Acta Polytechnica CTU Proceedings doi:10.14311/APP.2017.13.0049 Acta Polytechnica CTU Proceedings 13:49–54, 2017 © Czech Technical University in Prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app PLASMA TREATMENT IMPACT ON PHYSICAL AND CHEMICAL PROPERTIES OF POLYMERIC FIBERS Radim Hlůžeka, ∗, Zdeněk Prošeka, b, Jan Trejbala, Josef Fládra, Štěpán Potockýc a Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, 166 29 Prague 6, Czech Republic b University Centre for Energy Efficient Buildings of Technical University in Prague, Třinecká 1024, 273 43 Buštěhrad, Czech Republic c Czech Academy of Science, Institute of Physics, Na Slovance 1999/2, 182 21 Prague 8, Czech Republic ∗ corresponding author: radim.hluzek@fsv.cvut.cz Abstract. Presented work focuses on chemical and physical properties of plasma modified polymeric macro-fibers. Polyethylene terephthalate (PET) and polypropylene (PP) fibers having approx. 300 µm in diameter were modified using cold oxygen plasma in order to achieve their surface changes needed for durable bond and adhesion with cement matrixes. A duration of plasma modification differed between 5 to 480 seconds, where an effect of the treatment was examined. Fiber surfaces chemical changes were researched via wettability measurement with demineralized water (the measurement was repeated immediately and after 1, 7 and 30 days to find out the changes stability). Physical changes were studied by means of weight balance (determination of weight loss) and tensile strength tests. It was found that wettability was enhanced significantly – up to two times, while mechanical properties of treated fibers decreased only slightly. Keywords: polymer macro-fibers, plasma treatment, wettability, tensile strength. 1. Introduction Usage of man-made macro fibers as reinforcement of any composites become a commonality in majority of industry fields, civil engineering including. In the final consequence, a small bulk property improvement has strong impact in civil engineering due to mass pro- duction [1]. Fiber-reinforced materials exhibit good mechanical properties, e.g. high mechanical resistance (abrasion, impact resistance), ductility, water resis- tance etc. [2, 3]. The main task of the macro fiber addition is the dis- tribution of shrinkage into several small cracks in the case of materials based on shrinking binders (mainly lime and cement), and to prevent the single crack open- ings after linear elastic response in the case of loaded samples [1]. In the civil engineering, the macro fiber reinforcement is the most often used for a) production of watertight concretes and concretes exposed to risk of steel reinforcement corrosion (absence of cracks disallows water penetration), and b) for production of large-scale construction exposed to temperature or moisture changes, dynamic loads, point loads (the fiber reinforcement provides a compactness – hence usability – of materials after the crossing the material loading capacity) [4, 5]. The fiber reinforcement can be classified by fibers material, diameter, tensile strength and their modulus of elasticity. As material, polymeric fibers can be used. Both polymeric (in particular PET and PP) fibers have high tensile strength equal to about hundred or even thousands MPa. Their diameter is equal to tens or hundreds micrometers. Other characteristic prop- erty is low ratio of diameter to length (and related high specific surface enabling better stress transfer- ring from matrix to fibers) and favorable cost [6, 7]. Polymeric fibers reveal low surface wettability (hy- drophobicity). On the other hand, fiber reinforcement requires good adhesion between the fiber surface and matrix. To improve the mechanical strength of rein- forced materials, adhesion between the fiber surfaces and matrix must be unambiguously ensured [5, 8]. The required adhesion can be modified by fiber sur- face treatment by chemical and physical methods [7, 9– 13]. Currently, the newly introduced plasma treatment becomes popular as a progressive physico-chemical method. The low-pressure plasma treatment repre- sents a universal, efficient and eco-friendly alternative for surface modifications. Plasma can be defined as ionized gas (composed of electrons, ions, and neutral species). The mechanism for plasma surface modifica- tion relays on surface atoms replacement by oxygen atoms and formation of polar groups. The presence of polar or functional chemical groups enhances the reactivity with the matrix based on cement or lime binder (both contain water) [5]. In the present work, we report on the modification of polyethylene terephthalate (PET) and polypropylene (PP) fibers by oxygen plasma treatment. The influence of the plasma treatment on the contact angle, the weight and the tensile strength of the fibers is studied. 