to be related to the ultraviolet radiation present in the plasma, which leads to chain scission and the 7 14 formation of free radicals in the PE • • Owing to the competition between the recombination of the free radicals and the reaction with atomie oxygen, crosslinking can only occur when the ultraviolet intensity is high. Both the nature of the plasma gas and the input power have a strong influence on the ultraviolet intensity and may account for the differences described above.

Mechanica! properties The tensile strength of the PE tapes did not change significantly following anunonia- or 25 W air-plasma treatment (table 5.2). A 10 W air-plasma treatment on the other hand caused a small deercase (10%) in tensile strength. This was attributed to the accompanying change in surface roughness. The surface pits produced, are likely to act as flaws and hence reduce the tensile strength. Crosslinking of the upper surface layers increases the strength of these layers, thereby neutralizing the negative effects of the surface pits. As a result, higher values are found for the 25 W air-plasma-treated PE tapes, despite the similar surface topography of the 10 and 25 W air-plasma-treated PE tapes. The Young's modulus is not affected by either air-orammonia-plasma treatment. Air- and ammonia-plasma treatment 65

Table 5.2 Adhesion, tensile strength and modulus of plasma-treated PE tapes

Plasma Power (W) Pull-out strength Tensile strength Modulus (MPa)a (GPat (GPa)•

None 0.32 (0.08) 2.41 (0.06) 140 (7)

Air 10 2.25 (0.09) 2.15 (0.10) 145 (4) Air 25 2.68 (0.10) 2.29 (0.07) 138 (5)

Ammonia 10 2.44 (0.06) 2.32 (0.08) 141 (5) Ammonia 25 2.47 (0.07) 2.37 (0.09) 136 (6) aStandard deviation is given in parentheses

5.4.2 Adhesion and Failure Mode

Plasma treatment markedly enhances the acthesion of oriented PE tapes to epoxy resin, as illustrated by table 5.2. These results show a more than seven-fold increase in shear strength to 2.25 MPa after a 10 W air-plasma treatment. A further gain in acthesion to 2.68 MPa could be realized by raising the input power to 25 W. Ammonia-plasma-treated PE yielded shear strength values of about 2.45 MPa, independent of the input power used. To establish the failure mode, fracture surfaces were examined by SEM and XPS. As shown in figures 5.3a and 5.4a, there appears to be little difference between the surface topography of the PE tapes before and after pull-out. Furthermore, XPS analysis of the pulled PE tape and the groove leftafter pull-out showed a clean PE (table 5.3) and epoxy surface, respectively. These results indicate that slippage along the interface is the primary mode of 1 13 23 failure, as bas been observed previously by others - · . The debonded part of air-plasma­ treated PE tapes exhibits a rather smooth surface with spots of drawn material (fig. 5.4b). No trace of the original pitted surface is visible. These results suggest that a layer of material bas been removed owing to failure inside the PE tape. This is substantiated by the XPS results (table 5.3). Notrace of nitrogen could be detected, indicating that epoxy resin is notpresent at the surface of the air-plasma-treated PE tapes after pull-out. Matrix failure can therefore be excluded and interfacial failure would not have altered the surface composition significantly. Consequently, the sharp decrease in oxygen concentration points to shear faîlure inside the PE removing the highly oxidized surface layer. XPS was also used to unambiguously assess the failure mode of the ammonia-plasma-treated PE tapes. Following the same line of reasoning as above, the reduction in nitrogen content to virtually zero 66 Chapter 5

(:::;; 0.1 %) indicates shear failure within the PE tapes to be the primary mode of failure. The corresponding roughened fracture surface is shown in figure 5.4c. Although all the plasma­ treated PE tapes fail cohesively, failure occurs at different pull-out stresses (2.25-2.68 GPa), indicating differences in shear strength of the upper surface layers following plasma treatment. These differences are thought to arise from topographical features (pits) in actdition to crosslinking and/or chain scission caused by the plasma process.

Figure 5.4 Typical examples of scanning electron micrographs of the pulled PE tapes: (a) untreated; (b) air-plasma-treated and (c) ammonia-plasma-treated (drawing axis vertical for air-plasma-treated PE tapes, othervvise horizontal) Air- and ammonia-plasma treatment 67

Table 5.3 Suiface composition of pulled PE tapes

Plasma Power (w) Surface composition (atomie %)

c 0

None 97.9 2.1

Air 10 94.6 5.4 Air 25 93.4 6.6

Ammonia 10 94.5 5.5 Ammonia 25 95.6 4.4

5.4.3 Mechanism of Adhesion

As mentioned in the introduetion there exists much controversy about the nature of the changes brought about by plasma treatment and, consequently, the mechanism responsible for the increased adhesion. The discussion focuses on the importance of weak: boundary layers, wetting/wettability, surface topography and specific interactions. The role of each factor bas been investigated and will be discussed to some extent. Impurities in polymers, such as lubricants, antistatics and antioxidants, are known to migrate to the film surface and to take part in weak boundary layer formation. It bas been proposed that plasma treatment removes these impurities toa large extent (ablation), thereby increasing the adhesion. The removal of remaining solvent and stabilizer by extraction, prior to plasma treatment, should minimize the effect of weak: boundary layer formation. In fact, XPS analysis (table 5.1) indicates a clean surface for the as-made and hence extracted PE tapes. It is generally accepted that wetting of a substrate by the resin is a prerequisite, but is not necessarily sufficient, to obtain a good adhesion. In other words improper wetting always yields low acthesion values. In the case of PE, the wettability is low, owing to its non-polar surface. Plasma treatment might increase the wettability through the introduetion of polar 24 groups and/or surface roughening • As such, it can be argued that differences in wettingare the reflections of differences in these factors, and that wetting is not a separate factor. Nevertheless we investigated the wetting of the PE tapes by the epoxy resin used in this study. SEM micrographs of negative replicas of the grooves left after pull-out and subsequent extraction with boiling xylene (to remove any adherent PE) are shown in figure 5.5. A distinctive surface texture can be seen on the replicas of the grooves from air-plasma-treated PE tapes (fig. 5.5b). The resemblance between this micrographand those shown in figure 68 Chapter 5

5.3b and 5.3c is remarkable. The same resemblance is found between the micrographs of the untreated and arrunonia-plasma-treated PE tapes and the replicasof the corresponding grooves (fig.5.3a,d,e and fig. 5.5a,c). Even the typical microfribillar structure present at the surface of these tapes is faithfully replicated in the matrix. It thus appears that all, even the untreated, PE tapes are completely wetted by the epoxy resin, as observed by other investigators4.5. Hence, differences in wetting are an unlikely explanation for the differences in adhesion values.

Figure 5. 5 Typical examples of scanning electron micrographs of the replicas of the xylene­ extracted grooves leftafter pull-out of (a) untreated, (b) air-plasma-treated and (c) ammonia-plasma-treated PE tapes (drawing axis vertical for air-plasma­ treated PE tapes, othe1wise horizontal) Air- and ammonia-plasma treatment 69

SEM and XPS examination, in actdition to the foregoing discussion, reveal that three factors add to the acthesion following air-plasma treatment: the acthesion due to mechanica! interlocking (Tm); physico-chemical binding due to the introduetion of functional, oxygen containing groups (Tr); and non-polar dispersion farces (Td). The last factor is responsible for the acthesion of the untreated, non-polar PE tapes to epoxy resin. The pull-out strength is the sum of these three terms5 and can be written as:

T = Tm + Tr + Td

No significant changes in the non-polar dispersion component of the surface free energy of 25 26 carbon surfaces is observed after oxidation or amination · We therefore assume that the same is true for plasma-treated PE and that the contribution of the non-polar dispersion strength to the total shear strength is constant and equals 0.32 MPa. To clarify the

contributions of Tr and T m• air-plasma-treated PE tapes we re subjected to non-constrained annealing in a nitrogen atmosphere (130 oe, 45 h). Neither the surface chemica! composition nor the amount of different chemica! groups (C-0, C=O, 0-C=O) were altered by this treatment, as was shown by XPS. It resulted, however, inasmoothing of the surface texture. In fact, no observable differences o_n a micron sca)e (SEM) could be detected between the surface roughness of the annealed (fig. 5.6) and the as-made PE tapes (fig. 5.3a). Consequently, by employing these

