**2.2. Advantages and shortcomings of XPS technique for characterization of elastomer surfaces. Operating conditions, measurements and semi-quantitative analysis**

The standard XPS measurements are carried out under vacuum conditions by retardingfields techniques. The most commonly used X-ray sources are Al K (1486.6 eV) and Mg K (1253.6 eV). The X-ray lines from these sources are narrow (less than 0.9 eV) and provide good energy resolution for many applications. Initially, a survey scan or wide energy range scan, typically from 1000 to 0 eV, should be obtained in order to identify the elements present on the surface. As each element emits electrons at characteristic energies, it is possible to identify all the elements present in the sample surface, except hydrogen and helium which are not detectable by this technique. Elastomers usually contain a small number of elements, of which the most common are C, O, N, F and Cl. Other elements like sulphur and zinc can be detected in small quantities. Sulphur is a typical curing agent, whereas zinc is usually employed as a curing activator [22]. In most of the cases, these elements will not be taken into account as they have no real influence on the surfaces properties. Normally, the elements are uniformly distributed in the bulk; however, under certain circumstances surface segregation may take place.

It should be stressed that XPS is a semi-quantitative technique. In order to quantify the amount of each element the integrated area of a particular peak should be divided by the corresponding relative sensitivity factor. The following is a generalized expression for determination of atom fraction, *Cx*, of a constituent *x* in a sample:

$$\mathbf{C}\_{x} = \left(I\_{x} / \mathbf{S}\_{x}\right) / \left(\sum I\_{i} / \mathbf{S}\_{i}\right),\tag{2}$$

where *Ix* is the peak area and *Sx* is the atomic sensitivity factor of the *x-*th element. The denominator corresponds to the atomic fraction of other elements in the sample. Assuming a homogeneous distribution of elements, a strong line for each element in the spectrum should be analyzed. In case the requirement of homogeneity is not fulfilled, the assumption of homogeneity can be used as a starting point for further calculations. Reference published data on elemental sensitivity factors could be used for determination of *S*, although the type of instrument and analysis conditions should be considered. With this technique it is also possible to identify chemical states of a given element by measuring the high resolution or core level peaks.

Depth distribution of elements can also be obtained using XPS in destructive or nondestructive modes. In the first one, ion sputtering is used to remove surface layers. Sputtering and XPS can be applied consecutively or simultaneously. In the non-destructive mode, depth profiling is obtained by varying the detection angle of the emitted electrons. In this case the probed depth is limited to 3. More detailed information about both methods can be found elsewhere [20].

168 Advanced Aspects of Spectroscopy

specialized literature [20, 21].

**analysis** 

core level peaks.

1400 eV. Since inelastic mean free path of photoelectrons, , in solids is small [19], chemical information is obtained from the surface and few subsurface atomic layers. Quantitative information can be derived from the peaks areas, whereas chemical states can often be identified from the exact positions of the peaks and separations between them. The presence of chemical bonding causes binding energy shifts, which can be used to infer the chemical nature (such as atomic oxidation state) from the sample surface. Here, we limit ourselves to study elastomer samples. A complete description of XPS technique can be found in

**2.2. Advantages and shortcomings of XPS technique for characterization of elastomer surfaces. Operating conditions, measurements and semi-quantitative** 

certain circumstances surface segregation may take place.

determination of atom fraction, *Cx*, of a constituent *x* in a sample:

The standard XPS measurements are carried out under vacuum conditions by retardingfields techniques. The most commonly used X-ray sources are Al K (1486.6 eV) and Mg K (1253.6 eV). The X-ray lines from these sources are narrow (less than 0.9 eV) and provide good energy resolution for many applications. Initially, a survey scan or wide energy range scan, typically from 1000 to 0 eV, should be obtained in order to identify the elements present on the surface. As each element emits electrons at characteristic energies, it is possible to identify all the elements present in the sample surface, except hydrogen and helium which are not detectable by this technique. Elastomers usually contain a small number of elements, of which the most common are C, O, N, F and Cl. Other elements like sulphur and zinc can be detected in small quantities. Sulphur is a typical curing agent, whereas zinc is usually employed as a curing activator [22]. In most of the cases, these elements will not be taken into account as they have no real influence on the surfaces properties. Normally, the elements are uniformly distributed in the bulk; however, under

