Experimental techniques for surface characteristics analysis of polymers after plasma treatment. Comparative analysis
To control surface treatment results standards were developed based on two different methods. Optical method based on measurement of contact angle of test liquids on the solid surface and further calculation of free surface energy with its polar and disperse contributions is described in standard measurement procedures DIN 55660-2:2011-12 for inks and varnishes and DIN EN 828:2013-04 for glues. Standard procedure ISO 8296 describes test ink method that is widely used and consists in selection of an ink with known surface tension that will wet the solid surface completely. The value of surface tension of this ink is treated as value of surface free energy of solid sample.
The following polymers were used for the study: polymethyl methacrylate, acrylic or acrylic glass (PMMA), polyamide 6 (PA-6), polysiloxane (silicon), low density polyethylene (LD-PE). Polymers were divided in four groups according to the treatment type: control group, fluorination, treatment with propane - butane flame, Pyrosil treatment.
The control group samples were cleaned with isopropanol and were dried at the air. The fluorinated samples were contributed by the company INNOVENT Jena immediately after fluorination and were measured without any previous cleaning. During the fluorination process polymer samples are incubated in fluor saturated atmosphere. Thanks to its high reactivity fluor atoms partially substitute hydrogen atoms on the material surface and evoke increase of the value of surface energy and its polar part. Fluorination of polymer is irreversible and treated surface keeps its new characteristics during a long period of time. Flame treatment (propane – butane and Pyrosil) is especially efficient for the industry application due to its low manpower input. During the treatment process the burning head is passing over the treated surface during certain period of time and with a defined distance to the surface. The surface gets oxidized and becomes more polar characteristics .
Pyrosil method is based on the deposition of amorphous silicon oxide on the polymer surface. Precursor Pyrosil is dosed into the flame and while burning creates a thin (5–100 nm) and durable layer on the polymer surface. As the flame impact is quite short the sample surface does not heat up much. Before flame treatment the samples were cleaned with isopropanol and dried in the air.
Flame treatment was realized with a portable device GVE 2 HB from Sura Instruments. For pyrolyse one used the cartridges with a gas mixture propane-butane and with precursor Pyrosil. The burning head was conducted in a distance of 15–20 mm from the sample surface; every surface was treated only once. Further measurements were realizes after a complete cooling down of a sample.
For the investigations we used the product of Arcotest GmbH company. The preference was given to the non-toxic pink series of inks with the surface tension range of 28 to 60 mN/m. Inks are applied with a brush over the sample surface immediately after treatment. One begins with the ink of higher surface tension. If the edges of the stroke stay stable during at least 2 seconds than the value of ink surface tension is considered as equal to the surface free energy of a solid. If the edges of the stroke contract than this ink does not wet the surface and one should pass to the next ink with lower surface tension.
Optic contact angle measurement method
Optic method for contact angle measurement is based on the contour analysis of the drop places on the solid surface. The experiments were realized on contact angle measurement device ОСА 200 (Fig.1)
Program sets the tangent line in the point where the drop touches the solid surface (three-phase-point). The angle between sample surface and the tangent line to the drop surface is called contact angle (CA). The smaller CA is the better is the surface wettability.
As the force equilibration is achieved in the three-phase-point (Fig.2) surface free energy (SFE) can be calculated according to the Young-Laplace equilibration
σL – surface tension of the liquid, mN/m,
σS – surface free energy of the solid, mN/m,
σSL – interfacial tension between the liquid and the solid, mN/m,
ѲC – contact angle.
Knowing the values of polar and disperse parts of SFE gives more information about the wetting phenomena. The polar part is responsible for the surface activity in the processes of gluing or coating thatґs why pre-treatment methods are focused on the increment of precisely this parameter. Calculation of SFE and its contributions is realised in most cases according to the Owens-Wendt (OWRK) theory  that requires to know the contact angle values of liquids with known surface tension and with known polarity on the solid to measure. OWRK theory assumes that the solid surface to investigate is smooth without roughness, homogenous from the chemical and physical point of view and it does not react with the test liquids. Presenting the OWRK equation as a linear regression one can calculate regression coefficient RQ. RQ value less than 0.9 indicates that the surface does not match the assumptions of the OWRK theory or the test liquids do not suit the solid surface.
σL – liquid surface tension, mN/m,
σLd – disperse part of liquid surface tension, mN/m,
σLp – polar part of liquid surface tension, mN/m,
σSd – disperse part of SFE of a solid, mN/m,
σSp – polar part of SFE of a solid, mN/m,
ѲC – contact angle.