Figure 5.6 Scanning electron micrograph of an annealed air-plasma treated PE tape (drawing axis vertical) 70 Chapter 5

PE tapes, pull-out values are obtained free from the contribution of surface roughening. Table 5.4 summarizes the results of these tests and the calculated values forTrand Tm. The greatest contribution to the acthesion sterns from the functional groups, which accounts for about 76%. Somewhat lower values, i.e. 56-62%, are found for the contribution of functional groups in the adhesion of oxygen-plasma-treated melt-spun PE fibres to epoxy resin5 {calculated from the data given in reference 5). Of the oxidation products, the carbonyl and carboxylic acid 27 groups are among the most effective for improving the adhesion to epoxy resin • Raising the input power increases the concentration of these groups, which explains the higher absolute value of Tr for the 25 W air-plasma-treated PE tapes. The contribution of mechanical interlocking to the adhesion is much lower, with values of 9 and 12% for 10 and 25 W air­ plasma-treated PE tapes, respectively. Despite a similar surface topography, higher absolute values of Tm are found for the 25 W air-plasma-treated PE tapes. This, as well as the higher absolute value forT, is ascribed to the greater shear strengthof the 25 W treated PE tape, which is related to the crosslinking of the upper surface layer. The calculation presented above shows that the introduetion of functional groups is the major factor contributing to the increase in adhesion following air-plasma treatment. It accounts for 90% (10 W) and 84% (25 W) of the measured increase in adhesion. For melt-spun highly drawn PE fibres, more research has been conducted to elucidate the factors influencing the adhesion to epoxy resin after oxygen-plasma treatment. Although mechanica! interlocking was previously thought to be the major factor4, recent research indicates that both improved wetting and crosslinking 7 play a significant role • There are some major differences with respect to structure and crystallinity between these melt-spun highly drawn low-molecular-weight PE fibres and the

Table 5.4 Contributton of junctional groups (T1 ), mechanica/ interlocking (Tm) and non­ polar dispersion jorces (Td) to the adhesion (r)

Plasma Power (W) r (MPa) Taa (MPa) r/ (MPa) Tmb (MPa) r/ (MPa)

None 0.32 0.32 (100)

Air 10 2.25 2.06 1.74 ('l7) 0.19 (9) 0.32 (14) Air 25 2.68 2.35 2.03 (76) 0.33 (12) 0.32 (12)

Ammonia lO 2.44 2.12 (87) 0.32 (13) Ammonia 25 2.47 2.15 (87) 0.32 (13) a r3 =pull-out strengthof annealed air-plasma-treated PE tapes hcontribution (in per cent) is given in parentheses Air- and ammonia-plasma treatment 71

high-molecular-weight PE tapes used throughout this study. Little is known about the effect of molecular weight and structure on the different aspects of plasma treatment, i.e. crosslinking, oxidation, pitting and thus a direct comparison of the results between these high­ modulus PE structures is not possible. A somewhat different situation is encountered for the ammonia-plasma-treated PE tapes. The increase in acthesion is governed solely by the introduetion of amine groups, as shown by SEM and XPS examination. Table 5.4 lists the values for Tr· The introduced hydroxyl, carboxylic acid or amine groups by air- and ammonia-plasma treatment may participate in subsequent covalent bonding with a curing resin network. But even if this is not the case, these groups, as well as the carbonyl groups are capable of forming hydrogen bonds with the hydroxyl groups of the resin network. At present it is not clear whether covalent bonding takes place between the modified PE tapes and the epoxy resin or whether coulomb and/or hydrogen bonding forces prevail.

5.5 Conclusions

This study was aimed at investigating the changes brought about by air- and ammonia-plasma treatment of gel-spun, high-strength, high-modulus PE tapes and relating these changes to the increased acthesion to epoxy resins. It was found that:

- air-plasma treatment introduces hydroxyl, carbonyl and carboxylic acid groups at the surface of the PE tapes. It also results in severe etching of the surface, giving a pattem of short cracks perpendicular to the drawing direction. - ammonia-plasma treatment results in the incorporation of amine groups but does not alter the surface topography. - three factors are responsible for the acthesion of the air-plasma-treated PE tapes: (1) physico-chemical interactions between the various carbon-oxygen functionalities and the epoxy resin; (2) mechanica! interlocking; and (3) non-polar dispersion forces. The pull-out strength is the sum of these three terms and their contributions in this specific system are about 76%, 12% and 12%, respectively. - the acthesion of ammonia-plasma-treated PE tapes is determined by physico-chemical interactions in actdition tonon-polar dispersion forces. Their contributions to the acthesion in this specific system are 87% and 13%, respectively. the introduetion of functional groups and the subsequent increase in physico-chemical interactions is the major factor contributing to the increase in acthesion following air- and ammonia-plasma treatment. 72 Chapter 5

- the marked increase in acthesion following plasma treatment is evidenced by a change in faiture mode from interface controlled to cohesive failure inside the PE tape.

5.6 References

1. H.X. Nguyen, G. Rinhi, G. Wood andA. Poursartip, Proceedings of33thlnternational SAMPE Symposium, Anaheim (1988), p1721-1729 2. O.S. Kolluri, S.L. Kaplan and P.W. Rose, Eng. Plastics~. 127-133 (1992) 3. D.A. Biro, G. Pleizier and Y. Deslandes, J. Appl. Pol. Sci. 47, 883-894 (1993) 4. N.H. Ladizesky and I.M. Ward, J. Mater. Sci. 18, 533-544 (1983) 5. M. Nardin and I.M. Ward, Mat. Sci Technol. 2_, 814-827 (1987) 6. N.H. Ladizesky and I.M. Ward, Comp. Sci. Technol. 26, 129-164 (1986) 7. B. Tissington, G. Pollard and I.M. Ward, J. Mater. Sci. 26, 82-92 (1991) 8. S. Gao and Y. Zeng, J. Appl. Polym. Sci. 47, 2065-2071 (1993) 9. P.J.C. Chappell, J.R. Brown, G.A. George and H.A. Willis, Surf. Interface Anal. 17, 143-150 (1991) 10. J.R. Brown, P.J.C. Chappell and Z. Mathys, J. Mater. Sci. 27, 6475-6480 (1992) 11. S. Holmes and P. Schwartz, Comp. Sci. Technol. 38, 1-21 (1990) 12. J.R. Brown, P.J.C. Chappell and Z. Mathys, J. Mater. Sci. 27, 3167-3172 (1992) 13. Z-F. Li, A.N. Netravali and W. Sachse, J. Mater. Sci. 27, 4625-4632 (1992) 14. M. Hudis in 'Techniques and Applications of Plasma Chemistry' (Eds. J.R. Hollaban and A.T. BeU), John Wiley, New York (1974), p.l13-147 15. A.A.J.M. Peijs, H.A. Rijsdijk and P.J. Lemstra in 'Compósites, Design, Manufacture and Application' (Eds. S.W. Tsai and G.S. Springer), SAMPE, Covina (1991), p.ll-j-1 16. R. d' Agostino, R. Cramarossa, S. De Benedictis and G. Ferraro, Plasma Chemistry and Plasma Processing 1(1), 19-35 (1981) 17. D. Briggs in 'Practical Surface Analysis' (Eds. D. Briggs D. and M.P. Seah), Wiley, Chichester (1983), p.359-396 18. C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder in 'Handbook of X-ray Photoelectron Spectroscopy' (Ed. G.E. Muilenberg), Perkin-Elmer, USA (1979) 19. L.I. Gerenser, J. Adhesion Sci. Techno!. 1, 303-318 (1987) 303 20. T. Bahners and E. Schollmeyer, Angew. Makromol. Chem. 151, 19-37 (1987) 21. T. Bahners and E. Schollmeyer, Angew. Makromol. Chem. 151, 39-47 (1987) 22. Y. Yao, X. Liu and Y. Zhu, J. Adhesion Sci. Technol. 1. 63-75 (1993) 23. Chapter 4 24. W.D. Bascom in 'Advances in Polymer Science-85' (Ed. K. Dusek), Springer, Berlin (1988), p.89-124 Air- aru:l ammonia-plasma treatment 73