It should be stressed that XPS is a semi-quantitative technique. In order to quantify the amount of each element the integrated area of a particular peak should be divided by the corresponding relative sensitivity factor. The following is a generalized expression for

where *Ix* is the peak area and *Sx* is the atomic sensitivity factor of the *x-*th element. The denominator corresponds to the atomic fraction of other elements in the sample. Assuming a homogeneous distribution of elements, a strong line for each element in the spectrum should be analyzed. In case the requirement of homogeneity is not fulfilled, the assumption of homogeneity can be used as a starting point for further calculations. Reference published data on elemental sensitivity factors could be used for determination of *S*, although the type of instrument and analysis conditions should be considered. With this technique it is also possible to identify chemical states of a given element by measuring the high resolution or

*C I S IS x xx ii* / / / , (2)

Another important problem in XPS analysis is related with sample degradation due to X-ray radiation. In fact, this degradation comes from the secondary electrons emitted during the X-ray exposure [23, 24]. In most of the cases this degradation is slow enough as compared with time required for XPS analysis, thus the changes in composition due to X-ray can be neglected. Notwithstanding, this problem should be considered when analysing chemically unstable materials. In our work, no sample degradation due to X-ray radiation has been observed for all studied elastomers.

XPS measurements were performed in ultrahigh vacuum with base pressure of 2×10-10 mbar using a Phoibos 100 ESCA/Auger spectrometer with Mg K anode (1253.6 eV). To avoid Xray damage on the samples low X-ray power of 150 W was used. The core level narrow spectra were recorded using pass energy of 15 eV. For the data analysis, the contributions of the Mg K satellite lines were subtracted and the spectra were subjected to a Shirley background subtraction formalism [25]. The binding energy, *EB*, scale was calibrated with respect to the C 1s core level peak at 285 eV. The surface area subject to XPS analysis was around 5.6 mm2 that is large enough to obtain an average surface chemical composition. When modified samples were analyzed, the surface area subject to XPS analysis was smaller than the treated area, thus the contribution from untreated surfaces was negligible. The shape of C 1s core level peak (HR C1s) measured with high energy resolution was analysed using peak fitting in order to identify functional groups. Depending on the chemical environment of the carbon atoms, important chemical shift of C 1s peak can be observed. Decomposition of the experimental peak in components allows identification of the contribution from each component. For the analysis of HR C1s, the spectrum recorded from the untreated sample was used as a reference. The HR C1s was fitted leaving the full width at half maximum (FWHM) of the C–C/C–H component to vary freely while the other components were forced to adopt the same value. The fit of the treated samples was performed using the same values of FWHM and the binding energies (with uncertainty of 0.1 eV) as for untreated elastomer. The only remaining free parameter in the fit procedure was the area of the peaks. By doing so, new carbon species derived from the treatment processes could be identified. An example of the analysis of HR C1s is presented in Figure 2 where some spectra of untreated and modified elastomers are compared. Presence of new carbon species (in this case C-O and C-F bonds) can be identified from the shape of the HR C1s. The contribution from these groups varied depending on the surface treatment. However, identification of chemical groups can be difficult when different species produce similar chemical shifts (see Table 1 for *EB* of main carbon bonds identified in the present study). For example, C=C bond was included into the group of C-C/ C-H components since the shift between these two groups is only 0.3 eV [26] that is below the resolution limit of the experimental system used in this study. Detailed analysis of XPS spectra is presented in section 3.3.

**Figure 2.** High resolution C1s core level spectra of elastomer samples: a) untreated EPDM, b) EPDM after fluorination with CF4, c) EPDM after fluorination with SF6, and d) HNBR after the same fluorination as c)


**Table 1.** Components employed for the analysis of the C 1s core levels

#### **2.3. Complementary techniques for the interpretation of the results of XPS**

As we mentioned in the introduction section, surface and bulk properties of elastomers depend on the way the main constituents (C, H, O, N, etc.) are combined rather than on the presence of other chemical elements. Therefore, in some cases and depending on the light source employed for XPS analysis, it is difficult to distinguish between the presence of different functional groups, as occurs for HR C1s with C-O and C-N groups. Complementary information on surface chemistry of elastomers can be obtained from spectroscopy of inelastic scattering of light, e.g. Fourier Transformed Infra-Red spectroscopy (FTIR), Raman spectroscopy and others. Measurements of SFE of elastomer using sessile drop method is another very simple but powerful method which can provide valuable information on the type of the surface groups. The method is based on measuring the CA between a droplet of a certain liquid and an elastomer surface under well-controlled conditions. The CA is obtained from a balance of interfacial tensions between three phases: solid (S), liquid (L) and vapour (V) (Figure 3) and is defined from Young-Dupré equation:

