Surface energy increase of oxygen-plasma-treated PET

Materials Characterization 50 (2003) 209 – 215

Surface energy increase of oxygen-plasma-treated PET M.O.H. Cioffi a,*, H.J.C. Voorwald a, R.P. Mota b a b

DMT, Universidade Estadual Paulista, Av. Ariberto Pereira da Cunha, 333 Cep 12516-410, Guaratingueta´, Sa´o Paulo, Brazil DFQ, Universidade Estadual Paulista, Av. Ariberto Pereira da Cunha, 333 Cep 12516-410, Guaratingueta´, Sa´o Paulo, Brazil Received 11 February 2003; accepted 18 April 2003

Abstract Prosthetic composite is a widely used biomaterial that satisfies the criteria for application as an organic implant without adverse reactions. Polyethylene therephthalate (PET) fiber-reinforced composites have been used because of the excellent cell adhesion, biodegradability and biocompatibility. The chemical inertness and low surface energy of PET in general are associated with inadequate bonds for polymer reinforcements. It is recognized that the high strength of composites, which results from the interaction between the constituents, is directly related to the interfacial condition or to the interphase. A radio frequency plasma reactor using oxygen was used to treat PET fibers for 5, 20, 30 and 100 s. The treatment conditions were 13.56 MHz, 50 W, 40 Pa and 3.33  10 7 m3/s. A Rame´-Hart goniometer was used to measure the contact angle and surface energy variation of fibers treated for different times. The experimental results showed contact angle values from 47j to 13j and surface energies from 6.4  10 6 to 8.3  10 6 J for the range of 5 to 100 s, respectively. These results were confirmed by the average ultimate tensile strength of the PET fiber/ polymethylmethacrylate (PMMA) matrix composite tested in tensile mode and by scanning electron microscopy. D 2003 Elsevier Inc. All rights reserved. Keywords: Plasma treatment; PET/PMMA composite; Contact angle; Tensile strength

1. Introduction In composites, the reinforcement fibers sustain almost all of the applied tensile load whereas the role of the matrix is to adhere to the fibers, protect the fibers, and transfer the load through the interface [1]. The mechanical behavior of fiber-reinforced composites is dependent on the statistical fiber strength and the properties of the fiber/matrix

* Corresponding author. Tel.: +55-12-31232853; fax: +55-1231232852. E-mail address: [email protected] (M.O.H. Cioffi). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1044-5803(03)00094-9

interface [2]. For these materials two fracture modes are currently observed: (1) the interlaminar and intralaminar fractures usually caused by delamination of the fiber/matrix ply; and (2) longitudinal rupture along the fiber orientation or debonding between fiber and matrix due to the poor interfacial adhesion [3]. Considerable effort has been expended to predict the fracture behavior and to model these materials when loaded in fiber directions [4]. Reinforced materials have generated considerable interest, particularly for applications requiring various shapes such as dental implants, tissue replacement, tissue reinforcement and organ transplants, but their surfaces do not favorably interact with

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cells [5,6]. When adhesion and growth of tissue cells are required, material inertness is not desired [7]. It is the physical – chemical and morphological nature of polymers that governs the cell interaction at the interface. Therefore, synthetic polymers often require selective modification to introduce specific functional groups to the surface for the bonding of biomolecules [6]. The chemical inertness and low surface energy of the polyethylene therephthalate (PET) fibers produce an inadequate bond with the polymer matrix [8,9]. Many surface modification treatments of these fibers are used to enhance the interfacial shear strength between fibers and matrix, which consequently influence the mechanical properties of composites [10,11]. The oxygen plasma treatment can be used to modify the chemical and physical states of the material surface without altering the bulk properties. These attributes indicate that plasma treatment of the fiber surfaces can be an important technique controlling interfacial adhesion [8 –13]. The contact angle is a useful parameter for characterizing the average surface wettability of a treated material system [14]. The contact angle measurement is complex; because of the rough surface, there is a change from one point to another along the contact line. Research has been performed to identify a way to solve this problem. Marmur [15], for instance, obtained reliable information about this measurement by combining the contact angle measurements with theoretical calculations. In this work, the contact angle values permitted the assessment of the PET surface modification tendency, indicating the adequate condition for plasma treatment of the PET fibers.