25. S.P. Wesson and R.E. Allred in' Acid-Base lnteractions: Relevanee to Acthesion Science and Technology' (Eds. K.L. Mittal and H.R. Anderson Jr.), VSP, Utrecht (1991), p.145-170 26. H.P. Schreiher and F. St. Gennain in 'Acid-Base Interactions: Relevanee to Acthesion Science and Technology' (Eds. K.L. Mittal and H.R. Andersou Jr.), VSP, Utrecht (1991), p.273-286 27. A. Chew, D.M. Brewis, D. Briggs and R.H. Dahm in 'Adhesion 8' (Ed. K.W. Allen), Elsevier, London (1984), p.97-114 74 Corona grafting of acrylic acid 75

Chapter 6* Corona Grafting of Acrylic Acid

6.1 Introduetion

Oxidative processes such as acid etching1 and air-plasma2 treatment markedly improves the acthesion of high-modulus PE structures to epoxy resin, as measured by pull-out and evidenced by the change in failure mode from interface controlled to shear failure within the PE. By monitoring the changes in wetting, surface topography and surface composition, the factors responsible for the impravement in acthesion coulct be ictentified. The major factor contributing to the increase in acthesion was found to be the physico-chemical interactions between the various carbon-oxygen functionalities, introduced by the above-mentioned 1 2 pretreatments and the epoxy resin • . Of these oxidation products, carboxylic acid groups are 3 among the most effective for increasing the acthesion to epoxy resin . In this chapter, the corona grafting of high-modulus PE tapes with acrylic acid will be investigated, anct the effect on both the acthesion to epoxy resin and the mechanica! properties will be discussed.

6.2 Experimental

6.2.1 Polyethylene Tapes

The preparation of the oriented PE tapes, used throughout this study, bas been described in § 4.2.1.

*Reproduced in part from: F.P.M. Mercx, Polymer 34, 1981-1983 (1993) 76 Chapter 6

6.2.2 Corona Grafting

Tbe surface grafting was performed by first treating the PE tapes with a He/Ar corona, followed by exposure to acrylic-acid-saturated He gas. The home-built corona apparatus basically consistsof areaction chamber, a teflon-coated and ground aluminum plate and a ceramic-coated electrode. The standard experimental procedure consisted of evacoation of the reactor (P~ 1.3 mPa), after which He/Ar gas (83/17 vol/vol, P=9.1xlü4 Pa) was introduced. This step was repeated twice before initiating the corona (2,9 kV, 50Hz). The reactor was evacuated once again after the treatment, then acrylic-acid-saturated He gas was introduced. Aftera post-bleed of 15 min in this gas mixture (P=9.lxlü4 Pa) the reactor was evacuated again, then air was introduced to raise the pressure to environmental conditions. All treated samples were extracted with hot (50 oq water for 24 hours, dried and stored in a desiccator

over P20 5•

6.2.3 Characterization

Besides contact angles, X-ray photoelectron spectroscopy (XPS) and attenuated total reflection infrared (ATR-IR) were used to evaluate tbe effect of the corona treatment described above on the surface chemica! composition. For further details see § 3.2.2 and § 4.2.6, respectively. Scanning electron microscopy was performed using a Cambridge Stereoscan 200 microscope, operating at a voltage of 25 kV or less. The PE tapes were pressed in silverpaint to eosure good conductivity. The samples for SEM were coated with a gold/palladium layer approximately 15 nm thick. Details about the procedures used to determine the interfacial shear strength and the mechanica! properties of the treated and untreated PE tapescan be found in§ 5.2.3.

6.3 Results and Discussion

As a result of the interaction of inert gas electric discharges with PE, free radicals are 4 produced within the top surface layers , the number of which depends on the processing conditions. The parameters that could be changed during the course of the experiments were voltage and residence time. As the corona was only stabie within a narrow range of voltages, residence time was the only variabie taken into account. The optimum residence time was derived by measuring the contact angle with water after exposure of the corona treated 4 5 samples to air for 15 min. This leads to oxidation • , the degree of which is linked to the Corona grajting of acrylic acid 77

number of radicals present. Contact angles are a convenient way to follow the degree of oxidation and are inversely related to it. lt was found that the contact angle initially decreases with residence time, going from 68° for the virgin PE tape to 39° for PE tape treated for 1 min, after which it remains constant. A residence time of 1 minute presents an optimum and was employed in the surface grafting experiments described below.

6.3.1 Tape Characterization

X-ray photoelectron spectroscopy 6 The presence of oxygen can inhibit the grafting reaction , which is the reason for evacuating the corona reaction chamber twice befare initiating the corona, and for degassing the acrylic acid before use. The surface composition of the PE tapes and surface-modified PE tapes, as measured with XPS and expressed as the oxygen-to-carbon ratio, is given in table 6.1. Detailed information about the nature of the incorporated

Table 6.1 Surface composition and mechanica/ properties of untreated and acrylic acid­ grafled PE tapes

Treatment Surface Pull-out Tensile Tensile composition strengtha modulus• strengtha (0/C) (MPa) (GPa) (GPa)

None 0.03 0.32 (0.04) 138 (6) 2.40 (0.06) Acrylic acid grafted 0.35 2.52 (0.07) 141 (13) 2.43 (0.05)

•standard deviation given in parentheses

groups can be obtained from the high resolution and 0 1s spectra. The C1s spectra, shown in figure 6.1, are the most informative. The binding energy of carbon in hydrocarbons is 285 eV. Introduetion of oxygen induces a shift, for those carbon atoms chemically bonded to oxygen, to higher binding energies. A shift of 4.5 eV, as found in the surface-modified PE tapes, is characteristic of carbon atoms in a carboxylate environment (-0-C=O), i.e. carboxylic acid or ester groups6.7. Peak-fitting the spectrum provides evidence for the existence of C-0 (286.5 eV) and C=O (288 eV) functionalities, although they constitute less than 2% of the total amount of carbon-oxygen functionalities. Most likely these oxidation products are the result of the reaction of long-living radicals with oxygen. From the absence 78 Chopter 6

of a significant contribution at 286.5 eV (carbon singly bonded to oxygen), it was concluded, that the majority of the intensity of the peak at 289.5 eV sterns from carboxylic acid groups.

293

(a)

293 281 Binding energy (eV)

Figure 6.1 High resolution C1s spectra of (a) untreated and (b) acrylic acid-grafled PE tapes

From the peak area at 289. 5 e V, and allowing for the contribution to the hydrocarbon peak, the composition over the penetration depth was estimated to be 64% (poly)acrylic acid and 36% PE. This value is in good agreement with the composition calculated from the 6 0 1,/C1• ratio, i.e. 65% (poly)acrylic acid. Similar result were reported by Munro for the irradiation grafting of LDPE in the presence of acrylic acid vapour. Photoinitiated surface grafting of acrylic acid onto high-modulus PE fibres (Spectra 900)8 yielded lower values ( S: 50%) for the acrylic acid content of the surface (calculated from the XPS data given in reference 8). Corona grajting of acrylic acid 79

The C 1, peak for the virgin PE-tape is a single sharp peak (1.5 eV at half height) centred at 285 e V as expected for a clean PE surface. The broadening of the hydracarbon peak ( 1. 8 eV at half height) after corona graftingis indicative of sample damage, probably in the form of chain scission. lnfrared spectroscopy The reflection infrared spectrum of the surface-modified PE tapes shows two extra peaks compared to the reflection infrared spectrum of PE. Most prominent is a small peak at 1720 8 9 cm· 1, which is consistent with the stretching vibrations of C=O in acid groups . The relative 1 intensity of this C =0 peak compared to the CH2 (1465 cm- ) peak is 1.2%. A broad and hardly discernable peak at 3100 cm·1 is ascribed to 0-H stretching vibrations. The fact that 1 reflection infrared failed to detect the 0-H deformation (900-950 cm- ) and coupled C-0 1 9 stretching and 0-H deformation (near 1430 and 1300 cm· ) vibrations of carboxylic acids indicates that the grafting of acrylic acid is confined to the outermost surface layers.

Scanning electron microscopy The surface of the virgin PE tapes (fig. 6.2a) is rather smooth, except for the typical microfibrillar structure caused by the hot-drawing process. No change in surface texture was evident after the corona grafting of acrylic acid (fig. 6.2b).

