CC BY
33
UDC 678
E. Kraus, S. Horvat, B. Baudrit, O. V. Stoyanov, I. A. Starostina
RIGHT PRE-TREATMENT FOR ADHESIVE BONDING OF FLUOROPOLYMERS
Keywords: Adhesive bonding, adhesion analysis, bonded polymer joints, fluoropolymers, pre-treatment.
In this paper we report on the effect of different treatments on adhesion of four of the most commercially common fluoropolymers: fully fluorinated polymers, including polytetrafluoroethylene (PTFE) and perflyoroalkoxy alkane (PFA) and partially fluorinated polymers, including polyethylenechlorotrifluoroethylene (ECTFE) and polyvinylidene fluoride (PVDF).
Ключевые слова: адгезионное соединение, исследование адгезии, полимерное соединение, фторполимеры, предварительная
обработка.
Сообщается о влиянии различных методов обработки поверхности на адгезию четырех наиболее распространенных промышленных фторполимеров: полностью фторированных полимеров, в том числе политетрафторэтилена (ПТФЭ) и перфторалкокси алкана (ПФА) и частично фторированных полимеров, в том числе полиэтиленхлортрифторэтилена (ПЭХТФЭ) и поливинилиденфторида (ПВФ).
Introduction
Fluoropolymers are well-known materials with unique properties, which make them applicable in diverse fields. High thermal stability, chemical inertness, flame resistance, high coefficients of fraction, and low dielectric constants are some of the characteristics that ensured applications of fluoropolymers across the automotive, aircraft, construction, electrical, IT, household, food and medicinal industries [1]. The world consumption of these materials is in continuous increase. It is estimated that global fluoropolymers market in terms of revenue will rise from $7,159.3 million in 2012 up to expected $9,797.4 million in 2018. Outstanding properties lead to significant research towards new growth fields of fluoropolymers [2].
Such desired characteristics come from the electronic structure of the fluorine atom, the stable carbon-fluorine (C-F) bonds and the unique intramolecular and intermolecular interactions between the fluorinated polymer segments and the main chains. Fluorine molecules have little tendency for polarization or ionization, as well as no permanent dipole in its structure and no steric hindrance, resulting in low surface energy and low work of adhesion [3]. Poor wetting properties that are a consequence of the superficial dipoles are considered as the major limiting factors for the application of fluoropolymers. Therefore surface modifications are often used in enhancing properties, including plasma treatment, flame treatment, chemical etching, and surface priming [4]. Plasma treatment is a widely used method in surface modification and it has been reported to improve adherence properties of fluoropolymers by increasing the surface free energy (SFE) [3].
Theoretical aspects
Primary condition for successful adhesive bonding is good wetting of the substrate surface with an adhesive, which is the result of high SFE. Compared to hydrogen carbons, fluorocarbons have relatively low SFE. An explanation of this property can be found in the nature of the C-F bond. Substitution of F instead of H in polymers leads to an increased bond strength (5.03 eV) compared to C-H bonds (4.31 eV). Additionally, shared
electron pairs are situated closer to the F relative to the centre of the C-F bond, due to higher electronegativity of fluorine compared to carbon. Thanks to the relatively big size of the fluorine atom and length of the C-F bond, carbon atoms are surrounded by fluorine atoms, thus preventing any chemical attack of the weaker C-C bonds [3,5].
Plasma contains free electrons which in contact with the polymer surface break its covalent bonds and create free radicals. These interact with gas ions in plasma, thus modifying hydrophobic surfaces to hydrophilic [3].
Fig. 1 shows that energies of the plasma spectrum cover the range of C-F bonds energy. However, high energy of C-F bonds causes difficulties in oxidation of fluorinated polymers by plasma treatments, despite the fact that this treatment is considered as very effective in enhancing wettability of plastics.
eV
rim
100
10 1
0.1
0.01
CH4.
200- 7T-TU* " QHe
2000 1000" 300
Fig. 1 - Energy scheme for low pressure plasma [6]
Even defluorination may occur during ion bombarding at plasma treatment. C-F bonds could be formed immediately after the plasma exposure, resulting in hydrophobic recovery [7]. On the other hand, in partly fluorinated fluoropolymers oxidation and cross-linking take place in C atoms bonded with atoms different from F, such as H or Cl, leading to a more effectiveness of plasma treatment in adhesion properties [8].