The test liquids were diiodomethane, ethylene glycol and thiodiglycol. If the RQ value was too low water was used as a n additional test liquid. Test liquids have different polarity, they are not volatile, non-toxic and they have SFT high enough to form a contact angle comfortable to measure.
The SFE values obtained with ink method and with optical method are presented in the chart below Table 1.
Result comparison of SFE values obtained with two different measurement methods and after different surface treatments are presented in the charts below (Fig.3–6). There are no SFE values for untreated surfaces of silicone and LD-PE obtained with ink test method because there are no inks with SFT lower that 30 mN/m in a standard set.
Diagram analysis shows that the best results convergence between two methods is to see in case of the untreated surfaces measurements. Untreated polymer surfaces are non-polar and have low SFE due to its material characteristics. In case of treated surfaces test ink method shows lower SFE values than the values got with the optical method.
The test ink method is based on the statement that SFE of the solid surface is equal to the SFE value of the liquid that spreads completely on this solid surface. In this case the contact angle of this liquid will be equal zero and its cosinus is one. Analysing the Young equation from the point of view of the test ink method one can see that this equation stays valid only if σSL vector of intermolecular forces is equal zero and thus the second summand is zero as well. But the shematics (Fig.2) shows that even if the vectors of SFE and SFT are equal the contact angle value between them can be from 0˚ to almost 180˚. Whereby the σSL vector of intermolecular forces between solid and liquid can be zero in case of a complete wetting or it can have a value different from zero if the wetting is partial. It demonstrates that the test ink method does not take into consideration the vector of intermolecular forces that act between a liquid and a solid.
Connections between atoms and molecules that define the SFE depend on the interactions of different nature (polar and non-polar). The interactions induced by a temporary load fluctuation of an atom/molecule are defined as non-polar or van-der-waals force. Polar interactions are Coulombґs forces that act between constant and induced dipoles. The value of SFE and of SFT is the sum of these two contributes.
Intermolecular force between a liquid drop and a solid surface depends on the correlation between polar and disperse parts of SFE and SFT: the better the match is the higher is the possibility of molecular interactions between two phases that results in better adhesion and wettability (Fig.7).
In both cases the values of SFE and SFT are equal. In the first schematics (Fig.7a) there is a complete match between polar and disperse parts that determine the maximum force of molecular interaction on the phase boundary. Interfacial tension tends to vanish that results in CA equal zero (complete wetting). In the second schematics (Fig.7b)the polar and the disperse parts of SFE and SFT are different. The intermolecular force is weaker and the interfacial tension is higher. It results in CA value more that zero and in worse wettability.
Test inks measurements give correct results only if the correlation between polar and disperse parts of SFE of a solid and SFT of the ink are congruent. The information about ink polarity and the exact composition of inks is not indicated by manufacturer so this correlation not always takes place. From the physical pint of view the lower the ink SFT is the lower its polar part is. It means that the mostly true results of SFE measurements we can expect working with disperse surfaces. This fact explains the best SFE results correlation between the ink method and the optical method while working with untreated polymers which are almost completely disperse due to its chemical composition. Flame activation of a polymer surface enlarges its SFE and its polar part and precisely this polar part is impossible to determine with the test ink method. The difference between treatment methods and the resulting effects one can define only using the optical method because the test ink method shows the same values.
Comparative analysis shows that the SFE measurement results obtained with test inks are not always congruent with real value of SFE of a solid surface. Optical method based on contact angle measurement allows defining the value of SFE with more exactitude and repeatability and it makes possible the calculation of polar and disperse parts of SFE that is crucial to know before panting or coating the solid surface.
Contact angle measurement using the optical method and the further calculation of SFE with its polar and disperse parts makes possible to calculate the work of adhesion of a liquid to the solid surface.
Test inks are mostly toxic and volatile, there is a high risk of contamination while using the repetitively the same brush for different surfaces. It changes the ink composition and ink SFE that influence the measurement results. The result depends on the operatorґs interpretation of the brush stroke as well. Optical contact angle method allows obtaining high repetitive results independently from a human factor and it requires non-toxic test liquids.
Optical contact angle method has its advantage while working with the samples of small size: if the ink method requires enough space for several strokes wide as the brush optical method needs some square millimetres for several small drops. It is possible to generate drops of several picoliter volume and thus it is enough to have the sample area of about 0.01 mm2. The further advantage of the optical method is the possibility to work under higher or lower temperatures thank various climatic chambers.
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