Effect of surface free energy and wettability on the adhesion property of waterborne polyurethane adhesive

Abstract

According to van Oss–Chaudhury–Good's theory, some key adhesion parameters (involving surface free energy, interfacial free energy, adhesion energy, and the spreading coefficient, etc.) of waterborne polyurethane (WPU) adhesives on shoe substrates, styrene–butadiene–styrene rubber (SBS), ethylene vinyl acetate (EVA), and polyvinyl chloride (PVC) were calculated and the relationships among the surface free energy, interfacial free energy, wettability, and adhesion property of WPU adhesives to these substrates were investigated in detail. The results indicate that the surface free energy of the substrates is the dominant parameter to influence the wettability and adhesion strength, while wetting agents can efficiently improve the wettability via decreasing the surface free energy (γ_L) of the adhesive, although it cannot noticeably alter the adhesion strength. Further analysis found that surface modifications, such as plasma and halogenation treatments, can introduce some active groups (–OH, –NH, or –Cl) onto the surface of SBS, which increases the surface polarity of SBS and enhances the interfacial interaction and adhesion strength of the WPU adhesives, especially in the presence of an isocyanate-terminated hardener.

---

1. Introduction

The classical theories of adhesion, such as the adsorption theory, mechanical theory, electrostatic theory, diffusion theory, and the chemical bonding theory, have involved almost all the interactions between substrates and adhesives.^1,2 In practice, several types of interactions jointly contribute in a bonding system, which relies on different kinds of methods to enhance the adhesion, including roughening the surface of a substrate to increase mechanical interlocking, using a wetting agent to improve wettability, and introducing chemical groups to react with a cross-linking hardener.^3,4 Due to their low surface free energy, non-polar substrates, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and styrene–butadiene–styrene rubber (SBS), are difficult to be wetted and bound with adhesives. To remedy this problem, one efficient technique applied for industrial applications is surface modification, such as via wet chemical, flame, corona, or plasma treatment.^5,6 Among these, the wet chemical treatment is a conventional efficient method but it suffers the drawback that it is harmful to the environment because of the emissions of volatile organic compounds, whereas plasma treatment is considered as a new environmentally friendly method that involves no hazardous chemicals and has the additional benefit of only modifying the outermost surface layer without destroying the bulk properties.^7–10 Up to now, most work in the literature has been focused on solvent-based adhesion systems and the relationship among the key adhesion parameters, such as the surface free energy (γ_L), interfacial free energy (γ_SL), adhesion energy (W_A), spreading coefficient (S_C), and adhesion strength. In Krüss Company's report,¹¹ several basic parameters, including γ_L > 26 mJ m⁻², W_A > 65 mJ m⁻², S_C > 8 mJ m⁻², and γ_SL < 2 mJ m⁻², are often needed to be met for the solvent-based adhesion system to meet the required wettability and adhesion strength. However, this is not the case for waterborne adhesive systems, where these adhesion parameters (γ_L, W_A, S_C, and γ_SL) are quite different from those in organic solvent systems.

As the most important environmentally friendly adhesive, waterborne polyurethane (WPU) has been wildly applied to adhere various substrates, such as covering shoes, leather, glass, wood, plastics, rubber, metals, concrete, and ceramic, because of its unique performances and low environmental pollution.^12,13 However, WPU adhesives are usually difficult to spread and adhere on non-polar substrates. One reason is related to the low surface free energy (γ_S < 35 mJ m⁻²) of the substrate, which can be calculated from the wetting angles (contact angles) obtained from some model liquids according to van Oss–Chaudhury–Good's theory,^14–16 while another reason is related to the higher surface free energy of polyurethane and water (γ_L = 72.8 mJ m⁻²), which can be measured using the Du Nouy ring or Wilhelmy plate methods.^17,18 To improve the wettability and adhesion properties, some additives, such as wetting agents and cross-linking hardeners, are frequently employed with WPU adhesives, just the same way as with solvent-based adhesives.¹⁹ Unfortunately, to date, the correlations between the surface parameters of the substrates and WPU adhesive and their adhesion property are still ambiguous. Therefore, it is extremely necessary to summarize a relationship suitable for the WPU adhesion system from the point of view of the surface free energy, wettability, and adhesion strength.