Figure 6.2 Scanning electron micrographs of (a) untreated and (b) aCJylic acid-grafled PE-tapes 80 Chapter 6

6.3.2 Adhesion and Mechanical Properties

The introduced carboxylic acid groups may or may not participate in covalent bonding with the curing resin network. Even if this is not the case, these groups may form hydrogen bonds with the hydroxyl groups in the cured resin network. Although the exact mechanism of the improverneut is not yet clear, the adhesion to epoxy resin is markedly enhanced (table 6.1). The increase in adhesion by a factor of eight compares well with the improverneut found after 2 10 11 12 13 air- • • or oxygen-plasma • treatment, the most effective treatments reported so far. Moreover, and contrary to the case of air- or oxygen-plasma treatment, the tensile strength of the acrylic-acid-grafted PE tapes is not affected (table 6.1). Feng and Ranby8 found a three-fold increase in the adhesion of PE fibres grafted with acrylic acid by a photoirradiation method. As indicated before, the extent of acrylic acid grafting was far less for their procedure compared with corona grafting, and may account for the differences in adhesion observed.

6.3.3 Surface Treatment and Shear Strength

In this chapter as well as the previous two chapters, the effect of pretreatments, such as acid etching, plasma treatme:Ó.t and the corona grafting of acrylic acid, on the acthesion of high­ performance PE structures to epoxy resins have been investigated. Though all these pretreatments markedly improved the adhesion, some remarks can be made in eetrospeet which might be useful to future research. Fracture analysis of the pulled PE tapes indicates shear failure inside the PE to be the dominant mode of faiture for the KMn04/H20/H2S04-, air- and ammonia-plasma-treated PE tapes. Failure occurs at different pull-out stresses, indicating differences in shear strength. A question remains regarding the origin of these differences. On the one hand there are the corona-grafting of acrylic acid and the ammonia plasma treatment. These processes mainly introduce acrylic acid and amine groups, respectively, and are relatively mild with little or no chain scission. Their pull-out valnes are identical within experimental error and amounts toabout 2.5 MPa. On the other handthereare the oxidative processes such as acid etching and air-plasma treatment. Oxidation of PE yields functional groups like hydroxyl, carbonyl and carboxylic acid. The formation of the most effective 3 4 14 species i.e. carbonyl and carboxylic acid groups , is accompanied by chain scission • • It is believed that this chain scission negatively influences the shear strengthand is responsible for the lower pull-out values of the KMnOiH20/H2S04- and 10 W air-plasma-treated PE tapes. The recent observations by Gao et al. 15 support this view. They observed that u pon oxygen­ plasma treatment, the acthesion of high-modulus PE fibres to epoxy resin initially increased Corona of acrylic acid 81

with exposure time up to an exposure time of 300 s. At this stage, the failure mode changed from interfacial controlled to shear faiture inside the PE fibre. Ongoing oxidation leads to a rednetion of the mechanica! properties of the fibre surface, causing a decrease in the pull-out valnes measured. Additional support for the above view is derived trom the 25 W air-plasma-treated PE tapes. Even though oxidation induced chain scission is more prominent than in the case of a 10 W air-plasma treatment, UV induced crosslinking prevails leading to a crosslinked skin (see chapter 5). The pull-out value measured (2.68 MPa) does not differ significantly from the pull-out valnes found for the acrylic acid-grafted or ammonia-plasma-treated PE tapes. It thus seems that a pull-out value of about 2.5-2.6 MPa presents an upper limit which is related to the shear strength of the PE tapes produced.

6.4 References

1. Chapter 4 2. Chapter 5 3. A. Chew, D.M. Brewis, D. Brigs and RH. Dahm in 'Adhesion 8' (Ed. K.W. Allen), Elsevier, London (1984), p.97-114 4. S. Wu, 'Polymer Interface and Adhesion', Marcel Dekker, New York (1982), p.279- 336 5. C.H. Bamford and J.C. Ward, Polymer ~. 277-293 (1961) 6. H.S. Munro, Polymer Commun. 221-223 (1987) 7. D. Briggs in 'Practical Surface Analysis' (Eds. D. Briggs and M.P. Seah), John Wiley, Chichester (1983), p.358-396 8. Z. Feng and B. Ranby, Angew. Makromol. Chem. 17-33 (1992) 9. D.H. Wilhams and I. Fleming, 'Spektroskopische Methoden zur Strukturaufklärung', George Thieme Verlag, Stuttgart (1979), p.40-79 10. D.A. Biro, G. Pleizier and Y. Deslandes, l Mater. Sci. Letters 883-894 (1992) 11. O.S. Kolluri, S.L. Kaplan and P.W. Rose, Eng. Plastics 2,, 127-133 (1992) 12. B. Tissington, G. Pollard and I.M. Ward, Comp. Sci. Techno!. 44, 185- (1992) 185 13. N.H. Ladizesky and I.M. Ward, J. Mater. Sci. 3763-3773 (1989) 14. D.K. Owens, J. AppL Polym. Sci. 19, 265-271 (1975) 15. S. Gao and Y. Zeng, J. AppL Polym. Sci. 2093-2101 (1993) 82 The role of fibre anisotropy ... 83

Epilogue The Role of Fibre Anisotropy and Adhesion on Composite Performance

Carothers was one of the first to indicate that molecular orientation and chain extension is the route for increasing the strengthand stiffness of polymerie fibres. In this view, fully aligned polymerie ebains will inherently show the highest mechanical properties. This goal was long pursued by polymer chemists and physicists and finally realized in the seventies with the development of aramid and PE fibres. Typical 1-dimensional structures result, with strong covalent bondsin the chain direction and weak intermolecular interactions between the chains. Consequently, the mechanica! properties of these fibres are highly anisotropic, with unrivalled mechanica! properties in the chain direction but rather poor off-axis and compressive properties. The consequence of such an extreme anisotropy for composite performance was insufficiently realized in the mid-eighties. Particularly, because initia! experiments showed the lack of adhesion to be the prirnary cause of failure (see § 1.2). Top priority was thus given to the irnprovement in adhesion. However, with adequate methods developed, interest in aramid and PE fibre reinforeed polymers for structural applications was renewed. This resulted in a nurnber of investigations to assess the performance of these composites under 3-dimensionalloading conditions, as encountered in real practice. Ladizesky and Ward1 were the first to report on the mechanical properties ofunidirectional melt-spunldrawn PE-fibre-reinforced epoxies under different loading conditions (tensile, bending, shear) and the effect of plasma pretreatrnent of the PE fibres on these composite properties. The most thorough study was, however, conducted by Peijs et aU who studied the influence of adhesion on the off-axis and compressive properties of unidirectional (gel­ spun) PE-fibre-reinforced epoxies. The most important results are summarized in table 7 .1. Although plasma treatrnent markedly enhances the interface dominated off-axis properties such as the transverse and shear strength, the absolute values are still quite low when compared to (well-bonded) glass- and carbon-fibre-reinforced epoxies (table 7.2). Since fractographic studies indicate that the increase in properties is accompanied by a change in failure mode from interface controlled to fibre splitting, it has to be concluded that these low values are related with the low lateral strengthof the (gel-spun) PE fibres. In cornpression, 84 Epilogue

the poor performance is also the result of the low lateral strength of the PE fibre. The increase in adhesion following plasma treatment prevents the debonding and splitting of the PE composite as observed for untreated PE fibre reinforeed epoxies under compression, but at only marginally higher compressional stresses internat faiture via kink band formation occurs. Compared to the exceptional, and often mentioned, longitudinal tensile properties, the off-axis properties and on-axis compressive properties of, even well-bonded, PE-fibre­ reinforced composites are low and fall far bebind those of glass- and carbon-fibre-reinforced composites. Fibres which are isotropie (glass) or have a less pronounced anisotropic character (high-strength (HS)-carbon fibre).