$$
\gamma\_{SV} - \gamma\_{SL} - \gamma\_{LV}\cos\theta = 0 \,\, . \tag{3}
$$

When a droplet contacts a rough surface, the measured or apparent contact angle, may differ from the intrinsic one, i.e. the CA of the same liquid on an ideally smooth surface of the same material. Wenzel [33] proposed to introduce a roughness factor, *r*, which is the real contact area divided by the geometrical, or projected, area. Homogeneous wetting regime of a liquid on a rough surface is described by:

$$r\cos\theta\_d = \cos\theta \,. \tag{4}$$

The roughness factor can be determined numerically from 3D surface measurements obtained using appropriate technique, e.g. confocal microscopy, laser scanning profilometry, etc. [1]. Recently there were many criticisms on the Wenzel´s approach. In [34] it was demonstrated that CA behaviour is determined by interactions of the liquid and the solid at the three-phase contact line alone and that the interfacial area within the perimeter is irrelevant. They suggested that Wenzel´s equation is valid only to the extent that the structure of the contact area reflects the ground-state energies of contact lines and the transition states between them.

**Figure 3.** Schematic representation of contact angle

170 Advanced Aspects of Spectroscopy

section 3.3.

fluorination as c)

experimental system used in this study. Detailed analysis of XPS spectra is presented in

**Figure 2.** High resolution C1s core level spectra of elastomer samples: a) untreated EPDM, b) EPDM after fluorination with CF4, c) EPDM after fluorination with SF6, and d) HNBR after the same

> Bonds *EB* (±0.1 eV) Ref. C-C, C-H, C=C 285 [26, 27] C-O, -CH-CF2 286.3 [26-30] C=O, -CH-CF 288.1 [26-30] O-C=O, -CFH-CF2 289 [26-30] CF 289.8 [30, 31] CF2 291.8 [30, 31] CF3 293.3 [30-32]

**2.3. Complementary techniques for the interpretation of the results of XPS** 

As we mentioned in the introduction section, surface and bulk properties of elastomers depend on the way the main constituents (C, H, O, N, etc.) are combined rather than on the presence of other chemical elements. Therefore, in some cases and depending on the light source employed for XPS analysis, it is difficult to distinguish between the presence of different functional groups, as occurs for HR C1s with C-O and C-N groups. Complementary information on surface chemistry of elastomers can be obtained from spectroscopy of inelastic scattering of light, e.g. Fourier Transformed Infra-Red spectroscopy (FTIR), Raman spectroscopy and others. Measurements of SFE of elastomer using sessile drop method is another very simple but powerful method which can provide valuable information on the type of the surface groups. The method is based on

**Table 1.** Components employed for the analysis of the C 1s core levels

Depending on the specific method, the CA measurement allows determining total SFE, the polar and dispersive components of SFE (Fowke´s approach), apolar Lifshitz – Van der Waals (LW) and polar acid - base components (van Oss´s approach). According to van Oss´s approach, the surface tension could be resolved into components due to dispersion, induction and dipole-dipole forces, and hydrogen bonding [35]. For non-metallic solid surfaces, in addition to apolar LW interactions, electron acceptor – electron donor interactions, or Lewis acid-base (AB) interactions may often occur. In this case the total surface tension is the sum of two components: *LW* and *AB* [36]. Unlike LW interactions, polar interactions are essentially asymmetrical. The polar component of the free energy of interaction between solid and liquid can be expressed as [35]:

$$
\Delta \mathbf{F}\_{SL}^{AB} = -2 \left( \sqrt{\mathbf{\hat{\boldsymbol{\gamma}}\_S^+ \mathbf{\hat{\boldsymbol{\gamma}}\_L^-}} + \sqrt{\mathbf{\hat{\boldsymbol{\gamma}}\_S^- \mathbf{\hat{\boldsymbol{\gamma}}\_L^+}} } \right) \tag{5}
$$

where is the electron acceptor, or Lewis acid component, and  is the electron donor, or Lewis base component of the surface tension. Then, the expanded form of Young-Dupré equation can be obtained by combining (3) and (5):

$$0.5\gamma\_L^t \left(1+\cos\theta\right) = \sqrt{\gamma\_S^{LW}\gamma\_L^{LW}} + \sqrt{\gamma\_S^-\gamma\_L^+} + \sqrt{\gamma\_S^+\gamma\_L^-} \,. \tag{6}$$