In the present study, non-polar SBS, semi-polar ethylene vinyl acetate (EVA), and polar polyvinyl chloride (PVC) were selected as shoe substrates. The adhesion characteristics, including surface free energy (γ_S, γ_L), interfacial free energy (γ_SL), adhesion energy (W_A), spreading coefficient (S_C), and T-peeling strength, were investigated with and without wetting agents, based on which a relationship among the surface free energy, interfacial free energy, wettability, and adhesion property of the WPU adhesives system was established. Furthermore, two typical modification methods, namely plasma and wet chemical (halogenation) treatments, were employed to modify the surface of a non-polar substrate SBS. The changes of the adhesion parameters and enhancement of the adhesion property further confirmed this relationship. This work will help to understand the surface chemistry of these waterborne adhesive systems and offer guidance to assess their potential applications with various substrates.

2. Experimental

2.1 Materials

SBS rubber G1475 was supplied by Fuju Chemicals Co., China. The copolymer SBS used here was unvulcanized and the butadiene-to-styrene ratio was 60 : 40 wt%. EVA OR04 with 22% vinyl acetate was obtained from Changzhou Plastics Researching & Manufacturing Co., China. PVC P70A (=83 000) with a shore hardness of 70–75 A was purchased from Shenzhen Dersing Chemicals Co., China. All of the three polymers (SBS, EVA, and PVC) were used as shoe substrates. N-(2-Aminoethyl)-2-aminoethanesulphonic sodium salt (AAS-Na, 50% aqueous solution; Evonik Industries, Germany), wetting agent W77 (anionic/nonionic hydrocarbon surfactant without silicon or fluorine; Elementis Specialties Inc., UK), wetting agent KL245 (nonionic polyether siloxane copolymer; Evonik Industries, Germany), wetting agent FC4430 (20% aqueous solution of nonionic fluorocarbon surfactant; 3M Company, USA), and trichloroisocyanuric acid (TCI, Changzhou Junming Chemicals Co., China) were used as purchased. Poly(1,4-butanediol adipate)diol (PBA, =3000; Xuchuan Chemical Co., China) was dried at 110 °C under vacuum at a pressure of −0.098 MPa for 1 h before use. Isocyanate-terminated hardener (DN), isophorone diisocyanate (IPDI), and 1,6-hexamethylene diisocyanate (HDI) were supplied by Bayer Ltd., Germany. 2,2-Bis(hydroxymethyl) propionic acid (DMPA), 1,4-butanediol (BDO), triethylamine (TEA), acetone, ethylene glycol, and diiodomethane are all analytical reagents and were acquired commercially from Kelong Reagent Co. (Chengdu, China). Water was used after first being deionized.

2.2 Synthesis of the WPU adhesives

First, HDI (20.2 g, 120 mmol) and PBA (180 g, 60 mmol) were added in a 500 mL round-bottom, four-necked flask with a mechanical stirrer, a thermometer, and a condenser, and allowed to react at 80 °C for 2 h under a nitrogen atmosphere. Subsequently, IPDI (26.6 g, 120 mmol), BDO (5.4 g, 60 mmol), and TMP (3.6 g, 27 mmol, dissolved in 30 mL acetone) were injected into the mixture and stirred continuously for another 1 h at 60 °C. Then, DMPA (4.7 g, 35 mmol) was added to the pre-polymer and reacted at 60 °C until the residual NCO content reached the theoretical value, which was determined by the standard dibutylamine back-titration method. After the pre-polymer was cooled to 50 °C, a further chain extension reaction with 7.2 g AAS-Na (50% aqueous solution) was carried out at 50 °C for 1 h, followed immediately by neutralization with TEA (3.9 g, 39 mmol) for 10 min. Finally, 230 g distilled water was added under vigorous stirring conditions (2500 rpm), and the WPU latex was obtained after removing the acetone.

Three types of wetting agents were added into the WPU latex respectively for improving the wettability: hydrocarbon surfactant W77, with γ_L = 27.2 mJ m⁻²; polyether siloxane copolymer KL245, with γ_L = 24.6 mJ m⁻²; and fluorocarbon surfactant FC4430, with γ_L = 18.3 mJ m⁻². The formulas used to prepare the WPU adhesives are listed in Table 1. Herein, WPU-n refers to the WPU latex without any additive, while the WPU latexes with 0.5 wt% wetting agent W77, KL245, and FC4430 are abbreviated as WPU-77, WPU-245, and WPU-4430, respectively. WPU-DN represents the WPU latex mixed with 5 wt% isocyanate-terminated hardener DN.