2. Experimental PET fibers with a filament diameter of about 13 Am and elastic modulus of 14 GPa were provided by Montefiber Spa (Acerra, Naples, Italy). The PET filaments were treated in a cold plasma reactor as shown in the schematic diagram of Fig. 1. A 36  103-cm3 reaction chamber, which contain 13 cm diameter electrodes, provided a 2  103 cm3 of plasma supplied by a radio frequency generator.

Fig. 1. Plasma reactor schematic: (1) reaction chamber, (2) mechanical pump, (3) turbomolecular pump, (4) mass flow controller, (5) radio frequency generator, (6) impedance controller, (7) pressure controller, (8) plasma treatment region.

Oxygen gas was used to produce the etching mechanism. Plasma etching involves the chemical combination of the solid surface with the active gaseous species in the glow discharge [16]. The treatment was performed using the following conditions: 13.56 MHz excitation frequency and 50 W RF power; the pressure of treatment was kept at 40 Pa by a double stage mechanic pump, the mass flow controller maintained 3.33  10 7 m3/s gas flux and the treatment time varied from 5 to 100 s. A Rame´-Hart goniometer with a charged couple device (CCD) camera and a syringe was used for the contact angle measurements of fibers treated for different times. For these measurements, 0.2 Al of water was deposited on the fiber surface. To measure the surface energy, 0.2 Al of diiode methane was also used. The contact angle is the fluid drop tangent curve angle. However, due to the small fiber dimensions, these measurements were made on PET film, which provided the same physical and chemical characteristics of PET fibers. After fiber plasma treatment, PET/polymethylmethacrylate (PMMA) composites were produced using filament winding equipment. Tensile tests were performed in an INSTRON 4204 load frame and scan-

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Table 1 Contact angle measurements and superficial energy of oxygen plasma treated and untreated PET Treatment (s)

Contact angle (j) ( F 1.5j)

0 O2 O2 O2 O2

90 31 7 9 8

5 20 30 100

gsl Harmonic (J)

7.3  10 8.1  10 8.3  10 8.4  10

ning electron microscopy using a JEOL JSM5310 was performed for fractographic characterization.

3. Results and discussions Table 1 shows the contact angle (h) values for untreated PET and for oxygen-plasma-treated PET, the surface energy values and the work adhesion values for treatment times from 5 to 100 s. PET fibers exhibited a contact angle of 90j, which is associated with a low superficial energy. After the oxygen plasma treatment for 5 s, the contact angle of PET decreases to 31j as a result of surface modifications cause by the plasma treatment. For the treatment time of 20 s, the contact angle decreased from 31j to 7j. From the surface energy data, also shown in Table 1, there is a slight increase from 7.3  10 6 J for 5 s of treatment time to 8.1  10 6 J for 20 s, which indicates the influence of treatment time on the contact angle and, as a consequence, on the surface energy. Table 1 shows that for treatment times longer than 20 s, no significant influence on the contact angle

6

gsl geometric (J)

6.7  10 7.5  10 7.6  10 7.6  10

6 6 6

6 6 6 6

Wa (J/cm2)

13.5  10 14.5  10 14.5  10 14.5  10

6 6 6 6

occurred; the same for surface energy and for work adhesion. For 30 and 100 s no significant variation in the contact angle was observed, as shown in Fig. 2. The decrease from 90j to 31j for 5 s is a consequence of the interaction by etching of the involved reactive species of oxygen plasma with the PET fiber surface, which is constituted by carbon, oxygen and hydrogen, and the formation of volatile products. These products are expelled from the reactor by the vacuum system, but their formation changes the possible chemical links present on the fibers surface. For constant treatment conditions (power, pressure and gas flux), the influence of plasma etching on the fibers surface modification reached a maximum for a treatment time of 20 s. The reduction in the contact angle caused by the plasma treatment explains the increase in the tensile strength values for the oxygen-plasma-treated PET fiber/PMMA matrix composite, as shown in Table 2. The increase in adhesion was also evident by the fracture modes of the tensile specimens; the untreated fiber composite in Fig. 3 shows a brittle characteristic material in the same way as the PMMA [17]. Standard Table 2 Tensile strength values of the PET fibre/PMMA matrix composite Treatment (s)

j (MPa)

jM (MPa)

0

142 110 174 133 142 123 55 52 52 44 70 60

126

O2 5 O2 20 O2 30

O2 100 Fig. 2. Contact angle vs. time curve for oxygen-plasma-treated PET.

