WO2004087792A2

WO2004087792A2 - Surface oxyfluorinated polymide, substrates coated therewith and composite material incorporating it

Info

Publication number: WO2004087792A2
Application number: PCT/IB2004/050374
Authority: WO
Inventors: Lin Tu; Pieter Lodewikus Swart; Anatoli Aleksandrovich Chtcherbakov
Original assignee: Rand Afrikaans University
Priority date: 2003-04-01
Filing date: 2004-03-31
Publication date: 2004-10-14
Also published as: WO2004087792A3

Abstract

This invention relates to a modified polymeric material in the form of polyimide, which polyimide is surface oxyfluorinated and has improved wettability. The invention further relates to surface oxyfluorinated substrates such as optical fibres suitable for use in sensors and to composite materials incorporating the modified polymeric material.

Description

Surface oxyfluorinated polyimide, substrates coated therewith and composite material incorporating it.

Field of the Invention

This invention relates to a novel modified polymeric material. More specifically, but not exclusively, the invention relates to surface oxyfluorinated polyimides, to substrates such as optical fibres coated with such novel oxyfluorinated polyimides and to composite materials incorporating the novel modified surface oxyfluorinated polyimides.

Background to the Invention

Fibre optic sensors are used to monitor material and structural fatigue in fibre reinforced composite structures such as those used in aircraft. The optical fibres used are often coated with polymeric material so as to reinforce the optical fibre and prevent micro scratches on the surface thereof. As a result of the polymeric coating on the surface of optical fibres, the wettability of these fibres through liquid resin is poor.

The poor wettability is disadvantageous in that it prevents intimate interfacial contact between the optical fibre and the matrix of the material, such as liquid resin or thermosetting plastics, in which the optical fibre is to be embedded. In turn, this may negatively impact on the accuracy of the fibre optic sensors. Oxyfluorination

Fluorination of hydrocarbon polymers was first investigated in the nineteenth century, with numerous difficulties being encountered. Without wishing to be bound by theory it is believed that this was mainly because the reaction between fluorine and hydrocarbons is so rapid and exothermic that it often lead to fragmentation and combustion if the reaction was not controlled [2, 3]. In 1954, a process was described to fluorinate polymers by reacting polyethylene with elemental fluorine (on its own or diluted with an inert gas such as Argon). The polyethylene used in the reaction was in a form with a large surface area (e.g. powder) to dissipate the heat generated in the reaction.

In the early 1 970s, the La-Ma direct fluorination process was developed by Lagow and Margrave in which the principal feature of the process was initial infinite dilution of fluorine by a neutral gas followed by a gradual increase in the fluorine concentration [3, 4]. Through careful control of different reaction conditions, material ranging from partly fluorinated polymers to fully fluorinated polymers (per-fluorinated products) can be obtained.

Where an oxygen/fluorine gas mixture is used, the process is referred to as oxyfluorination. Where the oxyfluorination only effects the surface of a polymer so as to modify its surface properties, whilst leaving the bulk properties in tact, one generally refers to surface oxyfluorination.

Briefly, surface oxyfluorination entails the reaction between an oxygen and fluorine mixture and the surface of a polymer thereby to introduce fluorine atoms into the polymer to partially replace the hydrogen atoms on the molecular hydrocarbon chain. This in turn results in fluorocarbon and alkyl radicals. At the same lime, the molecular oxygen spontaneously reacts with the fluorocarbon and alkyl radicals to achieve functionalisation of the polymer, which incorporates oxygen containing groups into the polymer. [1 ]

The incorporation of oxygen in the polymer surface during surface oxyfluorination is observed in infrared spectra al the wave number of 1850 cm^"1 as acid fluoride groups (-COF), which are hydrolysed to relevant carboxyl (COO^") groups (1750 cm^"1 ) under moisture conditions [5]. Normally, incorporation of elemental fluorine partially replaces H atoms on polymer chains and produces C-F bonds, which appear as a broad absorption band at the wave number of 1200 cm^"1 [6].