Table 1 Formulas for the WPU adhesives

No.	Compositions of WPU adhesives^a (wt%)
	Wetting agent	Hardener DN	WPU latex
a All data refer to weight percentage.
WPU-n	0	0	100
WPU-77	0.5	0	99.5
WPU-245	0.5	0	99.5
WPU-4430	0.5	0	99.5
WPU-DN	0	5	95

---

2.3 Halogenation treatment of SBS

Halogenation was performed by immersing the SBS samples in chlorination solution (2 wt% of TCI in acetone) for 30 s. Subsequently, the samples were put into a drying oven at 60 °C for 5 min, then rinsed with ethanol water solution (25 wt%) for 30 s to remove the unreacted TCI. Finally, the samples were dried in the oven at 60 °C for 2 h. The halogenation-treated SBS was designated as SBS-h.

2.4 Plasma treatment of SBS

Plasma treatment was conducted in a compressed air plasma system from a Plasma Instruments reactor PTS-2000 (Nanjing Suman Electronic Co., China) at a frequency of 13 MHz for 120 s under atmospheric pressure. The power of the glow discharge was set at 200 W, and the distance between the nozzle and the surface was 10 mm. The samples after treatment were put into a vacuum drier and measurements were finished within 24 h. The plasma-treated SBS was named as SBS-p.

2.5 Characterization

Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) was performed using a Nicolet IS10 FT-IR spectrometer (Nicolet Instrument Inc., USA) with an ATR diamond unit (Golden Gate). The measurement was operated by applying the substrates onto the ATR diamond. The spectrum region is between 4000 cm⁻¹ and 650 cm⁻¹.

The wetting angle was measured by an optical goniometer (OCAH200, America) with computer-aided analysis of liquid drops. The read-outs were taken for 10 pictures for different drops and the arithmetic mean value was then reported. The surface free energy of the WPU latex was measured using the Wilhelmy plate method on a Krüss Processor Tensiometer K20 (Dachang Huajia Inc., China) at 25 °C. The surface free energy values quoted herein are the average of five measurements.

T-peeling strength tests of the adhesive joint were carried out on an Electronic Universal Testing Machine UTM6203 (Shenzhen Suns Inc., China) at an elongation rate of 100 mm min⁻¹. The adhesive process can be described as following: after a primary treatment, a quantitative amount of WPU adhesive was brushed on the surface of substrates with dimensions of 100 mm × 25 mm, desiccated at 25 °C for 5 min, and then dried at 75 °C for 5 min in a drying oven. Thereafter, the substrates were pressed under a pressure of 1.0 MPa at room temperature for 15 s. The samples were conditioned properly at 25 °C for 48 h before the T-peeling tests. The T-peeling strength values quoted herein are the average of three replicated experiments.

3. Results and discussion

3.1 Surface free energy parameters of substrates and WPU films

Surface free energy is a physical quantity that can precisely define the chemical nature of a material's surface. It is equal in respect of the value to the work required to create a new unit surface when separating two phases that stay in phase equilibrium in a reversible isothermal process.

The van Oss–Good model²⁰ assumes that the surface free energy (γ_i) can be presented as a sum of two components:

γ_i = γ^LW_i + γ^AB_i = γ^LW_i + 2(γ_i⁺γ_i⁻)^0.5 (1)

where, γ^LW_i is the Lifshitz–van der Waals (LW) component of surface free energy, γ^AB_i is the polar acid–base component of surface free energy, as resulting from the Lewis theory; γ_i⁺ is the component of γ^AB_i representing a Lewis acid; γ_i⁻ is the component of γ^AB_i representing a Lewis base; i = S represents a solid, while i = L represents a liquid.

In accordance with Young's eqn (2),²¹ the interfacial free energy of a solid to liquid is associated with the wetting angle of a liquid on a solid substrate.

γ_SL = γ_S − γ_L cos θ (2)

where, γ_S is the surface free energy of a solid in equilibrium with the saturated vapor of the liquid; γ_L is the surface free energy of a liquid in equilibrium with its saturated vapor; γ_SL is the interfacial free energy of a solid to liquid; θ is the wetting angle of a liquid to solid.