49 http://dx.doi.org/10.14311/APP.2017.13.0049 http://ojs.cvut.cz/ojs/index.php/app R. Hlůžek, Z. Prošek, J. Trejbal et al. Acta Polytechnica CTU Proceedings Figure 1. Digital camera photo of PET (left), PP fibers (right). 2. Materials 2.1. Polymer macro-fibers Two different types of fibers were used: PET and PP. Both of them were made by Spokar manufacturer (Czech Republic). They were primary made for the production of most types of brushes (tooth, paint etc.) and brooms [14]. The fibers had diameter equal to approx. 300 µm (measured using mechanical mi- crometer), original length was 1200 mm (subsequently chopped according to need). An industrial water flush- able sizing is standardly applied onto fiber surfaces as an integral part of their production. Fibers are shown in Figure 1. 3. Experimental methods 3.1. Plasma treatment To improve the wettability and related adhesion be- tween fiber surfaces and surrounded matrix, oxygen treatment in inductively coupled plasma was realized using device Tesla VT 214 (13.56 MHz). Parameters of the whole process differing only in exposition time from 5 to 480 seconds, the others were constant; total power of RF source 100 W, total gas pressure 20 Pa, oxygen flow 50 sccm. 3.2. Contact angle measurement According to reference [15], using horizontal direct optical method, contact angles were measured be- tween meniscus of distilled water (adhering on fiber surfaces) and fiber perpendicularly submerged into the water. Thus obtained results were averaged from 6 independent measurements. 3.3. Weight ratio analysis Due to ion bombardments and polymer oxidation and surface structuring (etching) during the plasma treatment, the fibers can be damaged. Weight ratio observation carried out before and after treatment enables to assess the fiber weight loss, which is con- nected to tensile strength capacity loss. In order to quantify the fiber weight loss, fibers were weighed before and immediately after plasma treatment on weighing-machine Kern ALJ 120-4 (d = 0.1 mg). 3.4. Tensile strength tests Physical and chemical processes mentioned earlier can influence the fibers tensile strength. To determine the extent of fibers mechanical properties damage, tensile strength test was carried out using loading frame Web Tiv Ravestein FP100. The test was displacement controlled at a constant rate equal to 6 mm/min in the case of PET and 3 mm/min in the case of PP fibers, respectively. The results were averaged from 6 independents experiments for each plasma treatment time of both fiber types. 4. Results and discussions 4.1. Contact angle measurement Contact angle measurement revealed 84.7±2.2◦ on untreated PET fibers and 88.3±0.6◦ on untreated PP fibers. The results provided on the treated fibers showed the unexceptionable positive plasma impact of the fibers wettability. All treated fibers independently on plasma exposition time showed significant decrease of contact angle. The angle measured immediately after plasma treatment on both types of fibers was moving between approx. 25–30◦. After 1, 7 and 30 days from the treatment, contact angle increased up to 65–70◦ for PET fibers and for the PP fibers up to 55–70◦. Besides, the results showed that it is not effec- tive to do any treatment longer duration than several seconds. Another finding is the fact that storage of fibers, even for one day, is very inefficient, and the contact angle increase rapidly towards the reference values. Dependence between contact angle, duration of plasma treatment and time of storage in atmo- spheric conditions is shown in Figure 2 and Figure 3. Based on these finding, it was decided that plasma treatment duration realized within other experiments throughout the study were shortened to 120 seconds only. Demonstration of the fibers wettability cap- tured during contact angle measurement is shown in Figure 4. 4.2. Weight ratio analysis Weight ratio observation performed on fibers before and after plasma treatment showed that the weight 50 vol. 13/2017 Physical and Chemical Properties of Polymeric Macro-fibers Figure 2. PET fibers – dependence between contact angle, duration of plasma treatment and time of exposure to atmospheric conditions. Figure 3. PP fibers – dependence between contact angle, duration of plasma treatment and time of exposure to atmospheric conditions loss is negligible. Fibers treated by 120 seconds exhib- ited practically no weight loss, just less than 0.25 %. Comparing both types of fibers, it can be said that the weight loss is the same until 30 seconds of plasma treatment. After 30 seconds of treatment, there are some small difference between PET and PP fibers. After the longest time of treatment (120 seconds), the maximum averaged weight loss was 0.06 % and 0.21 % for PET and PP fibers, respectively. Based on these findings, it can be assumed that mechanical properties of treated fiber were not affected by plasma treatment, which is a positive result. Graphic relation between weight loss and duration of plasma treatment is shown in Figure 5. 