Surface modification of high-performance aramid and ...and/or aramid fibres, are stilllimited intheir commercial use because of high material costs. However, they are widely applied

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

DOI:10.6100/IR455550

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https://doi.org/10.6100/IR455550https://doi.org/10.6100/IR455550https://research.tue.nl/en/publications/0091d6c7-6763-4312-848f-a34b393af1fc

SURFACE MODIFICATION OF

HIGH-PERFORMANCE ARAMlD AND

POL YETHYLENE 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 Fibre-Reinforced Polymers

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

1.3.1 Aramid Fibre-Matrix Adhesion 1.3.2 Polyethylene Fibre-Matrix Adhesion

1.4 Objective of the Present Investigation 1.5 Outline of the Thesis 1.6 References

Part A: Aramid Fibres

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

2.1 Introduetion 2.2 Experimental

2.2.1 Materials 2.2.2 Reactions 2.2.3 Characterization Methods

2.3 Results and Discussion 2.3.1 Chemica! Structure 2.3.2 Higher Homologues 2.3.3 Thermal Stability 2.3.4 Condusion

2.4 References

1

1 2 4 4 7

8

9 10

15

15

15

15

16 17

17

17

23 24 24 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

Chapter 5 Air- and Ammonia-Plasma Treatment

5.1 Introduetion 5.2 Experimental

5.2.1 Polyethylene Tapes 5.2.2 Plasma Treatment 5.2.3 Adhesion, Mechanica! Properties and

Chemica! Characterization

5.2.4 Scanning Electron Microscopy 5.3 Influence of Process Parameters 5.4 Results and Discussion

5.4.1 Tape Charaterization 5.4.2 Acthesion and Failure Mode 5.4.3 Mechanism of Acthesion

5.5 Conclusions 5.6 References

Chapter 6 Corona Grafting of Acrylic Acid

6.1 Introduetion 6.2 Ex perimental

6.2.1 Polyethylene Tapes 6.2.2 Corona Grafting 6.2.3 Characterization

6.3 Results and Discussion 6.3.1 Tape Characterization 6.3.2 Acthesion and Mechanica! Properties 6.3.3 Surface Treatment and Shear Strength

6.4 References

Epilogue The Role of Fibre Anisotropy and Adhesion on Composite Performance

iii

55

55 56 56 56 57

57 58 59 59 65 67 71

72

75

75 75 75 76 76 76 77

80 80 81

83

iiii Contents

Summary 88

Samenvatting 92

Curriculum Vitae 96

Dankwoord 97

Introduetion

Chapter 1 Introduetion

1.1 Fibre-Reinforced Polymers

1

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 of fibre-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

their commercial importance is growing2•3.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

premature failures6•7• 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

polyesters6•7• 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,

including epoxy resins, polyamides and even polyolefms6•7• 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 commercial importance8•9 . 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

as the aerospace, military and sporting goods2•3•5• 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 polymer chain and the low density of polymerie materials in general10·11 • 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

(N/Tex=GPa.p-1, p=density in g.cm-3)

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

agents12-16_ 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

do not penetrate or react with the aramid fibre 13 • Positive effects were only noted for highly

reactive coupling agents. Examples include the use of polyfunctional aziridines1\ 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

effective. Amino groups can be introduced by either ammonia19-21 , monomethyl amine19 or

nitrogenlhydrogen21 plasma treatment, a nitration-reduction cycle22·23 or bromination followed

by aminolysis22 • The amino groups introduced in these ways are almost exclusively attached

to the phenyl rings19-22 • Improved peel strength19·22 , pull-out strength20·23 and ILSS values2L24

up to 67-70 MPa we re attributed to improved physico-chemical interactions20•21 and

6 Chapter 1

c-Q-c_HU\._Z-J 11-11·~ 0 0 n

+ 2n Cf\-S-CHÏ No"'

8

R R I I CHONo CHONo

tc-o-J~_r.i. 11 li~J 0 0 n R'X

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

Figure 1.2 Grafting of aramid polymer

Introduetion 7

chemical22•23 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

oxidation of the phenyl groups21 • 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 the aramid fibre21 . A chemical methad for the selective introduetion of a variety of functional

groups was developed by Takayanagi25 • 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 acid25-27 , epoxy27 , allylic28 , acrylonitril26 and octadecyl26 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 years30•33 • 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 by Nguygen et al. 35 , Nardin et al. 36 , Kaplan et alY and Jacobs et al. 38 . 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 resulted in approximately a two-fold increase in interlaminar shear strength40•41 • 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

N-aroyloxamoyl chlorides2, 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

in coplanarity upon N-substitution3• In a coplanar structure, the carbonyl group exerts a de-

shielding effect". However, in non-coplanar structures a shielding effect ofthe carbonyl group

is observed4• 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

o-~-ë-o C=O I C=O I OH

2

Scheme 2.1

0 0 11 11

Cl-C-C-Cl .....

0

o-~-g-o c=o I C=O I Cl

0

o-~-ë-o~ c=o I C=O I OCH3

3

+ HCI

4

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 lying in the same plane as the amide group5•

The selective introduetion of ..

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

b

19

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

stretching vibrations (1650-1850 cm·1). 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

carboxylic acids6• The broadening is thought to be related to internal hydrogen bonding. H-

CH stretching vibrations at 2920 and 2860 cm·1, 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 ..

