This chapter delves into the challenges of bonding polyethylene fibers with cementitious matrices due to their hydrophobic nature. It introduces a rapid surface modification technique using tannic acid and sodium periodate, which enhances the fiber-matrix interface by creating a hydrophilic layer with active polar groups. The study demonstrates that this modification improves interfacial shear strength and pullout energy, leading to better mechanical interlocking and fiber-matrix adhesion. The chapter presents a detailed experimental methodology, including the preparation of modified fibers and cementitious matrix specimens, and provides comprehensive results and discussions based on microscopic analysis and mechanical testing. The findings highlight the potential of this rapid modification technique to enhance the performance of fiber-reinforced cementitious composites, making it a valuable resource for researchers and professionals in the field.
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Abstract
In cementitious composites, an application of various fibers can contribute to endow a controlled crack propagation, moderated brittle failure, superior tensile strength and higher energy absorption capacity. Fiber-matrix bonding properties play a key role in fiber strengthening efficiency and the final mechanical performances of the reinforced matrices. This is true specifically for high-performance polyethylene (PE) fibers which yield very high tensile strength and modulus of elasticity, but do not interact properly with cementitious matrix due to their inert hydrophobic surface lacking functional groups.
In the presented work, PE fibers are functionalized by using fast tannic acid modification technique to enhance the bonding properties between a cementitious matrix and the fibers. Environmental scanning electron microscopy (ESEM) confirmed the presence of polymer coating layers on the fiber surfaces. Micromechanical tests indicated that the modified fibers considerably improved the maximum fiber pullout force, interfacial shear strength and pullout work in comparison with the reference fibers. This enhancement in bonding properties could be traced back to the created functional layer on the PE surface triggering a better interaction with cement hydrates as well as a rougher surface enhancing fiber-matrix mechanical interlocking at interfaces. Overall, the introduced approach can be applied for different fibers to promote their bonding behavior with cementitious matrices resulting in an enhanced fiber reinforcing effect in composites.
1 Introduction
Cementitious materials are characterized by high compressive strengths, while possessing poor tensile properties, low strain capacity and meager resistance to cracking with noticeable brittleness [1]. As a consequence, different non-polymeric and polymeric fibers have been embedded into cementitious matrices to produce fiber reinforced cementitious composite (FRCC) with enhanced tensile strength, controlled crack propagation, mitigated brittle failure and higher energy absorption capacity [2]. Ultra-high molecular weight polyethylene (UHMWPE) fibers, a kind of special polyethylene (PE) fibers, represent superb modulus of elasticity and tensile strength along with outstanding stability in alkaline/acidic environments, making them as an appropriate candidate for FRCC among the other polymeric fibers [3]. Hence, they have also caused considerable attention to be used in strain-hardening cementitious composites (SHCC) [4]. Nonetheless, PE fibers possess hydrophobic property with no chemical active groups on their surface which yields weak adhesion properties between these fibers and their water-based cementitious matrices. Such low fiber-matric interaction is usually undesired [4].
As a composite material, it is well-known that the effectiveness of load transfer between the constituents, i.e. fibers and surrounding matrix, depends on fiber-matrix interfacial properties, particularly determining the degree of fiber reinforcing power in FRCC [5]. In general, it is preferable that the fibers tolerate applied forces through a gradual delamination process followed by full pullout from the matrix instead of either simple fiber pullout or pure fiber rupture. An optimized fiber-matrix bonding performance, hence, is required to exploit full advantages of utilization of fibers in FRCC/SHCC [6].
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As stated above, PE fibers do not have any functional groups and are therefore unable to adhere properly to cementitious matrices. On this basis, researchers have rendered a variety of fiber modification approaches comprising chemical and physical treatments [7]. In recent years, application of tannic acid (TA) as a cost-effective and plant-based polyphenol has significantly attracted huge interest to be used for surface modification of diverse substrates [8]. In comparison to dopamine (DA), which is widely applied for the same goal, TA has an analogous chemical structure but at a considerably lower price, and similarly shows an ability to form a polymeric layer having numerous hydroxyl groups on different surfaces under alkaline conditions [5]. There are frequent studies reported on the use of a 24-h TA modification or a 12-h TA modification on various substances to make them hydrophilic to be exerted for different applications [9, 10]. However, such long treatment durations hinder practical uses so far. To overcome this issue, sodium periodate (SP) has been introduced to accelerate the TA-based surface modification of different substrates [11, 12]. Since an oxidation step is necessary to form the TA hydrophilic layer, SP as a strong oxidizing can make TA self-polymerization easier and faster, shortening duration of the modification [11]. To the best of the author’s knowledge, no paper can be found in the literature stating rapid TA functionalization of PE fibers via using SP to be incorporated in cementitious matrices. Therefore, the current research suggests a simple and rapid technique to activate the PE fiber surface for improving interfacial adhesion properties in cementitious matrices for the first time.