If for a given liquid the components of surface tension *<sup>t</sup> L* , *LW L* , *<sup>L</sup>* , and *<sup>L</sup>* are known, (6) is a linear function of three unknown parameters corresponding to the components of surface tension of the solid surface *LW S* , *<sup>S</sup>* , and *<sup>S</sup>* . As this equation is underdetermined, the components of surface tension for the solid can be found by measuring CAs using at least three different liquids with known and different components of surface tension. If the values of the components of surface tension for the three liquids are close together, the calculated values for three parameters for the solid will be "unduly sensitive" [36] to small errors in the values of the parameters of surface tension of the liquids, and in the measured CAs. To overcome this problem, CA measurements should be performed with more than three liquids. These will constitute an overdetermined system of linear equations which can be solved by least-square method. In order to reduce the measurement error, each measurement of the CA should be repeated several times. Mean value, *<sup>i</sup>* , and standard error of mean, se*θi*, should be determined for each liquid from these measurements.

The resulting set of simultaneous equations is the following:

$$0.5\gamma\_{Li}^t \left(1+\cos\overline{\theta\_i}\right) = \sqrt{\gamma\_S^{LW}\gamma\_{Li}^{LW}} + \sqrt{\gamma\_S^-\gamma\_{Li}^+} + \sqrt{\gamma\_S^+\gamma\_{Li}^-} \tag{7}$$

where subscript *i* indicates the liquid. It can be written in the matrix form:

$$\mathbf{Y} = \mathbf{A}\mathbf{b}\_{\prime} \tag{8}$$

where **Y** is the matrix of independent variable (left side of eq. (7)), **A** is the (*n*3) matrix of known coefficients, *n* is the number of liquids used for CA measurements, and **b** is the vector of unknown parameters:

$$\mathbf{Y} = 0.5 \left[ \left( 1 + \cos \overline{\theta\_1} \right) \boldsymbol{\gamma}\_{L1}^{\boldsymbol{t}} \cdots \left( 1 + \cos \overline{\theta\_n} \right) \boldsymbol{\gamma}\_{Ln}^{\boldsymbol{t}} \right]^T \tag{9}$$

$$\mathbf{A} = \begin{pmatrix} \sqrt{\mathcal{V}\_{L1}^{LW}} & \sqrt{\mathcal{V}\_{L1}^{+}} & \sqrt{\mathcal{V}\_{L1}^{-}} \\ \cdot & \cdot & \cdot \\ \sqrt{\mathcal{V}\_{Ln}^{LW}} & \sqrt{\mathcal{V}\_{Ln}^{+}} & \sqrt{\mathcal{V}\_{Ln}^{-}} \end{pmatrix}' \tag{10}$$

X-Ray Photoelectron Spectroscopy for Characterization of Engineered Elastomer Surfaces 173

$$\mathbf{b} = \left[ \sqrt{\mathcal{V}\_S^{LW}} \sqrt{\mathcal{V}\_S^{-}} \sqrt{\mathcal{V}\_S^{+}} \right]^T. \tag{11}$$

Then, mean values of the surface tension components can be determined from the matrix equation:

$$\mathbf{b} = \left(\mathbf{A}^{\mathrm{T}} \mathbf{D}^{\mathrm{-1}} \mathbf{A}\right)^{-1} \mathbf{A}^{\mathrm{T}} \mathbf{D}^{\mathrm{-1}} \mathbf{Y} \;/\tag{12}$$

where 2 1 2 0 0 0 0 0 0 *<sup>n</sup> se se* **D**

172 Advanced Aspects of Spectroscopy

where 

value, *<sup>i</sup>* 

measurements.

 <sup>2</sup> *AB SL SL SL F* 

Lewis base component of the surface tension. Then, the expanded form of Young-Dupré

*L S L SL SL*

is a linear function of three unknown parameters corresponding to the components of

underdetermined, the components of surface tension for the solid can be found by measuring CAs using at least three different liquids with known and different components of surface tension. If the values of the components of surface tension for the three liquids are close together, the calculated values for three parameters for the solid will be "unduly sensitive" [36] to small errors in the values of the parameters of surface tension of the liquids, and in the measured CAs. To overcome this problem, CA measurements should be performed with more than three liquids. These will constitute an overdetermined system of linear equations which can be solved by least-square method. In order to reduce the measurement error, each measurement of the CA should be repeated several times. Mean

 , *<sup>S</sup>* 

, and standard error of mean, se*θi*, should be determined for each liquid from these

*Li i S Li S Li S Li*

where **Y** is the matrix of independent variable (left side of eq. (7)), **A** is the (*n*3) matrix of known coefficients, *n* is the number of liquids used for CA measurements, and **b** is the

0.5 1 cos 1 cos 1 1 *<sup>T</sup> t t*

1 11

*L LL*

 