Surface oxyfluorination of polymers

Attempts have been made to treat acrylic coated optical fibres and polypropylene fibres by surface oxyfluorination so as to improve their interfacial bonding with cementitious and polyester resin matrices. It was found that the oxyfluorination increased the shear bond strengths of the treated optical fibres with cementitious and thermosetting polymer resins by 22% and 66%, respectively [1 ].

However, in respect of acrylic coated optical fibres it was also found that the bonding between the acrylic coating and the surface of the quartz optical fibre cladding was not strong, which caused the acrylic coating layer to detach from the fibre during a fibre pullout test.

It has not been suggested previously to resort to surface oxyfluorination of polyimide coated optical fibre to increase its wettability and interfacial bonding. In fact, it would appear that the notion of surface oxyfluorination of polyimide as such has never before been suggested. It has now been found by the present inventors that such treatment has surprisin beneficial effects on the surface properties of the polyimide without negatively impacting on the bonding of the coating to the surface of the optical fibre.

Object of the Invention

It is an object of the invention to provide a surface oxyfluorinated polyimide having improved wettability.

It is a further object of the invention to provide substrates, such as optical fibres, coated with surface oxyfluorinated polyimide and to composite materials incorporating surface oxyfluorinated polyimide, wherein the surface oxyfluorinated polyimide has improved wettability.

Brief description of the invention

According to the present invention there is provided a novel modified polymeric material in the form of polyimide which is characterized in that it is surface oxyfluorinated.

According to a second aspect of the invention, there is provided a surface oxyfluorinated polyimide having a unique characteristic absorption band of between 1570 to 1630 cm^"1for the functional group (I) below:

The surface oxyfluorinated polyimide may have an interfacial shear bond strength when bonded to a hardened polyester resin of between 1 .3 MPa to 1 .8MPa, preferably 1 .4 MPa to 1 .6. MPa.

The surface oxyfluorinated polyimide may also have a contact angle of between 35° to 70°, preferably between 50° to 60°.

In an embodiment of the invention, the surface oxyfluorinated polyimide may be the conventional coating of a first substrate which is to be intimately bonded to a second substrate or alternatively incorporated into a second substrate such as a polymer matrix.

The first substrate may form part of a sensor means. In a preferred embodiment of the invention, the first substrate is an optical fibre which forms part of the sensor means. The surface oxyfluorinated polyimide coated optical fibre may incorporate a Bragg grating whereby the fibre is rendered suitable to be incorporated in a composite material to be used in stress monitoring, and in monitoring of material fatigue and of crack formations in fibre reinforced composite materials and concrete structures. Alternatively, the surface oxyfluorinated polyimide coated optical fibre may incorporate a long period grating. The sensor means may further be a Fabry-Perot sensor. It is also envisaged that the surface oxfluorinated coated optical fibre may form an arm of an interferometer.

The second substrate is preferably a fibre reinforced composite structure. The matrix may be a polyester based matrix, into which the first substrate coated with the surface oxyfluorinated polyimide is to be embedded.

In another embodiment of the invention, the polyimide may be provided in the form of a mounting member having exposed surfaces, wherein at least one of the exposed surfaces of the mounting member is surface oxyfluorinated. In one embodiment of the invention, the mounting member may be provided in the form of a polyimide strip. A monitoring means may be located between two polyimide strips, wherein the exposed surfaces of the polyimide strips are surface oxyfluorinated thereby to improve the wettability of the exposed surfaces and enhance the mounting thereof to a structure.

According to a third aspect of the invention, there is provided a surface oxyfluorinated polyimide coated substrate having improved interfacial shear bond strength. Preferably the substrate is an optical fibre.

According to a fourth aspect of the invention, there is provided the use of a surface oxyfluorinated polyimide coated optical fibre in a sensor means, wherein the surface oxyfluorinated polyimide coated optical fibre has improved interfacial bond strength relative to the second substrate to which it is to be bonded or in which it is to be embedded.

According to a fifth aspect of the invention, there is provided a method of surface oxyfuorinating a polyimide, comprising the steps of:

1. providing a polyimide;

2. exposing a surface of the polyimide to a gas mixture of fluorine and oxygen;

3. allowing the gas mixture to react with the surface of the polyimide thereby to form a surface oxyfluorinated polyimide. The gas mixture may comprise a fluorine to oxygen ratio of between 1 : 100 to 1 : 1 .