4000 2000 1800 1600 1400 1200 1000 800 600 cm-1

Figure 2.3 Infrared spectra of (a) 2, (b) 3 and (c) 4

21

a

b

c

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 13C-NMR spectra of the starting compounds and on tabulated increments 7 . 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

b a b

J lJ J 12 10 8 6 2 0

0 PPM

a a a a ao-~-~-oa a ,.......,

C=O I c=o I OCHsb

b

.I. 12 10 8 6 4 2 0

PPM a a 0 a a ao-~-~-oa a

C=O ,.....,..

I c=o I 0 I

cH-CHb I

dH-C, I 0 b c

6H-C/ 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.._~~~~~~""""""",_,.,.~--~.~ÜU DMF

1 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 groups onto the surface of aramid fibres was proposed in the previous chapter1. 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:

BPS p

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 the mass and the length of the fibres, assuming a crystal density of 1440 kg/m3• 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 reactions will proceed on the surface of the aramid fibres1, 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

f~-o-~-~-o-r.l + H HO oJ. n 0 0 11 H

Cl-C-C-Cl _.., t~-0-~J--0--~J c~c=o~ I I c c=o I I Cl Cl (o) n

I

~-0-~J-0-~j and/or C~C:O~ I I c c=o I l

OH OH

(b) n r~-o-~-g 0--~j c c=o~ I I H H

(c) n

t~-0-~J-0-~j c-~c=o~ I I c c:o I I 30 OCH3 (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,

(e) n

(f)

Scheme 3.1

0 I\

HO-CH,-C-C

Chopter 3

+HCI

Surface modijication of aramid fibres 31

XPS is a highly sensitive technique for surface analysis3•4 . 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

Treatment meas.

None 77

Oxalylchloride-water 70

Oxalylchloride-methanol 67

Oxalylchloride-ethylenediamine 65

Oxalylchloride-glycidol 70

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

cal cd.

77.8 64.3./72.7b

66.7

64.7

66.7

%N %0

meas. cal cd. meas. calcd.

8 11.1 15 11.1

5 7.1./9.1b 25 28.6./18.2b

5 6.6 28 26.7

17 17.6 18 17.6

6 5.6 24 27.7

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

eV for carbonyllaldehyde and 4.5 eV for carboxylic acid/ester groups3•4 • The chemica! shift

32 Chapter 3

of carbon in amide groups O=Ç-NH amounts to 3.5 eV3• 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 co valent 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

None

Oxalylchloride-water

Oxalylchloride-methanol

Oxalylchloride-ethylenediamine

Oxalylchloride-glycidol

"Standard deviation given in parentheses

Tensile strength (GPa)

2.2 (O.l)a

2.2 (0. 1)

2.2 (0. 1)

2.1 (0.1)

2.1 (0.1)

Bundie pull-out shear

strength (MPa)

28.3 (2.1)"

43.0 (1.6)

38.6 (1.8)

38.3 (1.2)

52.2 (2.1)

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

excellent results that were previously reported for amine-modified aramid fibres6·8 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, carboxylic acid and sulphonic acid groups are found at the surfaces of chromic acid1•4 ,

permanganate acid5 or potassiumchlorate acid5 treated polyethylene. The formation of

carbonyl and carboxylic acid groups increases wîth increasîng oxidative power of the acid

solutîon used and with the time of exposure2·3•5 and is accompanîed by chain scission1·5 • 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

the surface roughness1•2. 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

factors in improving the adhesion of oxidative acid treated PE4• 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 years clearly shows that this is not the casé6•

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-

drawing cast films as described previously10, 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 tapes assuming a crystal density of 103 kg/cm3• 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 c 0 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 brok en

44 Chapter 4

25 0 ,... .. CiS 20 0. è - 15 .c C)

~v-.-------x-------------.a Broken c

b

c: e -(/) 10 -:I a 0 •

"3 5 0.

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 ïn

c: 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,

KMn04/H20/H2S04, i.e., with the oxidation power ofthe acids applied5

•11

• 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

Treatment Time Pull-out strength 0/C atomie ratio (xl02)

(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 carboxylic acid groups12.1 3 • The preserree of sulphonic acid (S03H) groups was concluded

from the position of the S2P peak and tabulated data3

•12

•13

. The e1s 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

Silverstein in bis studies on the wetting14 , adhesion15 and failnre16 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

Chlorosulphonic acid

Chromic acid KMnO.fH20/H2S04

Time (min)

30

30

1

Tensile strength (GPa)

2.3 2.2 2.2

The etching of polyolefins and model compounds by chromic acid is well

documented1•4•17•18• 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;

or a combination of these2·6·19 . 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

improve the adhesion to epoxy resins19 • 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

20

0 ..... . CiS 15 a.. è E Cl c: 10 ! s 0

5 "S a.. 0

0 0

a

5

[J

a

C A

10

0/C * 100

A

Chapter4

A

x

15 20

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 of the resin used. For PE-reinforced epoxy and polyester composites mainly air-1•3 , oxygen-4• 10, and ammonia-plasma9-13 treatments are used to increase the level of adhesion.

Air or oxygen plasma contains a mixture of active oxygen species, mainly atomie oxygen14 , and leads to oxidation of the PE. As a result, a variety of functional groups are introduced onto the surface, including hydroxyl, carbonyl, ester and carboxylic acid groups1·3•7•9 . 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 from interface controlled to internat shear within