Table 7.1 Strengths of unidirectional PE-fibre-reinforced epoxies (lj= 50%)2

Untreated PE fibres Plasma-treated PE fibres

Longitudinal tensile strength (MPa) 910 1070

Transverse tensile strengtil (MPa) 2.5 8

Longitudinal compressive strengtil (MPa) 73 91

Transverse compressive strength (MPa) 21 44

Interlaminar shear strength (MPa) 14 30

Table 7.2 Strengths of unidirectional glass-3 and HS-carbon4-fibre-reinforced epoxies (lj=50%)

E-glass fibres HS.Carbon fibresa

Longitudinal tensile strengtil (MPa) 910 1780

Transverse tensite strength (MPa) 65

Longitudinal compressive strengtil (MPa) 827 750

Interlaminar shear strength (MPa) 76 100 aSurface treated high-strengtil carbon fibre, XA-S of Courtauld The role anisotropy ... 85

Thus, it has to be concluded that the highly anisotropic properties of the PE fibres, which result from the typical 1-dimensional structure with strong covalent honds in the chain direction and weak Van der Waals interactions between the aligned chains, are detrimental for composite performance under 3-dimensional toading conditions. Structural applications are thus limited. As a consequence, applications more and more focus on the outstanding energy absorption of the PE fibre. Since a poor fibre-matrix acthesion is beneficia] forthese applications, it seems that surface modifications of the PE fibres, as described in this thesis, are of minor practical importance for improving the applicability of PE fibres. This is true when only the mechanica! properties of PE fibre-based composites are considered. However, without surface treatment the impregnation of multifilament tows or fabrîcs with the resin is difficult if possible at all, thereby obstructing the commercial production of PE fibre based 5 composites • Surface treatment markedly improves the impregnation and surface-treated fibres are nowadays widely applied for the production of PE fibre-reinforced products such as 5 helmets, loudspeaker cones, fishing rods and radomes • So it can be concluded that the applicability of PE fibre in commercial products relied on the availability of effective surface modifications. A different situation is encountered for aramid-fibre-reinforced composites. Acthesion not only increases the interface dominated properties, it raises these properties to a level comparable to that of glass fibre reinforeed composites (table 7.3). Ultimately, the maximum attainable properties are determined by the fibre properties, as was the case for the PE fibre reinforeed composites. However, and because of the strenger hydrogen-bonding between the aligned aramid chains, the off-axis properties of aramid fibres and thus aramid-fibre­ reinforced composites lie at a distinctive higher level than those for PE fibres and PE-fibre­ reinforced composites. The major drawback for widespread structural application of aramid­ fibre-reinforced composites is the low compressive strength. Although aramid-fibre-reinforced composites outperfarm PE-fibre-reinforced composites in this respect, it cannot match the compressive properties of glass- or carbon-fibre-reinforced composites. Compressive failure 6 7 occurs via kink band formation within the aramid fibre • and explains why adhesion has little or no influence on the compressive strength measured. In contrast herewith, significant benefits from the improved acthesion are encountered durîng fatigue testing. Contrary to PE fibre reinforeed composites, the overall performance of well-bonded aramid fibre reinforeed composites justify the use in structural applications where compressive loads are absent or minimized. Otherwise, hybridization with glass or carbon fibre can offer a 8 9 solution • . Besides sports equipment, aramid-fibre-reinforced composites are widely used in aerospace. As fatigue is of major concern in these applications, a good fibre-matrix bond is essential. Thus is can be concluded that the development of an adequate acthesion was essential for the application of aramid fibres in these structural applications. 86 Epilogue

Just as the PE fibres, aramid fibres have an outstanding energy absorption and aramid­ fibre-reinforced composites are likewise used for impact and ballistic protection. Por these specific applications, a low fibre/matrix bond strength is generally beneficial.

1 13 Table 7.3 Typical strengths of epoxy based unidirectional composites (l't=60%) 0-

E-glass HS-carbon• HM-aramidb

Longitudinal tensile strength (MPa) 1000 1240 1380

Transverse tensile strength (MPa) 34 45 28

Longitudinal compressive strength (MPa) 550 830 20

Transverse compressive strength (MPa) 140 140 140

Interlaminar shear strength (MPa) 80 80-95 70

•High-strength carbon fibre, Torayca T-300 (Toray) t;(evlar 49 (DuPont) or Twaron D-1057 (Akzo), E=l25 GPa, ub=2,7 GPa

References

1. N.H. Ladizesky and I.M. Ward, Comp. Sci. Technol. 26, 129-164 (1986) 2. A.A.J.M .. Peijs, H.A. Rijsdijk, J.M.M. de Kok and P.J. Lemstra, Comp. Sci. Techno!. 52, 449-466 (1994) 3. Engineers' Guide to Composite Materials (Eds. J.W Weeton, D.M. Peters and K.L. Thomas), ASM International, USA (1987), p.6-45 4. A.A.J.M. Peijs, P. Catsman, L.E. Govaert and P.J. Lemstra, Composites 21, 513-521 (1990) 5. Private communications M.J.N. Jacobs, DSM High Performance Pibres, 1995 6. S.J. Deteresa, S.R. Allen, R.J. Parris and R.S. Porter, J. Mater. Sci. 19, 57-72 (1984) 7. H.P. Wu and J.R. Yeh, J. Mater. Sci. 27, 755-760 (1992) 8. G. Kretsis, Composites 18, 13~23 (1987) 9. N.L. Hancox, 'Pibre Composite Hybrid Materials, Applied Science Publishers, London (1981) 10. Composites, Engineered Materials Handbook-Vol. 1 (Eds. C.A. Dostal and M.S. Woods), ASM International, USA (1987), p.178 The roZe anisotropy ... 87

11. E.K. Drown, H. Al Moussani and L.T. Drzal, J. Acthesion Sci. Technol. ~. 865-881 (1991) 12. M. S. Madhukar and L.T. Drzal, J. Comp. Mater. 932-957 (1991) 13. F. Elkink and J.H.M. Quaijtaal in 'Integration of Fundamental Polymer Science and Technology 3' (Eds. L.A. Kleintjes and P.J. Lemstra), Elsevier, London (1989), p.228- 234 88

Summary

Owing to the superior properties per unit weight (specific properties), fibre reinforeed polymers more and more replace traditional construction materials as metals, wood and concrete indemanding applications. Initially applied in military and aerospace applications, fibre reinforeed polymers have now penetrated other segments of the market as well, including the automotive industry. The replacement of traditional materials was not achieved easily and was in fact proceeded by elaborate research to optimize the (mechanical) properties of fibre reinforeed polymers. The development of new high-performance fibres was but one important step. Besides the wen known glass fibres, three other high performance fibres have gained commercial importance. These are in chronological order of development, carbon fibres, aramid fibres and polyethylene fibres. In order to effectively use the strength and stiffness of these high-performance fibres, which are typically 20-200 times stiffer and 15-70 times stronger than the polymer matrix, they have to be strongly bonded to the matrix. However, the chernical nature of the as-made fibres in combination with the smooth surface provides only a modest acthesion at best. Therefore, the developments in the area of acthesion were at least equally decisive for the evolution of fibre reinforeed polymers to its present accepted status as competetive construction materials for metals, wood and concrete. For the two oldest high-performance fibres, glass and carbon, many surface treatments have been developed to overcome the initial weak bond strengthof the as-made fibres. In fact, the nowadays applied commercial treatments are more than adequate for bonding glass and carbon fibres to virtually all thermoset and thermoplastic polymers. A different situation is encountered for the two organic fibres. Having been introduced much later than glass and carbon fibres, the optimization of acthesive properties is still under investigation.

The main objective of the research described in this thesis is to improve the acthesion of high­ performance aramid and PE fibres to epoxy resins via surface modification of the reinforeing fibres. Besides the effect of these treatments on the mechanical properties, attention is focused on the relationship between the chemistry and topography of the fibre surface, the acthesion and the failure mode. In this way, the mechanisms responsible for the improved acthesion as well as the failure mode can be assessed. The latter indicate whether adhesion, the shear strength of the fibre or the shear strength of the matrix is the limiting factor in the 89

performance of these composites.

For the aramid fibres a novel chemica! modification procedure was developed, which is described in part A of this thesis (chapters 2 and 3). In partBof this thesis (chapters 4,5,6), acid etching, air- and ammonia-plasma treatment and corona-grafting of acrylic acid have been employed as pretreatments to improve the acthesion of the high-performance PE fibres and are described. Finally, in chapter 7, some concluding rernarks are made with respect to the effect of surface treatment on the shear strength of the upper surface layers of the PE tapes (§7.1) and the role of fibre anisotropy and acthesion on composite performance (§7.2).