The gas mixture may be introduced into a reactor, in which the polyimide is placed, al a pressure of between 2kPa to 70kPa. The temperature within the reactor may be between 0°C to 80 °C. In an embodiment of the invention, the polyimide surface is exposed to the above conditions for 0.1 minutes to 240 minutes.

The polyimide may be provided as a coating on a substrate or as a mounting member having exposed surfaces.

It has been found that oxyfluorinated polyimide coated optical fibres exhibit a much tougher surface and stronger coating / cladding bonding compared to the oxyfluorinated acrylic coated optical fibres, which possess a brittle surface and less strong coating / quartz bonding [1 ].

Brief Description of the Drawings

Without thereby limiting the scope of the invention, and by way of example only, an example of the invention will now be described with reference to the following figures:

Figure 1 : sketch of the optical fibre pullout test specimen;

Figure 2: Fourier Transform Infrared (FTIR) spectrum of an untreated polyimide coated optical fibre; Figure 3: FTI R spectrum of a polyimide coated optical fibre which is surface oxyfluorinated according to the invention.

Example of the Invention

Polyimide coated optical fibres of the type that are commercially available under the trade description of FiberCore HB-P were surface oxyfluorinated as follows:

The polyimide coated optical fibres were loaded into a reaction chamber (reactor) and a gas mixture of fluorine and oxygen was introduced into the chamber, the fluorine to oxygen ratio being between 1 : 100 to 1 :1 . The gas mixture was introduced into the reaction chamber at a pressure of between 2kPa to 80kPa, the reactor temperature being between 0°C and 80°C. Surface oxyfluorination was allowed to take place for between 0.1 minutes to 240 minutes during which a free radical chain reaction is initiated. The polyimide material is simultaneously fluorinated and functionalized.

The contact angles θ of treated and untreated optical fibres were measured on a Cahn DCA-322 dynamic contact angle analyzer using the dynamic method, i.e. testing advancing and receding angles by the micro- Wilhelmy technique. An optical fibre suspended from a microbalance is immersed into and then withdrawn from a liquid. According to the Wilhelmy relationship, the total force exerted on the fibre is the sum of wetting, gravitational and buoyancy forces [7], as shown in Eqs. (1 ) and (2). In these equations, γι is the surface free energy of the liquid; C is the circumference of the cross section of the fibre; dι is the density of the liquid; y is the immersion depth; and Af is the fibre cross-sectional area. Because F, γι and C are either known or measurable, cosθ can be calculated.

F = YiCcosθ + m_fg - digyA_f (1 )

F = YiCcosθ (2)

Fibre pullout tests were conducted using a dumbbell-shaped specimen prepared by casting polyester resin into two custom-made round polypropylene moulds. A hole was made at the half depth of the wall of a mould and an optical fibre was inserted 20 mm into each mould, as shown in Fig. 1 . After 24 hour curing, Fibre pullout tests were performed on a JJ T20K Tensile Testing Machine with a 500 N load transducer. The extraction rate was 2 mm/min. The adhesive interfacial shear bond strengths (τ_bond ) were calculated by using Eq. (3). The shear bond strengths obtained are the average values of five tests.

T _bond = P/(CL) (3)

where T ond is the adhesive interfacial shear bond strength, P is the peak pull-out load, C is the circumference of the cross-section of the fibre and L is the bonding length.

The FTI R spectra of the untreated and oxyfluorinated optical fibres are presented in Figs. 2 and 3.

Since there are carbonyl groups (C=0) attached to the benzene rings in the molecular structure of polyimide, polyimide has an absorption band at

1750 - 1850 cm^"1. II can be seen from the above figures that compared to untreated polyimide, surface oxyfluorinated polyimide has a higher and wider C=0 absorption band due to the increased C=0 group content introduced by oxyfluorination. It also has a unique NCFO absorption band at 1600 cm^"1. Untreated polyimide, as a typical nylon, already has broad absorption bands ranging from 1000 cm^"1 to 1250 cm^"1 and 1250 cm^'1 to 1340 cm^"1, mainly due to the absorption of CeHs-N bond and CβHs-H rocking, respectively.