Experimental
Fully fluorinated polymers (PTFE, PFA) and partially fluorinated polymers (ECTFE, PVDF) were treated with low-pressure plasma (LPP) PICO, manufacturer Diener Electronics GmbH (0.4 mbar, 60% 400 W) and atmospheric pressure plasma (APP), manufacturer Plasmatreat GmbH (400 W, distance nozzle - substrate 8 mm, OpenAir® - rotating nozzle). Air was used as a process gas. The influence of applied treatments on the SFE was evaluated by drop shape analysis (Kruss DSA30). The contacting liquids used were deionized water, ethylene glycol and diiodmethane, all of technical purity. The calculations of SFE were based on Owens-Wendt-Rabel-Kaelble (OWRK) method. The mechanical properties of bonded joints were tested by tensile shear tester Z010 manufacturer Zwick GmbH.
Results
The drop shape analysis (OWRK) revealed that not all treatments lead to increase in the SFE resulting in increased wettability of used fluorinated polymers. The achieved results are introduced in Table 1.
Table 1 - SFE for investigated fluorinated polymers before and after the treatment with LPP and APP
Polymer SFE [mN/m]
Without pretreatment APP (10 s) LPP (10 min)
ECTFE 28.94 ± 0.16 47.79 ± 0.54 45.04 ± 0.43
PVDF 34.39 ± 0.25 39.01 ± 0.18 38.09 ± 0.26
PTFE 15.29 ± 0.18 15.60 ± 0.30 23.22 ± 0.33
PFA 14.29 ± 0.16 14.60 ± 0.21 9.97 ± 0.14
A simple method to measure the surface tension of polymeric materials is its determination using test inks. For a quick examination of the effect for various treatment processes Arcotec test inks were used (Fig. 2).
Fig. 2 - Wetting of treated (left) and not treated (right) ECTFE-Surface with test ink 41 mN/m. The treated surface shows a SFE > 41 mN/m
The test ink is applied on the surface using the integrated pipette in the bottle. The measurements start by depositioning the test ink with a high surface tension (e.g. 72 mN/m) and become less onto the substrate surface after the pretreatment. Once the drop edges are stable for at least two seconds, the surface is easily wettable. In this moment, the value of substrate surface tension is the closest to the surface tension of the test ink. If the drop of the test ink contracts, the next lower test ink should be used.
Pretreatment and mechanical properties of partially fluorinated polymers (PVDF, ECTFE)
The studies on the adhesion ability with cold-curing 2-component adhesives showed a relative suitability of partially fluorinated polymers such as PVDF and ECTFE for bonding with 2C-epoxy (EP) and 2C-polyurethane (PUR) adhesives after pretreatments. Bond strengths of up to 4.8 MPa for ECTFE bonded with Loctite 9466 (2C-EP) after 10 s pretreatment with APP were achieved. The fracture tests according to DIN EN ISO 10365 (Adhesives -Designation of main failure patterns) showed thereby a fracture in the substrate (SF) for treated and adhesive failure (AF) for not treated surfaces. The maximum theoretical tensile shear strength of the bond was thus above the determined values.
8-| 760- 5-
Plastic - ECTFE Adhesive - Loctite 9466 Treatment - APP
Tensile-Shear-Test acc. DIN 1465
w
x
ra 9 5 2
1 0
2 4 6
APP Pretreatment (s)
Fig. 3 - Investigations on APP pretreatment for ECTFE. Tensile shear strength (acc. to DIN EN 1465) can significantly be increased by APP
For PVDF bonds with 2C-PUR-adhesive Weicon PU-90 showed a bond strength of 7.1 MPa, which was achieved after 10 s surface pretreatment with APP. The special cohesive failure (SCF) in the adhesive layer was determined according to DIN EN ISO 10365. The maximum tensile shear strength for used adhesive in combination with used substrate was reached (compare Fig. 4).
Fig. 4 - Investigations on APP pretreatment for PVDF. Tensile shear strength (acc. to DIN EN 1465) can significantly be increased by APP
Pretreatment and mechanical properties of fully fluorinated polymers (PFA and PTFE)
The common surface preparation methods could not be used for the investigation of fully fluorinated materials (PFA and PTFE) with desired results (Fig. 5). PFA bonds with 2C-PUR-adhesive Weicon PU-90 showed a tensile shear strength of 0.8 MPa, which was achieved after 10 min surface pretreatment with LPP. The adhesive failures (AF) in the interface were determined according to DIN EN ISO 10365.
An explanation of this behavior is very low surface energy of the fully fluorinated plastics with a very low polar fraction of the SFE. This low SFE prevent the surface wetting with the EP or PUR adhesive, which have relatively high surface energies with a high polar fraction. Poor wetting is followed by no interactions between the adhesive and the substrate which is reflected in a low adhesion and following subsequent poor mechanical properties. Since the breaking of C-F bonds requires a
high energy input, a plasma etching can occur during the pretreatment which leads to the formation of hydrophobic surface. Formation of highly hydrophobic surfaces were achieved already after 20 min pretreatment LPP (compare Fig. 6).