 

*Ln Ln Ln*

 

*LW*

*LW*

 

*L n Ln*

 **<sup>Y</sup>** , (9)

**A** , (10)

 , (7)

**Y Ab** , (8)

 

*S* 

 

is the electron acceptor, or Lewis acid component, and

0.5 1 cos *<sup>t</sup> LW LW*

equation can be obtained by combining (3) and (5):

surface tension of the solid surface *LW*

If for a given liquid the components of surface tension *<sup>t</sup>*

The resulting set of simultaneous equations is the following:

vector of unknown parameters:

0.5 1 cos *<sup>t</sup> LW LW*

where subscript *i* indicates the liquid. It can be written in the matrix form:

 

> 

> > *L* , *LW L* , *<sup>L</sup>*

, and *<sup>S</sup>*

, (5)

 . (6)

*-*

> , and *<sup>L</sup>*

is the electron donor, or

. As this equation is

are known, (6)

is the covariance matrix of errors of CA measurements.

The standard error of mean of the unknown parameters can be found from the main diagonal of the covariance matrix:

$$\mathbf{K} = \left(\mathbf{A}^{\mathrm{T}} \mathbf{D}^{\mathrm{T}} \mathbf{A}\right)^{-1}.\tag{13}$$

The calculated values of the parameters of the surface tension should be tested for statistical significance using *t*-test. In case some of the parameters are not statistically significant, it can be zero set and removed from **b**. Then, the calculation should be repeated using modified matrix **A**. By doing so, the standard error of the parameters of solid can be reduced.

Matrix method is also very useful for the analysis of surface tension variation in time, e.g. due to ageing. In this case, CA measurements are performed at different periods of time using a set of several liquids as described above. This constitutes a set of simultaneous equations at the selected points of time.

$$0.5\gamma\_{Li}^t \left(1 + \cos\overline{\theta\_i}(t\_j)\right) = \sqrt{\gamma\_S^{LW}(t\_j)\gamma\_{Li}^{LW}} + \sqrt{\gamma\_S^-(t\_j)\gamma\_{Li}^+} + \sqrt{\gamma\_S^+(t\_j)\gamma\_{Li}^-} \,. \tag{14}$$

Therefore, **Y** and **b** change to (*np*) matrixes, where *p* is the number of time points:

$$\mathbf{Y} = 0.5 \begin{pmatrix} (1 + \cos \theta\_{11}) \boldsymbol{\gamma}\_{L1}^{t} & \dots & (1 + \cos \theta\_{1j}) \boldsymbol{\gamma}\_{L1}^{t} & \dots & (1 + \cos \theta\_{1p}) \boldsymbol{\gamma}\_{L1}^{t} \\ \vdots & \vdots & \vdots & \vdots & \vdots \\ (1 + \cos \theta\_{n1}) \boldsymbol{\gamma}\_{Ln}^{t} & \dots & (1 + \cos \theta\_{nj}) \boldsymbol{\gamma}\_{Ln}^{t} & \dots & (1 + \cos \theta\_{np}) \boldsymbol{\gamma}\_{Ln}^{t} \end{pmatrix} \tag{15}$$

$$\mathbf{b} = \begin{pmatrix} \sqrt{\mathcal{I}\_{S1}^{LW}} & \cdots & \sqrt{\mathcal{I}\_{Sj}^{LW}} & \cdots & \sqrt{\mathcal{I}\_{Sp}^{LW}} \\ \sqrt{\mathcal{I}\_{S1}^{-}} & \cdots & \sqrt{\mathcal{I}\_{Sj}^{-}} & \cdots & \sqrt{\mathcal{I}\_{Sp}^{-}} \\ \sqrt{\mathcal{I}\_{S1}^{+}} & \cdots & \sqrt{\mathcal{I}\_{Sj}^{+}} & \cdots & \sqrt{\mathcal{I}\_{Sp}^{+}} \end{pmatrix} . \tag{16}$$

Assuming that all measurements have the same error, the matrix of parameters of solid surface can be found from the following equation:

$$\mathbf{b} = \left(\mathbf{A}^{\mathsf{T}}\mathbf{A}\right)^{-1}\mathbf{A}^{\mathsf{T}}\mathbf{Y}\_{\mathsf{A}} \tag{17}$$

and standard errors of the unknown parameters can be found from the main diagonal of the covariance matrix:

$$\mathbf{K} = \mathbf{s}^2 \left(\mathbf{A}^T \mathbf{A}\right)^{-1},\tag{18}$$

where *s*2 is the sample variance determined as:

$$s^2 = \frac{1}{n-p-1}(\mathbf{Y} - \mathbf{A}\mathbf{b})^T(\mathbf{Y} - \mathbf{A}\mathbf{b})\,. \tag{19}$$

Although different substances can be used as probe liquids, the following five liquids are the most widely used: water, glycerol, diiodomethane, formamide, and ethylene glycol [1, 29, 37, 38]. The values of the components of surface tension for these liquids are listed in Table 2.