Although the applicant does not necessarily wish to be bound by theory, it is believed that it is easier to fluorinate polyolefins due to the existence of terminal H atoms on the molecular chains when compared to polyimides. Benzene rings in polyimide are chemically very stable and there are no existing terminal H atoms so that under normal oxyfluorination conditions, the extent of fluorination (replace H with F) on the polyimide is very limited compared to olefin polymers. Therefore, C-F absorption (around 1200 cm^'1) cannot be clearly verified by the spectrum in Fig. 3. Comparing the above two spectra in Figs 2 and 3, it can also be seen that the oxyfluorinated polyimide surface appears more hydrophilic than the untreated polyimide surface.

Table 1 presents values of the advancing contact angles of untreated and oxyfluorinated polyimide coated optical fibres. These fibres were ultrasonically cleaned before performing the contact angle measurements on them. The results in Table 1 show that oxyfluorination reduces the contact angles of polyimide coated optical fibres significantly, indicating a large increase in surface wettability. 11

Table 1. Contact angles of untreated and oxyfluorinated polyimide coated optical fibres.

Table 2 summarises the measured interfacial shear bond strengths between untreated and oxyfluorinated polyimide coated optical fibres and the polyester matrix. Surface oxyfluorination leads to an increase in interfacial bonding by 74 %. The increase in interfacial bonding arises from two factors. Firstly, the increased optical fibre surface wettability leads to an intimate contact between the polyimide coating and the polyester resin, which results in a complete covering of the optical fibre by liquid resin when the fibre is embedded in the resin. Secondly, there is a possible interfacial reaction between the oxyfluorinated polyimide layer and the polyester resin. We address this mechanism in the next paragraph.

Table 2. Adhesive interfacial shear bond strengths of untreated and oxyfluorinated optical fibres with the hardened polyester resin.

Untreated fibre Shear bond Oxyfluorinated Shear bond strength fibre strength

It is further pointed out, again without necessarily being bound by theory that a substance or functional group can be classified into either Lewis acidity or Lewis basicity according to its capacity of accepting or donating electrons. The Lewis acidity or Lewis basicity of a substance cannot contribute by itself to the cohesion of the substance because of the absence of an electron donor or acceptor. But when a two-phase system exists, such as a polyimide / resin interface, the acidity of one phase and the basicity of the other are complementary to each other and result in strong acid-base interactions across the interface [8]. The acid-base interaction can be classified as an electron share chemical bond.

Polyimide has the following monomer structure formula:

The benzene rings in the polyimide structure are chemically very stable due to their closed big π bond, which allows the electron cloud to be evenly distributed between each carbon atom. This reduces the energy of the whole benzene structure. However, the bond between the C-atom of carbonyl groups and the benzene ring (indicated by a star) may be a weak point, which can be attacked by fluorine radicals (° F) during oxyfluorination. The reason is that due to higher negativity of O compared to C, the electron cloud of the C = O double bond is attracted towards the O-atom of the C = O group, which leads to partial positive charge on the C-atom and partial negative charge on the O-atom. The phenyl group as a whole also impels the C = O double bond towards the O-atom, which makes the C-atom have a more positive charge. Therefore, under the attack of • F, the bonds between the C-atoms of the carbonyl groups and the benzene ring are opened and partial fluorination occurs. In one monomer structure, there are four such weak bonds. During the oxyfluorination process, functionalization also occurs partially on the polyimide chain as the molecular oxygen reacts spontaneously with the fluorocarbon and other radicals generated in the fluorination process, resulting in a free radical chain reaction. The oxyfluorinated polyimide structure may be typically described as follows:

Under moist conditions (such as moisture in the air), - COF groups are hydrolyzed to -C(OH) = O groups. The O-atom of the - OH group possesses higher electronegativity compared to the H-atom, which attracts the electron towards the O-atom and leads to electropositivity on the H-atom. Besides, the conjugation between the lone electron pair on the O-atom of the - OH and the double bond of the C=0 also draws the electron cloud of the OH bond towards C = O. Thus, the H-atom in -C (OH) = 0 becomes highly electropositive, i.e. higher electron accepting capacity (Lewis acidity). Therefore, surface oxyfluorination makes polyimide coating of optical fibre become more Lewis acidic.