8-. 76-
ni
CL 5 ■ S
i 4
С
ш
55 3
X
го 2 •
Plastic - PFA Adhesive - Weicon PU-90 Treatment - LPP
Tensile-Shear-Test acc. DIN 1465
LPP Pretreatment (min.)
Fig. 5 - Investigations on LPP pretreatment for PFA. Tensile shear strength (acc. to DIN EN 1465) cannot be significantly increased by LPP
Fig. 6 - Hydrophobic PFA surface after 20 min. pre-treatment
Resume
These investigations have shown the importance and the necessity of using appropriate pretreatment methods to improve the adhesion and optimization of plastic surfaces as well as choosing the right adhesive to produce stable bonded joints with high mechanical properties. Drop shape analysis (OWRK) revealed that not all treatments lead to an increase in the SFE resulting in increased wettability of used fluorinated polymers.
Literature
1. Ebnesajjad S.: Introduction to Fluoropolymers. Applied Plastics Engineering Handbook. Elsevier Inc. pp. 49-60, 2011.
2. R Market and Markets: Fluoropolymer Market By Type (PTFE, PVDF, FEP, Fluoroelastomers) & Application (Automotive, Electrical & Electronics, Chemical Processing, Industrial) - Global Trends & Forecast to 2018: http://www.marketsandmarkets.com/Market-Reports/fluor-polymer-market-497.html, 2013.
3. Ebnesajjad S.: Fluoroplastics, Volume 1: Non-Melt Processible Fluoropolymers - The Definitive User's Guide and Data Book. 2nd Ed. Elsevier Inc. pp. 24-37, 321-333, 2015.
4. Siperko L. M., Thomas R. R.: Chemical and Physical Modification of Fluoropolymer Surfaces for Adhesion Enhancement: A Review, Journal of Adhesion Science and Technology, V3(1), pp. 157-173, 1989.
5. Xiu Y.: Fabrication of Surface Micro- and Nanostructures for Superhydrophobic Surfaces in Electric and Electronic Applications. Doctoral dissertation. Georgia Institute of Technology, USA, pp. 11-17, 2008.
6. Fraunhofer IAP. Surface Research, Low pressure plasma: http://www.polymer-surface.com/plasma, 2010.
7. Tanaka K., Takahashi K., Kogoma M.: Defluorination of Polytetrafluoroethylene Surface by Combination of Atmospheric Pressure Glow Plasma Treatment and Chemical Transport Method. Journal of Photopolymer Sci. and Techn, V24(4),pp. 441-445, 2011.
8. Friedrich J.: The Plasma Chemistry of Polymer Surfaces: Advanced Techniques for Surface Design. Wiley-VHC Verlag & Co. KGaA, Weinheim, Germany, pp. 202-210, 2012.
© E. Kraus - Master of Science, Ph.D.-Student, Kazan National Research Technological University, Research Assistant and Deputy Business Unit Manager "Joining" in German Plastics Centre (SKZ), Wuerzburg, Germany, S. Horvat - Master of Science, Research Assistant in SKZ - German Plastics Center, Wuerzburg, Germany, B. Baudrit - Doctor of Natural Science, Business Unit Manager "Joining" in German Plastics Centre (SKZ) Wuerzburg, Germany, O. V. Stoyanov - Doctor of Engineering, Full Professor, Dean of Technology, Processing and Certification of Plastics and Composites Faculty, Head of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, ov_stoyanov@mail.ru; I. A. Starostina - Doctor of Chemistry, Full Professor of Physics Department, Kazan National Research Technological University, Kazan, Russia
© Э. Краус - магистр, аспирант Казанского национального исследовательского технологического университета, научный сотрудник и заместитель руководителя Немецкого центра пластмасс, Вюрцбург, Германия, С. Хорват - магистр, научный сотрудник Немецкого центра пластмасс, Вюрцбург, Германия, Б. Баудрит - доктор естествознания, руководитель Немецкого центра пластмасс, Вюрцбург, Германия, О. В. Стоянов - доктор технических наук, профессор, декан факультета Технологии, переработки и сертификации пластмасс и композитов, заведующий кафедрой технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия, ov_stoyanov@mail.ru; И. А. Старостина -доктор химических наук, профессор кафедры физики, Казанский национальный исследовательский технологический университет, Казань, Россия.