51 R. Hlůžek, Z. Prošek, J. Trejbal et al. Acta Polytechnica CTU Proceedings Figure 4. Contact angles captured during the wettability measurement Figure 5. Relation between fibers weight loss and duration of plasma treatment obtained before and after plasma treatment. Figure 6. Dependence between fibers tensile strength and duration of plasma treatment. 52 vol. 13/2017 Physical and Chemical Properties of Polymeric Macro-fibers 4.3. Tensile strength tests Tensile strength (more precisely load carrying capac- ity) of reference and plasma treated fibers was ap- proximately constant for all plasma treatment times (measured on reference, 10, 60 and 120 seconds treated fibers). PET fibers reached on approx. 20 N, while PP fibers on approx. 32 N. As it was already shown by weight loss ratio mentioned earlier, fiber tensile strength test confirmed that the plasma treatment had no significant effect on fiber mechanical perfor- mance. The relation between fibers tensile strength and duration of plasma treatment is shown in Figure 6. 5. Conclusions Polypropylene (PP) and polyethylene terephthalate (PET) commercial macro fibers Spokar having ap- prox. 300 µm in diameter were surface treated by mean of low-pressure cool oxygen plasma treatment in order to achieve their chemical and physical surface changes. The aim of the executed treatment was to improve fiber surface wettability (from hydrophobic to hydrophilic) and morphology (from smooth to rough- ened), both standardly required for strong interphase interaction with cement matrix, when fibers used as reinforcement. The plasma modification differed in duration (from 5 to 480 seconds). An effect on physico-chemical changes on fiber surfaces was examined by wettability measurement, wight loss ratio (fibers were weighed before and after plasma treatment) and finally by fiber tensile strength tests, when the main focus of the re- search was targeted to impact of plasma treatment on fiber mechanical performance. Besides, time-stability of chemical changes on modified fibers was researched – wettability between fiber surfaces and distilled water was examined using direct optical set for contact an- gle measurement immediately, 1, 7 and 30 days after plasma modifications. It was found out that: • The wettability of plasma treated (regardless to duration of treatment time) both PP and PET fibers increased almost three times, if compared to reference samples. • The modified wettaility was very short-term. It was degraded almost to reference values already after one day, when fibers exposed to standard atmospheric conditions. • Impact of realized treatment did not have no ob- vious effect on fiber mechanical performance, as demonstrated by no weight loss and no tensile strength changes for much as treated fibers. Acknowledgements This work was financially supported by Czech Technical University in Prague – SGS project SGS16/201/OHK1/3T/11, by the Czech Science Founda- tion research project 15-12420S and by the Ministry of Education, Youth and Sports within National Sustainabil- ity Programme I, project No. LO1605. The work occurred in frame of the LNSM infrastructure (no. LM2015087). References [1] P. K. Mallick. Materials. In Fiber-reinforced composites, materials, manufacturing and design, chap. 2. CRC Press, 2007. [2] J. Trejbal, L. Kopecký, S. Potocký, J. Fládr. The effect of glass fiber reinforcement on flexural strength of lime-based mortars. 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Trejbal, L. Kopecký, S. Potocký, Z. Remeš. Direct optical method for contact angle measurement on micro fibers. In Nanomaterials and nanotechnology in civil engineering 2015 – conference proceedings, pp. 89–90. Czech Technical University in Prague, 2015. 54 http://www.spokar.com/cz/technologie/vyroba-vlaken http://www.spokar.com/cz/technologie/vyroba-vlaken Acta Polytechnica CTU Proceedings 13:50–55, 2017 1 Introduction 2 Materials 2.1 Polymer macro-fibers 3 Experimental methods 3.1 Plasma treatment 3.2 Contact angle measurement 3.3 Weight ratio analysis 3.4 Tensile strength tests 4 Results and discussions 4.1 Contact angle measurement 4.2 Weight ratio analysis 4.3 Tensile strength tests 5 Conclusions Acknowledgements References