Part A: Aramid fibres

In chapter 2, organic reacrions on aromatic amides as model compounds for aramid fibers have been studied to evaluate the feasibility of a two step chemical modification procedure for the selective introduetion of specific organic groups. The first and key step in the modification procedure is the reaction of the aromatic amides with oxalylchloride yielding a highly reactive intermediate. In the second step, the remairring acid chloride group is converted into different organic groups according to known methods. Examples shown include the reaction with water, methanol and glycidol to introduce acid, ester and epoxy groups. All reaction products were characterized by infrared, 1H-NMR and 13C-NMR spectroscopy in addition to elemental analysis, proving the almost quantitative conversion of the model compounds. The thermal stability of the reaction productsis about 140-150 °C.

The actual surface modification of aramid fibres, according to the procedure developed in chapter 2, is discussed in chapter 3. The evidence presented by X-ray photoelectron spectroscopy (XPS) verifies that the methodology developed is effective for the selective introduetion of carboxylic acid, ester, amine and epoxy groups at the surface of aramid fibres. The acthesion to epoxy resins, as measure by a multifilament pull-out test, was markedly improved by the introduetion of the afore-mentioned groups and fibre properties were not affected. Of the different groups introduced, the epoxy groups are by far the most effective and raise the pull-out strength by 70%. As evidenced by the extensive fibrillation of the epoxy-modified aramid fibres subjected to the pull-out test, shear faiture inside the aramid fibre occurs, indicating that acthesion is no longer the limiting factor in these composites. 90

Part B: Polyethylene fibres

Oriented PE tapes were employed in this study instead of fibres as they offer a better signal to noise ratio in the spectroscopie techniques used, allowing the accurate determination of the surface chemica! composition after the adhesion measurements and related herewith the identification of the failure mode. lt should be noted here, that the morphology and the mechanical properties of these tapes are identical to those of the gel-spun fibres.

The adhesion of high-performance PE structures to epoxy resin is greatly enhanced by pretreatment with oxidizing acids (chapter 4). For KMn04/H20/H2S04 treatment a maximum increase in adhesion, as determined by the pull-out test, of 600% was found. Chrornic and chlorosulphonic acid treatment improve the adhesion with 550% and 300%, respectively. Moreover, this improverneut was obtained without substantially affecting the mechanical properties. By monitoring the changes in surface topography (SEM) and surface composition (XPS) the improverneut in adhesion could be related to the introduetion of oxygen-containing groups, i.e. hydroxyl, carbonyl, carboxylic acid. Furthermore, at the highest level of adhesion obtained the failure mode changed from interface controlled to shear failure within the polyethylene tape.

Air- and ammonia-plasma treatments markedly improve the adhesion of gel-spun, high­ modulus PE tapes to epoxy resin, as measured by pull-outand evidenced by the change in failure mode from interface controlled to shear failure within the PE (chapter 5). By monitoring the changes in wetting, surface topography and surface composition, the factors responsible for the adhesion could be identified. These results suggest that for air- plasma­ treated PE tapes the adhesion depends on (1) the physico-chernical interaction between the various carbon-oxygen functionalities and the epoxy resin, (2) mechanical interlocking and (3) non-polar dispersion forces. The pull-out strength is the sum of these three terms and their contributions in this specific system are 76%, 12% and 12%, respectively. The adhesion of ammonia-plasma-treated PE tapes to epoxy resin is determined by physico-chernical interactions of the amine groups and non-polar dispersion forces only, with contributions of 87% and 13%, respectively. Summary 91

Chapter 6 describes the grafting of high-modulus PE tapes with acrylic acid using a two-step procedure. The tapes were first subjected to a He/ Ar corona discharge, immediately foliowed by exposure to acrylic acid saturated He gas. Evidence for the grafting was provided by X­ ray photoelectron spectroscopy, which showed the surface of the treated tapes to consist of 64% acrylic acid and 36% PE. The grafting of acrylic acid is confined to the outermost surface layers as indicated by reflection infrared spectroscopy. Pull-out tests showed that the corona grafting of acrylic acid impraves the acthesion to epoxy resins by a factor eight. Moreover, the increased acthesion is not achieved at the expense of a decrease in mechanica! properties of the high-modulus PE tapes. A comparison between the different treatments described in this section of the thesis, revealed that oxidative processes such as acid etching and air-plasma treatment negatively influence the shear strengthof the PE tapes.

In the epilogue some remarks are made with respect to the role of fibre anisotropy and acthesion on composite performance. PE and aramid fibres typically consist of aligned polymerie chains, withstrong covalent bondsin the chain direction and weak intermolecular interactions (Van der Waals or hydragen bonding) between the chains. The mechanica! properties are consequently extremely anisotrapic, with unrivalled mechanica! properties in the chain direction but rather poor off-axis and compressive properties. Parallel research recently showed that ultimately, not the adhesion, but the low off-axis and compressive properties of aramid and, particulary, PE fibres limit the performance of aramid and PE fibre reinforeed composites under 3-dimensionalloading conditions as encountered in real practice. 92

Samenvatting

Vezelversterkte polymeren combineren een hoge sterkte en stijfheid met een lage soortelijke massa. De specifieke treksterkte en stijfheid, d.w.z. de voornoemde mechanische eigenschappen per eenheid van massa, zijn gelijkwaardig aan of overtreffen die van traditionele constructiematerialen zoals staal, aluminium, hout en beton. Het is dan ook niet verwonderlijk dat deze vezelversterkte polymeren, die initieel vooral in de militaire sector en de ruimtevaartindustrie werden toegepast, in toenemende mate hun weg vinden in de transportsector, de bouw en sportartikelen. De sterke groei van de vezelversterkte polymeren, ten koste van de traditionele constructiematerialen, werd voorafgegaan door een grote onderzoeksinspanning om de mechanische eigenschappen van deze materialen te optimaliseren. Essentieel was de ontwikkeling van nieuwe hoogwaardige vezels, die de eigenschappen van de al langer toegepaste glasvezels ver overtreffen. Drie van de vele ontwikkelde nieuwe vezels hebben een commerciële status bereikt. In chronologische volgorde van ontwikkeling zijn dit de koolstofvezels, de aramide-vezels en de polyetheenvezels. Om de sterkte en stijfheid van de versterkingsvezels optimaal te benutten moeten de uitwendige spanningen via de polymere matrix op de vezels worden overgedragen. Dit vraagt om een goede en, onder alle omstandigheden, blijvende hechting tussen matrix en vezels. De chemische samenstelling en het gladde oppervlak van de hoogwaardige versterkingsvezels vormen echter een belemmering voor het verkrijgen van een goede hechting. Vandaar dat resultaten op het gebied van de hechtingsverbetering mede bepalend zijn geweest voor de ontwikkeling van vezelversterkte polymeren als concurerend constructiemateriaal voor metalen, hout en beton. Voor glas- en koolstofvezels, de twee oudste versterkingsvezels, zijn vele oppervlaktebehandelingen ontwikkeld ter verbetering van de hechting. In feite zijn de huidige commercieel beschikbare methoden meer dan adequaat voor het goed hechten van deze versterkingsvezels aan nagenoeg alle thermoplastische en theemobarde polymeren. Ook voor de jongere aramide- en polyetheenvezels zijn verscheidene, min of meer effectieve, behandelingen ontwikkeld. In tegenstelling tot de glas- en koolstofvezels zijn de problemen met de hechting echter nog niet volledig opgelost Het optimaliseren van de vezel­ matrixverbinding vormt dan ook een belangrijk onderwerp van onderzoek. Samenvatting 93

Het belangrijkste doel van het in dit proefschrift beschreven onderzoek is het verbeteren van de hechting van aramide- en polyetheenvezels aan epoxy-harsen via oppervlaktemodificatie van de versterkingsvezel. Aangezien chemische modificaties de mechanische eigenschappen van de vezels nadelig kunnen beïnvloeden, zijn de sterkte en stijfheid van de vezels voor en na de behandelingen gemeten. Daarnaast is veel aandacht besteed aan de relatie tussen de chemische samenstelling en topografie van het oppervlak, de hechtsterkte en het bezwijkgedrag. Hiermee kan het mechanisme achter de verbeterde hechting worden achterhaald. De wijze waarop de proefstukken bezwijken geeft aan of de vezel-matrix hechting, de afschuifsterkte van de vezel of de afschuifsterkte van de matrix de limiterende factor is in de prestaties van deze vezelversterkte polymeren.