154 132 54

58

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despite the low adhesion, a significant increase in mechanical strength occurred. In Fig. 4 the fracture surface of PET/PMMA composite specimen oxygen plasma treated for 5 s is presented. It is observed that the cracks followed the fibers, leaving a smooth fiber surface and propagated through the resin matrix, which means a delamination failure in Fig. 4a. Fig. 4b shows the fracture morphology of the broken fiber ends. Fractured fibers with no preferred crack direction are observed, indicating a combination of interlaminar and intralaminar failure. The low amount of PMMA resin that remained on the PET fiber surface indicates low adhesion.

Fig. 3. SEM fractography of untreated PET fibers/PMMA matrix composite specimen. a)  1500 b)  1000.

branches [18] on the matrix due to the crack propagation is observed in Fig. 3a, and in Fig. 3b a clean surface of fibers with no adhered matrix is shown. Surface analysis indicated fracture of specimens parallel to the load direction in the tensile tests. As a consequence, due to the fracture orientation, it is possible to conclude that the composite exhibited weak fiber/matrix adhesion. In comparison to the isolated matrix with an average ultimate tensile strength of 75 MPa, the untreated fiber composite showed an average ultimate tensile strength of 126 MPa, which indicates that

Fig. 4. SEM fractography of PET fibers/PMMA matrix composite specimen oxygen plasma treated for 5 s. a)  1000 b)  1000.

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independent of average ultimate tensile strength, which is not the highest according to Table 2. The average ultimate tensile strength was higher than that of the untreated material and, in the case of the isolated filament [19], the treatment damaged the outer surface but the average ultimate tensile strength exhibited no significant difference in comparison to the untreated material, 984 and 998 MPa, respectively. Step formation, observed in Fig. 6, is a consequence of a combination of translaminar and interlaminar failure with extensive delamination, and was also reported by Cioffi and Voorwald [20]. This case

Fig. 5. SEM fractography of PET fibers/PMMA matrix composite specimen oxygen plasma treated for 20 s. a)  1000 b)  1000.

The occurrence of a transverse failure mode shows that there is a stronger fiber/matrix adhesion for this treatment condition than for the untreated composite. Fig. 5a shows the fracture surface of PET/PMMA composite specimen oxygen plasma treated for 20 s. A bundle of fractured fibers with high amount of resin adhered in the boundary of fibers indicates strong interface. The fracture surface morphology of fiber ends can be seen in Fig. 5b, in which the ductile character of the fiber PET is observed. This condition was considered to be an adequate treatment for PET fibers/PMMA matrix composite,

Fig. 6. SEM fractography of PET fibers/PMMA matrix composite specimen oxygen plasma treated for 30 s. a)  2000 b)  1000.

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4. Conclusions The plasma treatment provides a reduction in the contact angle of fiber surfaces compared to the untreated fibers; consequently, this increases the superficial energy and the work adhesion of the PET fibers. The plasma treatment was responsible for stronger fiber/matrix adhesion, mainly after oxygen plasma treatment for 20 s, according to observations from the fractography analysis obtained by the scanning electron microscopy. The average ultimate tensile strength of treated fiber composites exhibited higher values in comparison to the untreated fibers. Acknowledgements The authors thank Mrs. Maria Lu´cia Brison de Mattos, INPE Sa˜o Jose´ dos Campos, for scanning electron microscopy characterization. The support provided by FAPESP through the process number 97/01160-7, is acknowledged. References

Fig. 7. SEM fractography of PET fibers/PMMA matrix composite specimen oxygen plasma treated for 100 s. a)  500 b)  750.

exhibited the lowest average ultimate tensile strength, 54 MPa as indicated in Table 2. Fig. 7a shows the broken ends of a bundle of fibers in transverse failure. A high amount of adhered resin on the fibers surface, which indicates good adhesion, can be observed. The difference of the ductile/brittle character between fiber and matrix with respect to the failure mode, is observed in Fig. 7b. This condition will not be considered due to the low average ultimate tensile strength. The treated PET filament in this condition showed an intense degradation of the fibers surfaces due to the exposure time.

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