2 Experimental
2.1 Materials
A kind of special PE fiber i.e. UHMWPE fiber (SK62) was provided from Dyneema® (DSM, The Netherlands) with an average diameter of 18 µm according to microscopic measurements. TA (ACS reagent, CAS-Number: 1401–55-4), SP (NaIO4, ACS reagent, ≥ 99.8%) and ethanolamine (EA, ≥ 98%, CAS Number: 141-43-5) were provided by Sigma-Aldrich (St. Louis, MO, USA). The fibers were washed in deionized water/isopropanol solution inside a sonication bath for 1 h and the washed fibers were then dried in ambient temperature for later use.
2.2 Rapid Functionalization of PE Fibers by TA, EA and SP
To rapidly modify PE fiber surface, the washed PE fibers (0.10 g) were placed into an aqueous 200 mL-solution containing a certain amount of 4 g/L of TA. After that, 4 mL of EA was added to the above solution followed by addition of a constant SP amount of 8 g/L. The final solution was kept under mild shaking at ambient temperature for various durations. Ultimately, the modified fibers were taken out from the solution, washed several times in deionized water, dried in room temperature and put inside a storage bag for the tests. The samples with modification durations of 30 min, 1 h and 3 h were labeled as PE-TA-EA-SP-30 min. PE-TA-EA-SP-1 h and PE-TA-EA-SP-3 h, respectively (one-step method).
In addition to the mentioned one-step process, a two-step modification was further performed. The same TA solution including 0.10 g of PE fibers was prepared and then 8 g/L of SP was added to the solution. The provided solution was kept under mild shaking at room temperature for 1 h. Afterwards, 4 mL of EA was inserted and the experiment was proceeded for 2 h. The modified fibers were finally taken out from the solution, washed several times in deionized water, dried in room temperature and kept in a storage bag for the tests This sample was named as PE-TA-SP/EA-3 h (two-step method).
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2.3 Fabrication of Specimens for Single Fiber Pullout Tests
A high-strength cementitious matrix with a water-to-binder ratio of 0.18 was used according to the literature regarding SHCC with PE fibers [13]. By means of a hand mixer containing a planetary rotating blade, the dry components including 296.25 g of cement powder (CEM I 52,5 R-SR3/NA, Holcim Technology Ltd., Zurich, Switzerland), 57.22 g of silica fume (Elkem Microsilica 971, Elkem ASA Silicon Materials Company, Oslo, Norway) and 29.42 g of quartz sand (0.06–0.2 mm, Quarzwerke Strobel, Germany), were mixed as the first step. Low, medium and high speeds were used for 30 s for each in sequence. The mixing process was proceeded by the addition of 7.10 g of superplasticizer (Glenium ACE 460, BASF, Germany) and 63.92 g of water to the solid powders for an additional 5 min under high speed. A specifically designed mold as reported in Ref [14] was used to produce specimens for the single fiber pullout test with a fiber embedded length of 2 mm. The prepared mortar mixture was cast into the mold in which the casting process was conducted under vibration for 40 s to remove air bubbles. The specimens were subsequently demolded after one day and stored at standard climate conditions for 14 days to be tested.
2.4 Characterization Methods
In order to evaluate surface changes after rapid TA functionalization of PE fibers, an environmental scanning electron microscope (ESEM) Quanta 250 FEG by the FEI Company (Hillsboro, OR, the Netherlands) was used.
Investigation of micro-mechanical properties i.e., single fiber pullout tests was done by means of a Zwick line testing machine (ZwickRoell GmbH, Ulm, Germany), utilizing a 10-N load cell and a constant displacement rate of 0.01 mm/s. Furthermore, single fiber pullout specimens were prepared with a 2-mm free length according to [14]. At least eight samples were considered to be tested for each modification series. Besides, interfacial shear strengths (τ) were calculated using Eq. (1) with regard to Pmax, d, and L as the maximum pullout force, diameter of the PE fiber and the fiber length embedded in the matrix, respectively [3]:
$$\tau = \frac{{P}_{max}}{\pi dL}$$
(1)
3 Results and Discussion
3.1 Investigation of PE Fiber Surface Changes After Rapid TA Modification
The surface changes before and after rapid TA modification of PE fibers were evaluated by means of ESEM analysis and the acquired images are shown in Fig. 1. For the pristine sample, long grooves over the fiber length along with slight micro-pits can be seen on the fiber surfaces implying relatively smooth surface. In contrast, all the rapid TA-modified PE fibers clearly indicate a TA coating layer on their surfaces which reveals that the PE functionalization process was successfully performed. Additionally, it can be obviously seen that the surfaces of the treated fibers become rougher through deposition of TA on the fibers’ surfaces.
According to Fig. 1, it can be further realized that TA modification durations and order of EA addition to the synthesis solution can affect the surface morphologies of the treated PE fibers. An uneven surface with partially distributed TA coatings was observed in the modified sample with the lowest treatment time, i.e. sample PE-TA-EA-SP-30 min, while a relatively smoother coated surface with more TA coating was found for the modified sample with the longest modification time, i.e. sample PE-TA-EA-SP-3 h. Indeed, a longer modification duration can trigger more polymerization of TA in the presence of EA deposited on the PE fiber surface as reported in Ref [5] as well. Moreover, it seems that sample PE-TA-SP/EA-3h showed an inhomogeneous surface with the highest surface roughness among the modified samples. This can be likely explained by a better chance of Schiff-base and/or Michael-type addition reactions [5] between the EA as an amine compound and formed quinones of TA in presence of SP when EA is added after 1 h of addition of SP compared to simultaneous addition of SP and EA. It is worthwhile to note that this surface roughness seen in the treated fibers will directly influence fiber-matrix interfaces, reflecting on interfacial properties between fibers and their matrix.
