Abstract: Plasma polymer films of tetravinylsilane were deposited on unsized glass fibers using an RF glow discharge operated in pulsed mode to improve ...
Plasma coating of glass fibers used for polymer composites L. Hoferek, P. Janecek, V. Cech Institute of Materials Chemistry, Brno University of Technology, Purkynova 118, CZ-612 00 Brno, Czech Republic Abstract: Plasma polymer films of tetravinylsilane were deposited on unsized glass fibers using an RF glow discharge operated in pulsed mode to improve wettability and adhesion to the polymer matrix of composite sample. The adhesion was characterized by fiber-bundle pull-out test. The interfacial shear strength in composite sample can be controlled by deposition condition. Keywords: composite, glass fiber, polyester resin, adhesion, plasma treatment 1. Introduction Fiber reinforced composites combine high strength of rigid reinforcements with high toughness of flexible matrix. In fiber composites, both the fiber and matrix retain their original physical and chemical identities, yet together they produce a combination of mechanical properties that cannot be achieved with either of the constituents acting alone, due to the presence of an interphase between these two constituents [1]. We can distinguish two interfaces at the interphase region. One of them at the fiber surface is relatively sharp and other at the matrix is diffused one. The primary function of the interphase is to transmit stress from the matrix to the fibers and to protect the fibers from environmental damage. An ability of this region to transmit stress depends upon the interphase strength, as well as the mechanical properties of fiber, matrix, interphase and the bonding forces (adhesion) at interfaces. The surface of reinforcements has to be modified to improve wettability and adhesion to the matrix for sophisticated composites. The surface modification of reinforcements is a useful way to influence the chemical and physical structures of their surface layer, tailoring fiber-matrix stress transfer, but without influencing their bulk mechanical properties. Silane coupling agents are applied to glass fiber surface to promote their adhesion to the polymer matrix. However, the formed siloxane bonds are hydrolytically unstable, which results in worsening of mechanical properties of the composite in the presence of water and, eventually, in failure of the material. This problem may be resolved with plasma surface modification [2]. Low temperature plasma technology is a new technique used for surface modification [3] and PECVD (Plasma-Enhanced Chemical Vapor Deposition) is useful method able to prepare controlled interphases. 2. Experimental A fiber-bundle pull-out test was used to evaluate adhesion at the fiber-matrix interface. The design of the composite sample was constructed with respect to result of
Finite Element Analysis (FEA). The bundle was terminated by the disk at the bottom side and by a polymer dumb bell (ISO 527) at the other side. The bundle of tested glass fibers (GF) was placed in the axis of polymeric disk. Such a specimen was subjected to a tensile test, where the dumb-bell-shaped part of the specimen is hold by grips and the disk is hold by a test fixture using an edge. The experiment consists of an increasing normal force, which is applied to the fiber bundle in order to pull it out of the polymer disk. The glass fibers were used unsized (clean) fibers (type E) in roving form, the diameter of single fiber was 19 µm. Polymer matrix was prepared from unsaturated polyester resin Viapal VUP 4649 E(M). This polyester resin was cured from oligomers on the base of isophtalic acid with mixture of styrene, initiators, stabilizer and separator. Curing was carried out in the oven with programmable temperature regime. The axial pull-out test using a universal testing machine (Z010/TH2A, Zwick) was employed to evaluate adhesion at the fiber-matrix interface. Two kinds of series of composite samples were prepared and tested. One of them was fabricated from unsized GF and the other one from fibers modified by PECVD. Plasma discharge was generated by inductive coupling system worked at RF (13,56 MHz) in pulsed mode. The pulsed mode means, that plasma was controlled by changing the ratio of time, when plasma was switched on (ton) to the time when plasma was switched off (toff). Afterwards we can define the effective power Peff = ton/T×Ptotal, where the period was defined as T = ton+toff and the total power of generator was Ptotal = 50 W [4]. First, the surface of unsized glass fibers was pretreated by oxygen plasma to improve the film adhesion. Next, we used tetravinylsilane (TVS, purity 97%, Sigma Aldrich) as the monomer in a mixture with oxygen gas to surface modify a bundle of glass fibers by thin film. Finally, we kept modified glass fibers in a flow of argon gas for purpose of recombination of all active radicals. The fibers and composite samples were also analyzed by Scanning Electron Microscopy (SEM) (Fig. 3, 4) and Confocal Laser Scanning Microscopy (CLSM).