When the acidic oxyfluorinated polyimide is in contact with the polyester resin, the Lewis acid - base interfacial interactions may occur at the interface between the increased Lewis acidity of the oxyfluorinated polyimide and the Lewis basicity of the polyester resin, i.e. electron share bonds are formed.

The invention has accordingly demonstrated that surface oxyfluorination treatment effectively improves the surface wettability of polyimide coated optical fibres, which promotes an intimate interfacial contact (increased bonding area) and thus the mechanical bonding between the optical fibre and the polyester resin. The increased acidity of the polyimide coating layer due to surface oxyfluorination can also lead to a Lewis acid - base interfacial interaction with the basicity of polyester resins, by which the electron share bonding is formed. Therefore, surface oxyfluorination treatment not only improves the mechanical bonding, but is also believed to introduce chemical bonding between the polyimide coated optical fibre and polyester resin.

The surface oxyfluorinated polyimide coated optical fibres provided with Bragg grating properties may be incorporated in the conventional manner into polymeric composite materials, such as materials based on fibre reinforced composites having a polyester matrix, or concretes. Once so incorporated it may be used in the conventional manner for stress monitoring and similar purposes. Many variations of the invention may be devised without thereby departing from the spirit of the invention. For example it is envisaged that a surface oxyfluorinated polyimide coated optical fibre may form part of a sensor means such as a Bragg grating sensor, a long grating sensor, a Fabry- Perot sensor or may be an arm of an interferometer.

References

1 . P.L. Swart, L. Tu and A. A. Chtcherbakov, Effect of surface oxyfluorination on the bonding between acrylic coated optical fibre and cementitious and polyester resin matrices, Opt. Eng. 39 (4), 1 038-1043 (2000).

2. R.J. Lagow and J.L. Margrave, Direct fluorination: A new approach to fluorine chemistry, Prog. Inorg. Chem. 26, 161 - 209 (1 979). 3. M. Anand, J.P. Hobbs and I.J. Brass, Surface fluorination of polymers, Organofluorine Chemistry: Principles and Commercial Applications, Plenum Press, New York (1994).

4. R. J. Lagow and J. L. Margrave, The control reaction of hydrocarbon polymers with elemental fluorine, Polym. Lett., 12, 177 - 184 (1974).

5. L Tu, D. Kruger, J.B. Wagener and P.A.B. Carstens, Wettability of surface oxyfluorinated propylene fibre and its effect on interfacial bonding with cementitious matrix, J. Adhes. 62, 187 - 21 1 (1997).

6. F.J. du Toit and R.D. Sanderson, Surface fluorination of propylene 1 . Characterisation of surface properties, J. Fluorine Chem., 74, 107

- 1 14 (1999).

7. CM. Chen, Polymer Surface Modification and Characterization, Hanser Publishers, Munich, Vienna, New York (1994).

8. R.J. Good, M.K. Chaudhury and C.J. Van Oss, in Fundamentals of Adhesion, L.H. Lee Ed., Plenum Press, New York , 153 - 172

(1991 ).

Claims

, „„„„„ O 2004/08779217Claims

1 . A modified polymeric material in the form of polyimide, wherein the polyimide is surface oxyfluorinated.

2. The modified polymeric material according to claim 1 , wherein the polyimide which is surface oxyfluorinated has a characteristic absorption band of between 1570 to 1630 cm^"1 for the functional group of formula (I) below:

3. The modified polymeric material according to any of the preceding claims, wherein the polyimide which is surface oxyfluorinated has an interfacial bond strength, when bonded to a hardened polyester resin, of 1.3 MPa to 1.8 MPa.