**Table 2.** The components of surface tension for different probe liquids (from [29, 37])

Additionally, the Fowke's model can be used to determine the polar and dispersive components of surface energy. The following is the set of simultaneous Young-Dupré equations corresponding to the measurements of the CA for *p* liquids at time *tj*:

$$0.05\gamma\_{Li}^t \left(1 + \cos\overline{\theta\_i}(t\_j)\right) = \sqrt{\gamma\_S^d(t\_j)\gamma\_{Li}^d} + \sqrt{\gamma\_S^p(t\_j)\gamma\_{Li}^p}, i = 1...p. \tag{20}$$

Since there are two unknown parameters in this model, the number of liquids used for the CA measurements can be smaller than for van Oss´s model. After corresponding modification of **A** and **b**, solution of (20) can be found by the matrix method described above.

#### **3. Case studies**

#### **3.1. Characterization of elastomer surface subject to ageing**

Though synthetic elastomers like EPDM are very attractive to industry due to their high chemical stability and low permeability for water, they are sensitive to oxidation at elevated temperatures. Understanding of the chemical mechanisms of elastomer degradation is a key for designing advanced elastomers with higher resistance to oxidative degradation. Therefore, XPS and SFE analysis were employed to elucidate chemical changes produced on EPDM elastomer surfaces due to ageing.

174 Advanced Aspects of Spectroscopy

covariance matrix:

Table 2.

surface can be found from the following equation:

where *s*2 is the sample variance determined as:

**3. Case studies** 

*s*

Assuming that all measurements have the same error, the matrix of parameters of solid

and standard errors of the unknown parameters can be found from the main diagonal of the

1

 <sup>1</sup> <sup>2</sup> *<sup>s</sup>*

Although different substances can be used as probe liquids, the following five liquids are the most widely used: water, glycerol, diiodomethane, formamide, and ethylene glycol [1, 29, 37, 38]. The values of the components of surface tension for these liquids are listed in

> Liquid *γ<sup>t</sup> γLW γAB γ- γ<sup>+</sup>* Water 72.80 21.80 51.00 25.50 25.50 Glycerol 64.00 34.00 30.00 57.40 3.92 Formamide 58.00 39.00 19.00 39.60 2.28 Ethylene glycol 48.00 29.00 19.00 30.10 3.00 Diiodomethane 50.80 50.80 0.00 0.00 0.00

Additionally, the Fowke's model can be used to determine the polar and dispersive components of surface energy. The following is the set of simultaneous Young-Dupré

> 0.5 1 cos ( ) ( ) ( ) , 1 . *<sup>t</sup> d d p p Li i j S j Li S j Li*

Since there are two unknown parameters in this model, the number of liquids used for the CA measurements can be smaller than for van Oss´s model. After corresponding modification of **A** and **b**, solution of (20) can be found by the matrix method described above.

Though synthetic elastomers like EPDM are very attractive to industry due to their high chemical stability and low permeability for water, they are sensitive to oxidation at elevated

 *t t ti p* (20)

 

( ) <sup>1</sup>

**T T b AA AY** , (17)

**<sup>T</sup> K AA** , (18)

*n p* **Y Ab Y Ab** . (19)

<sup>2</sup> <sup>1</sup> <sup>T</sup>

**Table 2.** The components of surface tension for different probe liquids (from [29, 37])

equations corresponding to the measurements of the CA for *p* liquids at time *tj*:

 

**3.1. Characterization of elastomer surface subject to ageing** 

XPS wide energy range scans were obtained for EPDM samples aged at 80 ºC and 120 ºC during up to 100 days. Surface chemical composition of the samples determined from these spectra as a function of ageing duration is shown in Table 3. The high carbon content in all samples arises from the contribution from the backbone structure of the elastomer. Oxygen, nitrogen, silicon and zinc are generally attributed to curing agents, amine-based accelerators and additives [1, 14].