In deel A van dit proefschrift wordt een tweestaps chemische modificatieprocedure voor aramidevezels beschreven (hoofdstukken 2 en 3). In deel B komt de modificatie van polyetheenvezels door behandeling met oxiderende zuren, blootstelling aan lucht- en ammoniakplasma's evenals de corona geïnduceerde introduktie van acrylzuur-groepen aan bod. De (mechanische) eigenschappen van aramide- en polyetheenvezels zijn uitgesproken anisotroop. Het effect hiervan op de prestaties van met deze vezels versterkte polymeren wordt in de epiloog besproken, waarbij tevens de rol van vezel-matrix hechting nadrukkelijk aan de orde komt.

Deel A: Aramide vezels

In hoofdstuk 2 wordt de uitvoerbaarheid van een tweestaps chemische modifcatieprocedure voor aramidevezels onderzocht aan de hand van modelreacties, waarbij aromatische amiden als model voor aramide fungeren. De essentiële stap in de modificatieprocedure is de reactie van de aromatische amiden met oxalylehloride waarbij een intermediair met één reactieve zuurchloride-groep ontstaat. Via bekende organische methoden kan deze zuurchloride-groep in vervolgreacties worden omgezet naar voor de hechting bruikbare groepen. Tot de voorbeelden die gegeven worden, behoren de reacties met water, methanol en glycidol waarbij zuur-, ester- en epoxy-groepen geïntroduceerd worden. De resulaten wijzen op een nagenoeg kwantitatieve omzetting van de modelstoffen en ondersteunen de uitvoerbaarheid van de voorgestelde tweestaps modificatieprocedure.

De daadwerkelijke modificatie van aramidevezels, op basis van de in hoofdstuk 2 ontwikkelde methodologie, wordt beschreven in hoofdstuk 3. Oppervlakteanalyse van de vezels voor en na de verschillende behandelingen laat de introduktie zien van zuur-, ester-, amine- en epoxy­ groepen,en daarmee de toepasbaarheid van de modificatieprocedure. Alle gemodificeerde 94

vezels vertonen een aanmerkelijk betere hechting aan epoxy-harsen, zoals vastgesteld m.b.v. pull-out testen. De epoxy-groepen blijken van de verschillende geïntroduceerde groepen het meest effectief te zijn en verhogen de pull-out sterkte met 70%. Opvallend is de sterke fibrillatie van de epoxy gemodificeerde aramidevezels na de pull-out test. Dit is een indicatie dat niet langer de hechting maar de interne afschuifsterkte van de aramidevezel de limiterende factor zal zijn in de prestaties van de op deze vezels gebaseerde composieten.

Deel B: Polyetheen vezels

Het in dit deel van het proefschrift beschreven onderzoek werd uitgevoerd met hooggeoriënteerd polyetheen {PE) tapes, gemaakt via het verstrekken van PE-gelen. De morfologie en de mechanische eigenschappen komen nagenoeg overeen met die van gelgesponnen PE vezels. De belangrijkste reden voor het gebruik van tapes in plaats van vezels ligt in de betere signaal/ruis verhouding van de tapes in de toegepaste spectroscopische technieken. Dit maakt een nauwkeuriger bepaling van de chemische samenstelling van het oppervlak na de hechtingsexperimenten mogelijk. Hierdoor kan het bezwijkgedrag nauwkeuriger vastgesteld worden.

De hechting van PE tapes aan epoxy-harsen neemt sterk toe na voorbehandeling van de PE tapes met oxiderende zuren (hoofdstuk 4). Voorbehandeling met KMn0iH20/H2S04 gaf een maximale verbetering van 600% in de, met pull-out tests gemeten, hechtsterkte. Iets lagere waarden werden gevonden voor de overige in het onderzoek betrokken oxiderende zuren, chroomzuur (550%) en chlorosulfonzuur (300%). Belangrijk is verder dat deze voorbehandelingen de mechanische eigenschappen van de vezel niet of nauwelijks negatief beïnvloeden(~ 10%). Verscheidene oppervlakteanalyse-technieken werden ingezet om de veranderingen in de topografie en chemische samenstelling van het oppervlak van de PE tapes te volgen. Dit maakte het mogelijk om de verbeterde hechting te relateren aan de introduktie van zuurstofhoudende groepen, waaronder hydroxyl, keton en zuurgroepen. Met dezelfde oppervlakteanalyse-technieken werd het breukgedrag in kaart gebracht. Bij de hoogst bereikte waarde van de hechtsterkte bezwijkt de vezel intern. In alle andere gevallen vindt onthechting tussen vezel en matrix plaats.

Een sterke toename in de hechtsterkte wordt gemeten na voorbehandeling van de PE tapes met lucht- (maximaal 900%) of ammoniak-plasma's (800%), waarbij een verandering in breukgedrag optreedt van breuk aan het vezel-matrix grensvlak naar afschuiving in de PE tape (hoofdstuk 5). Uit de veranderingen in topografie, chemische samenstelling van het oppervlak Samenvatting 95

en bevochtigingsgedrag konden de factoren die een rol spelen in de hechting van deze plasma­ behandelde tapes worden vastgesteld. Lucht-plasma's hebben een sterk oxiderende werking. De aan het oppervlak gevormde oxydatieprodukten zorgen voor een sterke toename in de fysisch-chemische interacties met de epoxy- hars. De bijdrage van deze fysisch-chemische interacties in de hechting is 76%, tegenover 12% van de mechanische verankering als gevolg van de sterk toegenomen oppervlakteruwheid. Terwijl de hechting van de niet behandelde PE tapes volledig valt toe te schrijven aan de apolaire Van der Waals krachten, spelen deze krachten met 12% een ondergeschikte rol in de hechting van de luchtplasma behandelde PE tapes. De hechting van de ammoniak-plasma behandelde tapes wordt bepaald door fysisch­ chemische interacties van de geïntroduceerde amine-groepen met de epoxy-hars (87%) en apolaire Van der Waals krachten (13%).

Een tweestaps modificatieprocedure werd toegepast om selectief acrylzuur-groepen te introduceren aan het PE oppervlak (hoofdstuk 6). Hiertoe worden de PE tapes onmiddelijk na een He/ Ar corona-ontlading blootgesteld aan met acrylzuur verzadigd He gas. Oppervlakteanalyse-technieken bevestigen de introduktie van de acrylzuur-groepen en de hechtsterkte neemt met een factor 8 toe. Bovendien tast de procedure de mechanische eigenschappen van de PE tapes niet aan. Uit een vergelijking van de in dit deel van het proefschrift beschreven behandelingen blijkt dat oxydatieve behandelingen zoals, zuuretsen en lucht-plasma behandeling een negatieve invloed heeft op de afschuifsterkte van de PE tapes.

In de epiloog wordt nader ingegaan op het belang van de sterke vezelanisotropie en de hechting op het functioneren van de polyetheen- en aramidevezel versterkte polymeren in de praktijk. Kenmerkend voor de hoogwaardige aramide- en polyetheenvezels is dat deze zijn opgebouwd uit hooggeoriënteerde polymeerketens, parallel aan de vezelrichting. De consequentie hiervan is dat de eigenschappen sterk anisotroop zijn. Immers tegenover de sterke covalente bindingen in de ketemichting staan de relatief zwakke intermoleculaire (waterstofbruggen of van der Waals) interacties loodrecht op de ketemichting. Hierdoor zijn de eigenschappen loodrecht op de vezelrichting orden van factoren lager, wat onder meer tot uiting komt in de lage compressie- en afschuifsterkte ten opzichte van glas- en koolstofvezels. Recent uitgevoerd parallel onderzoek laat zien dat uiteindelijk niet de hechting maar de lage compressie en afschuifsterkte van deze I-dimensionale vezels beperkend zijn voor het functioneren van de polyetheen- en aramidevezel-versterkte polymeren onder 3-dimensionele belastingssituaties, zoals die in de praktijk voorkomen. Een betere hechting blijft echter in alle gevallen tot betere prestaties leiden. 96