4. The modified polymeric material according to any of the preceding claims, wherein the polyimide which is surface oxyfluorinated has an interfacial bond strength, when bonded to a hardened polyester resin, of 1 .4 MPa to 1.6 MPa.

5. The modified polymeric material according to any of the preceding claims, wherein the polyimide which is surface oxyfluorinated has a contact angle of 35° to 70°. O 2004/087792

18

6. The modified polymeric material according to any of the preceding claims, wherein the polyimide which is surface oxyfluorinated has a contact angle of 50° to 60°.

7. The modified polymeric material according to claim 1 , wherein the polyimide which is surface oxyfluorinated is provided in the form of a coating on a first substrate which is to be bonded to a second substrate or incorporated into a matrix.

8. The modified polymeric material according to claim 7, wherein the first substrate forms part of a sensor means.

9. The modified polymeric material according to claim 7, wherein the first substrate is an optical fibre.

10. The modified polymeric material according to claim 9, wherein the optical fibre coated with a polyimide which is surface oxyfluorinated, incorporates a Bragg grating whereby the optical fibre is rendered suitable to be incorporated in a composite material to be used in stress monitoring, and in monitoring of material fatigue and of crack formation in fibre reinforced composite materials and concrete structures.

1 1 . The modified polymeric material according to claim 9, wherein the optical fibre coated with a polyimide which is surface oxyfluorinated, incorporates a long-period grating whereby the optical fibre is rendered suitable to be incorporated in a composite material to be used in stress monitoring, and in monitoring of material fatigue and of crack formation in fibre reinforced composite materials and concrete structures. O 2004/087792

19

12. The modified polymeric material according to claim 9, wherein the optical fibre coated with a polyimide which is surface oxyfluorinated forms part of a Fabry-Perot sensor.

13. The modified polymeric material according to claim 9, wherein the optical fibre coated with a polyimide which is surface oxyfluorinated constitutes an arm of an interferometer.

14. The modified polymeric material according to claim 7, wherein the second substrate is a fibre reinforced composite structure.

15. The modified polymeric material according to claim 7, wherein the matrix is a polyester based matrix.

16. The modified polymeric material according to claim 1 , wherein the polyimide which is surface oxyfluorinated is provided in the form of a mounting member having exposed surfaces, wherein at least one exposed surface thereof is surface oxyfluorinated.

1 7. The modified polymeric material according to claim 16, wherein the mounting member is a polyimide strip, wherein at least one exposed surface is surface oxyfluorinated.

18. A first polyimide mounting member and a second polyimide mounting member, wherein a monitoring means is located between the first polyimide mounting member and the second polyimide mounting member, the first polyimide mounting member and second polyimide mounting member being characterised in that at least one exposed surface thereof is surface oxyfluorinated.

19. An optical fibre coated with a modified polymeric material in the form of polyimide, wherein the polyimide is surface oxyfluorinated according to claim 1 .

20. The surface oxyfluorinated polyimide coated optical fibre according to claim 19, wherein the optical fibre forms part of a sensor means

21 . The surface oxyfluorinated polyimide coated optical fibre according to claim 20, wherein the sensor means is chosen from the group consisting of Bragg grating sensors, long grating sensors, Fabry-

Perot sensors and interferometers.

22. A method of improving the wettability of a polyimide, the method including the steps of:

a. providing a polyimide;

b. exposing a surface of the polyimide to a gas mixture of fluorine and oxygen;

c. allowing the gas mixture to react with the surface of the polyimide thereby to form a surface oxyfluorinated polyimide coated substrate.

23. The method according to claim 22, wherein the gas mixture comprises a fluorine to oxygen ratio of 1 :100.

24. The method according to claim 22, wherein the gas mixture comprises a fluorine to oxygen ratio of 1 : 1 .

25. The method according to claim 22, wherein the gas mixture is introduced into a reactor in which the polyimide is placed at a pressure of between 2 kPa to 70 kPa.

26. The method according to claim 22, wherein the gas mixture reacts with the surface of the polyimide at a temperature of between 0°C to 80°C.

27. The method according to claim 22, wherein the gas mixture is allowed to react with the surface of the polyimide for 0.1 minutes to 240 minutes.