Fig. 1.
ESEM images of pristine PE fibers and various rapid TA-modified PE fibers
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In addition to formation of surface roughness as a physical effect of fast TA functionalization of PE fibers, the chemical impact of such modification can be described as follows:
a)
Highly reactive quinones can be generated via oxidation of numerous phenolic hydroxyl groups of TA in presence of SP as a strong oxidizing agent which subsequently follows a self-crosslinking reaction between the aryl rings [5]. More reactivity of the radicals produced by SP namely superoxide and hydroxyl radicals than O2 (conventional TA treatment in presence of air) can result in forming carboxyl groups in addition to hydroxyl groups through oxidative cleavage of TA aromatic units during the modification process [11].
b)
A Michael addition reaction and/or a Schiff base reaction can take place by addition of EA as an amine compound leading to formation of amino groups on the fiber surface as well. Finally, a thin TA polymeric layer containing active polar groups is generated on the PE fiber surfaces [5, 6].
3.2 Micromechanical Assessment of Rapid TA-Modified PE Fibers/cementitious Matrix Interactions
Interfacial shear strength values of the specimens containing pristine and modified PE fibers were achieved from single fiber pullout tests and the calculated results are shown in Fig. 2(a). A poor fiber-matrix interaction is seen for the pristine PE fibers with an interfacial shear strength of 0.69 MPa. In contrast, the rapid TA-modified PE fibers showed improved fiber-matrix adhesion properties with interfacial shear strengths from 1.29 MPa to 1.73 MPa, expressing a minimum improvement of 87% compared to the pristine samples. The fast TA modification, in fact, appropriately activates PE fiber surfaces through forming hydroxyl, carboxyl and amino groups [5, 11] leading to more reactions between these polar groups and cement hydrates which cause superior interfacial properties. With regard to modification time for one-step modified samples, sample PE-TA-EA-SP-30 min had the highest interfacial shear strength. It can be traced back to the shortest modification time for this sample which led to an uneven and partially covered fiber surface by TA layer as observed in ESEM images which is beneficial for fiber-matrix interfacial properties. With increasing modification duration up to 1 h, the shear strength decreased which was followed by an increase for 3-hmodification. This can be explained by the smoothening/roughening effect of reproducing a TA layer on the fiber surface over the time. For the sample using the two-step method, i.e. sample PE-TA-SP/EA-3h, the maximum shear strength was gained among all the modified samples. A better chance of reaction between the reactive TA layer and EA, and a higher surface roughness as seen in ESEM images resulted in enhanced fiber-matrix interactions for this sample.
Moreover, pullout energy values of the pristine and modified samples are represented in Fig. 2(b). The pullout energy is 0.062 N*mm for the pristine PE fibers indicating a weak interfacial performance with the cementitious matrix. Contrarily, the modified samples demonstrated a promoted pullout energy ranging from 0.068 N*mm to 0.146 N*mm. Deposition of a reactive TA layer along with created roughness on the PE fiber surface, indeed, not only improved the fiber-matrix interactions chemically but also strengthened the mechanical interlocking of the fiber-matrix interface. It can be seen that the longest modification duration, i.e. 3 h, led to maximum pullout energy values owing to completion of TA polymerization and TA/EA reaction in a longer synthesis duration. Furthermore, the one-step modification method showed greater pullout energy than the two-step modification, while the trend was reversed for interfacial shear strength. This suggests that the physicochemical surface phenomena at fiber-matrix interfaces induced by these two different methods are dissimilar affecting interfacial performances of the specimens.
Fig. 2.
Interfacial shear strength and fiber pullout energy values of the pristine and rapid TA-modified PE fibers
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4 Conclusions
The current research proposes a simple method to enhance interfacial performances between PE fibers and a cementitious matrix through functionalization of PE fiber by rapid TA surface modification. The findings are as follows:
The PE fiber surface became rougher after rapid TA surface modification as shown by ESEM images. It was further observed that the two-step modification method resulted in more surface roughness and more TA deposition on the fiber surface.
Both interfacial shear strength and pullout energy of the modified samples were improved by rapid TA modification due to forming polar active groups on the fiber surface which can react with cement hydrates as well as creating surface roughness for better mechanical interlocking at fiber-matrix interfaces.
It was seen that even the 30-min TA modification was effective for enhancing PE fiber-cementitious matrix interaction; nonetheless, the 3-h TA modification yielded more pronounced improvement in both interfacial shear strength and pullout energy parameters.
Acknowledgements
The authors express their gratitude to the German Research Foundation (Deutsche Forschungsgemeinschaft - DFG) for the financial support of the project 455631638.
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