With the increasing ageing duration, the O/C ratio also increases (last column of Table 3). In addition, for both temperatures there was certain increase in nitrogen and silicon concentrations for the 100 days ageing. As XPS is a superficial analysis technique, the variation of these elements present in small amounts on the surface could be related to diffusion processes during ageing and segregation of impurities on the surface. Being a thermally activated process, migration of additives is faster at higher temperatures, thus surface concentration of silicon after ageing at 120 ºC is higher than at 80 ºC.


**Table 3.** Chemical composition of some EPDM samples obtained from XPS wide energy range scan (with permission from [1]) \*

traces The results of curve fitting procedure of the HR C1s are shown in Figure 4. The broad carbon peak in the range of *EB* from 283 eV to 289 eV can be attributed to different carbonbased surface functional groups. C 1s peak was fitted with four Gaussian/ Lorenzian components with the maximum intensity at *EB* of 285 eV, 286.3 eV, 288.1 eV and 289 eV. According to the literature, these energies can be assigned to C-C or C-H, hydroxyl (C-O/ C-OH), carbonyl (C=O) and carboxyl (O-C=O), respectively (see Table 1). Most part of carbon was in form of C-C / C-H. For the samples aged at 80 ºC the amount of carbon bonded to oxygen, especially in form of hydroxyl, increased with the increase of ageing duration. After 100 days at 80 ºC, carbon-oxygen bonds were composed of hydroxyl (20% with respect to carbon), small portion of carbonyl (4%) and traces of carboxyl (1%). This effect was similar to the evolution of the oxygen content registered in the wide energy range scan (Table 3). Similar behaviour was also observed by [39] and [7]. When comparing the samples aged during 100 days at 80 ºC and 120 ºC the portion of carbon-oxygen functional groups was lower at higher temperature, though both samples had similar surface contents of oxygen. According to [39] C-OH bonds are the main product of EPDM ageing as inferred from the C 1s core level peak. However, the results obtained in our work suggested that ageing above 100 ºC could cause hydroxyl desorption. This could explain the lower C-O content registered after the treatment at higher temperature.

Variations in the components of the SFE for the elastomer as a function of the ageing parameters were determined using acid-base regression method with the five liquids listed in Table 2. The results are shown in Figure 5. For both ageing temperatures was statistically insignificant, so this term was omitted from the model. Since the component is null, EPDM surface is mainly *<sup>S</sup>* monopolar. In the absence of a parameter of the opposite sign, energy parameters of a monopolar surface do not contribute to the total surface energy (energy of cohesion) since the polar component 2 0 *AB* [35]. Therefore, the total SFE is controlled solely by LW interaction. However, monopolar surfaces can strongly interact with bipolar liquids.

**Figure 4.** C 1s spectra of EPDM: a) as received, b) aged 5 days at 80 ºC, c) aged 50 days at 80 ºC, d) aged 100 days at 80 ºC, e) aged 100 days at 120 ºC. Dots – experimental data, solid lines – fitting (with permission from [1])

At 80 ºC, *LW S* increased exponentially with ageing duration reaching almost stable values after 60 days. Parameter *<sup>S</sup>* had an induction period of approximately 5 days. These results agree with previous works in which the induction period during thermal oxidation of EPDM was determined as 130 h at 80 ºC [40] and 150 h at 150 ºC [7]. Variations of the induction period in different works can be due to differences in the EPDM composition, more specifically, in the carbon black and antioxidants content. After induction period, *<sup>S</sup>* increased rapidly and reached the maximum in 30 days. Then, it remained almost constant with a slightly decreasing tendency, which, however, was within a standard error. The solid line connecting the filled circles in Figure 5a was obtained by fitting the experimental data with an exponential function having a time constant of 17.70.4 days.

176 Advanced Aspects of Spectroscopy

registered after the treatment at higher temperature.

surfaces can strongly interact with bipolar liquids.

is null, EPDM surface is mainly *<sup>S</sup>*

permission from [1])

*S* 

after 60 days. Parameter *<sup>S</sup>*

At 80 ºC, *LW*

during 100 days at 80 ºC and 120 ºC the portion of carbon-oxygen functional groups was lower at higher temperature, though both samples had similar surface contents of oxygen. According to [39] C-OH bonds are the main product of EPDM ageing as inferred from the C 1s core level peak. However, the results obtained in our work suggested that ageing above 100 ºC could cause hydroxyl desorption. This could explain the lower C-O content

Variations in the components of the SFE for the elastomer as a function of the ageing parameters were determined using acid-base regression method with the five liquids listed

> was

monopolar. In the absence of a parameter of the

 [35].

in Table 2. The results are shown in Figure 5. For both ageing temperatures

statistically insignificant, so this term was omitted from the model. Since the component

opposite sign, energy parameters of a monopolar surface do not contribute to the total surface energy (energy of cohesion) since the polar component 2 0 *AB*