The relationship between the interfacial free energy and the surface free energy of the solid to liquid can be formulated as the following eqn (3):

γ_SL = ((γ^LW_S)^0.5 − (γ^LW_L)^0.5)² + 2((γ_S⁺γ_S⁻)^0.5 + (γ_L⁻γ_L⁺)^0.5 − (γ_S⁺γ_L⁻)^0.5 − (γ_S⁻γ_L⁺)^0.5) (3)

According to the eqn (1)–(3), the wetting angles θ and the surface free energy of the solid to liquid should satisfy the following relation:

(γ^LW_Sγ^LW_L)^0.5 + (γ_S⁺γ_L⁻)^0.5 + (γ_S⁻γ_L⁺)^0.5 = γ_L(1 + cos θ)/2 (4)

In order to solve eqn (4), the wetting angles (θ) of three model liquids (redistilled water, ethylene glycol, and diiodomethane, representing polar, semi-polar, and non-polar solvents, respectively) with known parameters²² (γ_L, γ^LW_L, γ^AB_L, γ_L⁺, γ_L⁻, as shown in Table 2) to the solids must be measured first, then the surface free energy parameters (γ_S, γ^LW_S, γ^AB_S, γ_S⁺, γ_S⁻) of the solids (substrates and WPU films) can be calculated according to eqn (4). The results are tabulated in Table 3.

Table 2 Surface free energy parameters of three known model liquids

Model liquids	Surface free energy parameters (mJ m^−2)
	γL	γ^LWL	γ^ABL	γL^+	γL^−
Redistilled water	72.8	21.8	51.0	25.5	25.5
Ethylene glycol	48.0	29.0	19.0	1.92	47
Diiodomethane	50.8	50.8	0	0	0

---

Table 3 Surface free energy parameters of the substrates and WPU films

Parameters	Substrates			WPU films
	SBS	EVA	PVC	WPU-n	WPU-77	WPU-245	WPU-4430
γS (mJ m^−2)	31.9	36.47	44.33	48.44	47.86	47.74	47.38
γ^LWS (mJ m^−2)	31.9	35.93	42.70	42.93	42.94	42.98	42.71
γ^ABS (mJ m^−2)	0	0.54	1.63	5.51	4.92	4.76	4.67
γS^+ (mJ m^−2)	0	0.10	0.29	0.46	0.18	0.15	0.12
γS^− (mJ m^−2)	0.01	0.71	2.31	16.63	33.92	37.82	44.16
Surface polarity (%)	0	1.47	3.70	11.38	10.28	9.97	9.86

---

As shown in Table 3, the surface free energy (γ_S) of SBS is below 35 mJ m⁻², while the Lewis acid/base component (γ^AB_S) is almost zero, which convinces us that SBS belongs to a non-polar substrate. EVA, with polar ester groups, possesses a relatively higher surface free energy, and so is regarded as a semi-polar substrate, while PVC, with the highest γ_S value (>40 mJ m⁻²) due to its polar group –Cl, belongs to a polar substrate. As for the adhesives, WPU films show a higher surface polarity and γ_S (>47 mJ m⁻²) compared with the SBS and EVA substrates. This large difference in γ_S will yield a large interfacial tension, which is adverse to adhesion between WPU films and these substrates (SBS and EVA). Comparatively, the similarity of the surface free energy parameters (γ_S, γ^LW_S, and γ^AB_S) between WPU films and PVC is believed to facilitate the adhesion of the WPU-PVC joints.²³

It is noteworthy that wetting agents have an ability to change the other surface free energy parameters of WPU films but no discernable reduction in γ_S is observed. Among the three types of wetting agents tested here, the decreasing ability shows the order: fluorocarbon surfactant (FC4430) > polyether siloxane (KL245) > hydrocarbon surfactant (W77), which is consistent with the order of γ_L of FC4430 (18.3 mJ m⁻²) < KL245 (24.6 mJ m⁻²) < W77 (27.2 mJ m⁻²).

3.2 Relationship between the surface free energy and wettability

Wettability is an ability of a liquid to maintain contact with a solid surface. The degree of wetting (wettability) can be determined by the wetting angle (θ) and spreading coefficient (S_C), which is the balance between the adhesion energy (W_A) and cohesion energy (W_C). The adhesion energy between a liquid and solid causes a liquid drop to spread across the surface, while the cohesion energy within the liquid makes the drop to ball up and avoid contacting with the surface.²⁴ These useful parameters are calculated as:

W_A = γ_S + γ_L − γ_SL (5)

W_C = 2γ_L (6)

S_C = W_A − W_C = γ_S − γ_L − γ_SL (7)

According to eqn (2) and (5)–(7), when S_C ≥ 0, θ = 0° or does not exist, the liquid wets the solid surface completely, and when 0 > S_C > −2γ_L, 0° < θ < 180°, the liquid partially wets the solid surface. In particular, when S_C = −2γ_L, W_A = 0, the liquid will contract into a liquid globule and cannot wet the surface at all.