Curriculum Vitae

Frans P.M. Mercx, geboren 29 augustus 1959 te Halsteren, behaalde het diploma Atheneum­ B in 1977 aan het Mgr. Frencken College te Oosterhout. Aansluitend begon hij met de studie Scheikundige Technologie aan de Technische Hogeschool Eindhoven, tegenwoordig Technische Universiteit Eindhoven (TUE) geheten. Het ingenieursdiploma werd in augustus 1983 behaald na het voltooien van het afstudeerproject 'Koolstofgedragen kobaltkatalysatoren' in de sectie Anorganische Chemie. In datzelfde jaar ontving hij hiervoor de KIVI Petroleum Techniek prijs. Van october 1983 tot en met october 1987 was hij als wetenschappelijk onderzoeker in dienst van de stichting Zuiver Wetenschappelijk Onderzoek (ZWO) sectie Scheikundig Onderzoek Nederland (SON) en als zodanig gedetacheerd bij de vakgroep Kunststoftechnologie van de TUE. Het onderzoek richtte zich aanvankelijk op moleculaire composieten, maar werd midden 1986 uitgebreid met het onderzoek naar de vezel-matrix hechting in een nieuwe generatie hoogwaardige composieten. Hiermee werd de basis gelegd voor het in dit proefschrift beschreven onderzoek. Een deel van het promotieonderzoek werd bij het TNO Kunststoffen en Rubber Instituut uitgevoerd, waar de auteur vanaf 16 november 1987 tot 1 juni 1995 werkzaam was. Aanvankelijk in de werkgroep Composieten, met als specialisatie vezelversterkte thermoplasten en later in de werkgroep Functionele Polymeren en Toepassingen. Het accent lag hierbij vooral op de smeltverwerkbare vloeibaar kristallijne polymeren, een nieuwe generatie van hoogwaardige kunststoffen. Van 1 juni 1995 tot 1 februari 1996 werkzaam bij GASTEC N. V. in de sectie materialen van de unit gasdistributietechniek. Het onderzoek aldaar richtte zich vooral op de invloed van plastische vervorming op de levensduur van PE gasbuizen en het niet-destructieve onderzoek van lasverbindingen in PE leidingsystemen. Vanaf 1 februari 1996 als 'Product Development Specialist- Valox' werkzaam bij General Electtic Plastics. 97

Dankwoord

Op deze plaats wil ik iedereen hartelijk bedanken die hetzij direct, hetzij indirect heeft bijgedragen aan het tot stand komen van dit proefschrift. Een aantal personen en instellingen wil ik daarbij met name noemen.

De Technische Universiteit Eindhoven en het TNO Kunststoffen en Rubber Instituut voor de mogelijkheden die zij mij hebben geboden voor het uitvoeren van experimenten.

Prof. dr. Piet Lemstra voor zijn blijvende steun ondanks de grote tijdsspanne van het onderzoek en prof. dr. ir. Jan van Turnhout voor de vrijheid en de mogelijkheid om allerlei inniatieven uit te werken.

Jan Buijs en Bas Damman voor het kritisch doorlezen van de manuscripten, de inhoudelijke discussies en vooral voor de steun en vriendschap in de afgelopen jaren.

Jos Goosens (ex-TNO, M&E) en Herman Ladan (TUE) ben ik erkentelijk voor de electronen­ microscopische opnamen; Abder Benzina voor de reflectie-infraroodopnamen en de interessante discussies tijdens de lange avonden/nachten achter het ESCA apparaat.

Collega's en ex-collega's van de werkgroepen FPT en Composieten van het TNO Kunststoffen en Rubber Instituut voor hun enthousiasme en prettige werksfeer. Met name het wekelijkse voetballen en de vele wetenschappelijke discussies/weddenschappen met taart als inzet zal ik niet licht vergeten. Daarnaast ben ik de medewerkers en (ex-)collega promovendi van de TUE (vakgroepen TCK en WFW) erkentelijk voor de steun en het feit dat ik me er altijd welkom voelde.

De in alle opzichten aanwezige steun en belangstelling van mijn vrienden was van niet te onderschatten belang. Dit geldt met name voor het 'Culinair Eetgenootschap' (resp. Walter Arts, Paul Lemmen, Barbara van der Est, Wim Courage, Rob Fey en Jan Doomink), dat me voldoende gelegenheid bood om stoom af te blazen. Verder dank ik mijn familie voor hun morele steun tijdens mijn promotie. Stellingen

behorend bij het proefschrift Surface rnodification of high-performance aramid and polyethylene fibres for improved adhesive bonding to epoxy resins

1 Voor de beoordeling van de grootte en nauwkeurigheid van oriëntatieparameters bepaald uit de azimutbale intensiteitsverdeling bij Röntgen-verstrooiing is het van essentieel belang te vermelden op welke wijze de intensiteit bij grote hoeken van de achtergrondintensiteit onderscheiden wordt. A. Bruggeman wui J.A.H.M. Buijs, 'Drawing and orientation development of aromatic polyesters with terphenyl units in the main chain wui aliphatic side chains ', submitted to Polymer G. Gustaftson, 0. lnganäs, H. Osterhalm and J. Laako, Polymer 32, 1574-1580 (1991) C-P. Lafrance, M. Pézolet and R.E. Prud'homme, Macromolecules 24, 4948-4956 (1991)

2 Bij de verklaring van de door hen waargenomen verschillen in absolute sterkte van gel-gesponnen polyetheen vezels en thermotroop gesponnen poly(fenyl-p­ fenyleentereftalaat) vezels houden Gorshkova et aL ten onrechte geen rekening met verschillen in ketendichtheid. I. A. Gorshkova, A. V. Savitsky and A. Yu. Bilibin, Intern. J. Polymerie. Mater. 22, 135-142 (1993)

3 Bij het gebruik van optische deeltjestellers voor het bepalen van de vervuilingsgraad van olie is het onjuist de meetfout te geven als afwijking in het aantal deeltjes. De juiste weergave is de fout in de grootte. B.M. Verdegan, K.McBroom, B. W. Schwandt, C.E. Holm, L. Liebmann, Advances in oilfilter test methads, paper presenred at the 1nternationalFluid Power Exposition wui Technica! Conference, 24- 26 March 1992

4 De door Y oon et al. gevonden grote sprong in de warmtegeleidingscoëfficiënt bij het bereiken van het glaspunt van een uithardend epoxy systeem is het gevolg van een onjuiste analyse/verwerking van de meetgegevens. H.N. Yoon, C.N. Yoon and H.C. Kim, J. Polym. Sci., Polym. Phys. 29, 1081-1084 (1993) P. Korpiun, B. Merté, G. Fritsch, R. Tilgner wui E. Lilscher, Colloid Polym. Sci. 261, 312-318 (1983) 5 Hoewel het polyamide-imide TorlonR (Amoco) een starre hoofdketen bezit is er geen sprake van een stoajachtige keten. Daarom is het gebruik van de term 'moleculair composiet' voor blends van dit polymeer met flexibele keten polymeren zeker niet terecht. S. Palsule and J.M. G. Cowie, Polymer Bulletin 33, 241-247 (1994) S. Palsule, Pol. Mat. Sci. Eng. 72.. 595-596 (1995)

6 De door Roebroeks en Van Dreumel gepresenteerde resultaten van het effect van een mechanische oppervlakte verruwing van aramide weefsels op de hechtsterkte zijn misleidend, daar ze te weinig rekening houden met de in 'echte' composieten aangetroffen situatie. Dit proefschrift: Hoofdstuk 1 G. Roebroeks and W.H.M. van Dreumel in Mater. Sci. Monographs: 35 (EDs. K. Brunsch, H-D. Gölden and C-M. Herkert), Elsevier, Amsterdnm (1986), p. 95-102

7 Uit het feit dat doorBeersen Ramirez een kruip 'nul' gerapporteerd wordt voor bundels (of touwen) van VectranR (Hoechst) vezels mag niet geconcludeerd worden dat individuele filamenten van dit vloeibaar-kristallijne polymeer geen kruip vertonen. D.E. Beers and J.E. Ramirez, J. Text. lnst. 81, 561-574 (1990) J.A.H.M. Buijs and J. Breen, Plast. Rubber and Comp. Proc. Appl. 23, 311-317 (1995)

8 Concentratie op details is het essentiële kenmerk van promoties.

F.P.M. Mercx Eindhoven, 7 maart 1996