3. Results and discussion Prepared composite samples with unsized and plasma modified glass fibers were tested by universal testing machine. Displacement rate was 1 mm per minute and the force response was measured. When the bundle of glass fibers was pulled-out from polymeric disk of composite sample, the tensile test was ended. The displacement’s curves for unsized glass fibers are shown in Fig. 1, Table 1 and includes all results of the test (h is the height of polymeric cylinder, F is the applied force for pulling-out the bundle of tested glass fibers from disk, σ is the maximum of stress in longitudinal surface section). Data from this table were averaged. We prepared 5 sets of composites samples with the bundle of glass fibers modified by TVS plasma in different condition (such as flow rates and deposition time) (Tab. 2). The force-displacement curves corresponding to the sets are shown in Fig. 2 and Table 3. The best results were obtained from the set of number 5. The deposition conditions of this set were as follows: deposition time 30 min., the ratio of pulse plasma discharge 1:19 (ton = 1 ms; toff = 19 ms), the oxygen flow 0.46 sccm and TVS flow 0.12 sccm. Table 1. Composite samples with unsized glass reinforcements.
Table 2. Deposition conditions of different sets of plasma modified GF.
Maximum stress reached in pull-out test of unsized glass fibers was 18.6 MPa and of plasma modified was 26.2 MPa. It means, composites fabricated from plasma coated glass fibers showed an increase of adhesion by 40% with respect to unsized reinforcements. The tangent of load-displacement curves for plasma treated reinforcements was a bit higher and the relative displacement was increased too, from 1.02 mm to 1.30 mm with respect to unsized reinforcements. These all results give us information, that the adhesion between fibers and matrix was strongly increased due to presence of a thin plasma polymer film. Even more, the composite with plasma modified reinforcements is in global scale much more ductile and flexible. We can see the SEM micrograph of unsized glass fibers embedded in polyester resin after tensile test in Fig. 3. We can observe clear and naked glass fibers and very poor adhesion to polymer matrix. The fibers were easily Table 3. The confrontation of tensile test results of sets of plasma modified versus unsized glass fibers.
300
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plasma modified GF, ser. 1 plasma modified GF, ser. 2 plasma modified GF, ser. 3 plasma modified GF, ser. 4 plasma modified GF, ser. 5 unsized GF
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Displacement [mm] Fig. 1 Applied force versus displacement corresponding to sets of composites with unsized GF.
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Displacement [mm] Fig. 2. Applied force versus displacement corresponding to composite specimens with plasma modified GF compared with those of unsized GF.
pulled-out from the polymer matrix without any rupture. And the matrix had no signs of the cracks. Different result can we observe in the SEM micrograph (Fig. 4) of plasma modified glass fibers after tensile test. The fibers are coated by thin plasma polymer film and completely saturated by polyester resin. There is better adhesion due to plasma treated interface and no signs of cracks.
nized for preparation controlled interphase in composites and nanocomposites. The fiber coating method for toughening composites seems to be one of the most effective methods for achieving simultaneously high strength and high toughness when an appropriate polymer is chosen [5]. Theoretical and experimental studies have shown that the coated material should be ductile or flexible with an interlayer modulus lower than that of the matrix [6]. Plasma polymerization and plasma treatment have a great ability to vary and control many physicochemical properties over wide ranges resulting in an extraordinary potential for useful application. Acknowledgement: This work was supported in part by the Czech Ministry of Education, grant no. ME09061 and MSM0021630501, the Academy of Sciences of the Czech Republic, grant no. KAN101120701, and the Czech Science Foundation, grant no. 104/09/H080.
Fig. 3 Micrograph of unsized glass fibers embedded in polyester resin after tensile test.
Fig. 4 Micrograph of plasma modified glass fibers embedded in polyester resin after tensile test.
4. Conclusion Plasma surface modification is an effective technique to influence physical and chemical properties of interphases in fiber reinforced polymer composites. The tensile strength of single fibers is sensitive to both the plasma treatment and plasma polymerization. Strong adhesion is promoted by increasing the surface energy of fibers, as a result of plasma modification, decreasing contact angle of the matrix, and thus the wettability of fibers by the polymer matrix is improved. The organosilicon plasma polymers are widely recog-
References [1] J.-K. Kim and Y.-W. Mai, Engineered Interfaces in Fiber Reinforced Composites, Elsevier, Amsterdam (1998). [2] V. Cech, N. Inagaki, J. Vanek, R. Prikryl, A. Grycova and J. Zemek, Thin Solid Films 502, 181 (2006). [3] N. Inagaki, Plasma Surface Modification and Plasma Polymerization, Lancaster, Technomic Publ. (1996). [4] V. Cech, J. Studynka, N. Conte, V. Perina, Physico-chemical properties of plasma-polymerized of tetravinylsilane, Surface & Coating technology 201, 5512 (2007). [5] J. K. Kim and Y. W. Mai, High strength, high fracture toughness fiber composites with interface control-a review, Compos. Sci. Technol. 41, 333 (1991). [6] L.J. Broutman and B.D. Agarwal, A theoretical study of the effect of an interface on the properties of composites, Polym. Eng. Sci. 14, 581 (1974).