Therefore, the total SFE is controlled solely by LW interaction. However, monopolar

**Figure 4.** C 1s spectra of EPDM: a) as received, b) aged 5 days at 80 ºC, c) aged 50 days at 80 ºC, d) aged 100 days at 80 ºC, e) aged 100 days at 120 ºC. Dots – experimental data, solid lines – fitting (with

agree with previous works in which the induction period during thermal oxidation of EPDM was determined as 130 h at 80 ºC [40] and 150 h at 150 ºC [7]. Variations of the induction period in different works can be due to differences in the EPDM composition,

increased exponentially with ageing duration reaching almost stable values

had an induction period of approximately 5 days. These results

At 120 oC (Figure 5b), *LW S* raised up at the beginning of ageing and then followed almost linear increasing behaviour with low rate. Surprisingly, after 100 days ageing at 120 ºC *LW S* was approximately 5% smaller than for ageing at 80 ºC. However, after 100 days *LW S* still maintained the linear growth, while at 80 ºC it stabilized.

The evolution of *<sup>S</sup>* was similar to that of *LW S* , although the initial increase was not as steep as for *LW S* component. There is a large difference in the behaviour of *<sup>S</sup>* for both ageing temperatures. On short ageing periods *<sup>S</sup>* was notably smaller at the higher temperature, but this difference vanished on large ageing periods. In addition, at 120 oC the induction period was not observed. Probably, the induction period at higher temperature was less than one day, so it could not be measured in these tests. This finding is consistent with [40] who reported shortening of the induction period to 10 h at 120 ºC.

**Figure 5.** Components of SFE as function of ageing duration: a) ageing temperature 80 ºC, b) ageing temperature 120 ºC (with permission from [1])

The initial value of *<sup>t</sup> S* , which is equal to *LW S* in our case, is consistent with the findings of [39] for EPDM before weathering test at ambient temperature. They observed that *<sup>t</sup> S* first increased and then stabilized at 23.8 - 25.4 mJ m-2. These values are almost two-fold smaller than in our thermal ageing experiments. This fact supports the hypothesis of a thermally activated nature of the processes responsible for the increase in the surface energy [1].

During ageing of EPDM, two competitive processes typically occur: (i) oxidation of the elastomer chains and (ii) crosslinking between the chains. The oxidation process resides in chain scission and recombination accompanied by formation of oxygen functional groups and radicals. Since double bonds are more chemically active due to the presence of a πbond, cross-linking and oxidation at the initial stage of ageing mainly involves rupture of double bonds. Characteristic times for cross-linking of EPDM at 80 ºC and 120 ºC are 100 h and 12.5 h, respectively [40]. After these periods, the material is considered fully crosslinked (at given temperature) that implies significant reduction of the concentration of double bonds. Also, it is reasonable to expect that with the increasing temperature the degree of cross-linking increases and the residual concentration of double bonds decreases. During the induction period, cross-linking is the dominating process as can be inferred from the behaviour of *<sup>S</sup>* , O/C ratio and very high activation energy for oxidation of EPDM, which ranges between 143.4 and 171.4 kJ mol-1 [41]. Further ageing of cross-linked elastomer is accompanied with slower oxidation of carbon chains. The higher reactivity of residual double bonds for EPDM aged at 80 oC can explain the steeper increase in SFE and higher concentration of oxygen after induction period. The evolution of SFE for ageing at 80o C is described by a first-order reaction with the activation energy between 63.5 and 83.7 kJ mol-1[1]. These values are higher than those reported in [42], but similar to the activation energy for oxidation of long hydrocarbon chain alkanes and aromatics such as in heavy fuel oil [43]. For ageing at 120 ºC the linear increase in SFE is described by a zero-order reaction. Zeroorder reaction was reported also for surface degradation of fully cross-linked EPDM under artificial weathering conditions [39].

In conclusion, oxygen functional groups, mainly hydroxyl, were identified on EPDM surface after ageing. The presence of these groups was more pronounced after the treatment at 80 ºC than at 120 ºC. Higher ageing temperatures lead to faster cross-linking processes. At lower temperature C=C bonds are not fully consumed due to cross-linking [3], hence the oxidation processes at lower temperature is more intensive than at higher temperature. In addition, ageing at long durations promotes changes in the surface chemical composition of EPDM. These changes can be attributed to migration of additives towards the surface as reflected by the increase in Si and N concentrations after 100 days ageing at both temperatures.