From Tables 3 and 4, it can be seen that the γ_S of SBS, EVA, and PVC are 31.9 mJ m⁻², 36.47 mJ m⁻², and 44.33 mJ m⁻², correspondingly, while the wetting angles of the WPU-n adhesive on SBS, EVA, and PVC are 86.8°, 78.3°, and 54.0°, respectively. This phenomenon reveals that PVC with a high γ_S and low θ is favorable for wetting; nonetheless, SBS with the lowest γ_S and largest θ is hard to be wetted by the WPU-n adhesive. Fortunately, γ_L and the wettability of a liquid to solid can be revised by using wetting agents. Fig. 1 and Table 4 list the changes of θ and γ_L of WPU adhesives with and without wetting agents. Take the case of SBS, the θ decreases from 86.8° to 64.2°, 60.9°, and 48.2° with a decline of γ_L from 49.7 mJ m⁻² to 37.0 mJ m⁻² for WPU-W77, 35.2 mJ m⁻² for WPU-245, and 29.7 mJ m⁻² for WPU-4430, respectively, showing a proportional responsive relationship. Similar results are observed for the substrates of EVA and PVC. All the evidence manifests that wetting agents have the ability to decrease the γ_L of WPU adhesives, and thus increase their wettability to substrates.

Table 4 Wettability of WPU adhesives on various substrates

Substrates	Adhesives	θ (°)	WA (mJ m^−2)	SC (mJ m^−2)	γSL (mJ m^−2)
SBS	WPU-n	86.8	52.47	−46.93	29.13
	WPU-77	64.2	53.04	−20.95	15.58
	WPU-245	60.9	52.32	−18.08	14.78
	WPU-4430	48.2	49.50	−9.90	12.10
EVA	WPU-n	78.3	59.78	−39.62	22.27
	WPU-77	51.2	60.18	−13.82	13.29
	WPU-245	49.0	58.29	−12.11	13.38
	WPU-4430	38.1	53.07	−6.33	13.10
PVC	WPU-n	54.0	78.91	−20.49	15.12
	WPU-77	39.8	65.43	−8.57	15.90
	WPU-245	36.9	63.35	−7.05	16.18
	WPU-4430	27.5	56.04	−3.36	17.99

---

Fig. 1 γ_L of wetting agents and WPU adhesives.

The wettability of a liquid to solid can also be determined by the spreading coefficient (S_C) and interfacial free energy (γ_SL) according to eqn (5)–(7). As shown in Table 4, the S_C of all the samples have a negative value, which means that the adhesion energy (W_A) on these substrates are lower than the cohesion energy (W_C) of the WPU adhesives, corresponding to partial wetting. In contrast to the polar PVC substrate with a relatively high S_C of −20.49 mJ m⁻², the S_C of the WPU-n adhesive on SBS (−46.93 mJ m⁻²) and EVA (−39.62 mJ m⁻²) are so low that they are difficult to be wetted. With the use of wetting agents, the S_C on all three substrates increase dramatically and thus the wettability is strikingly improved obviously, among which the improvement of wettability follows the order: fluorocarbon surfactant (FC4430) > polyether siloxane (KL245) > hydrocarbon surfactant (W77). Further analysis reveals that when the S_C was above −20 mJ m⁻² and the γ_SL below 18 mJ m⁻², all the samples could be wetted easily by WPU adhesives. That is to say, to achieve better wettability on the substrates, a higher S_C (>−20 mJ m⁻²) or a lower γ_SL value (<18 mJ m⁻²) is necessary for waterborne adhesives.

3.3 Effect of the surface free energy and wettability on the adhesion property

As reported in the previous ref. 25, the adhesion force is composed of complicated physicochemical interactions including strong chemical bonds (covalent bonds and ionic bonds) and weak intermolecular interactions (van der Waals forces) as well as some specific interactions (like hydrogen bonding). Fig. 2 shows the adhesion strength (T-peeling strength) of the WPU adhesives on various substrates. As stated qualitatively, the T-peeling strength follows the order: PVC > EVA > SBS, which is in good agreement with the order of surface free energy (γ_S), the adhesion energy (W_A), and wettability (S_C). This phenomenon is probably attributed to the higher γ_S of the substrate exhibiting a higher surface polarity, which is efficient by offering more polar interactions with the WPU adhesive, thereby resulting in a higher T-peeling strength.

Fig. 2 T-peeling strength of WPU adhesives on various substrates.

Concerning the addition of wetting agents, they can significantly improve the wettability, but they have no apparent impact on the T-peeling strength, as shown in Fig. 2. In fact, the T-peeling strength, in most degrees, is determined by the surface polarity or interfacial interaction between the substrate and adhesive. Nevertheless, the addition of any wetting agent does not change the surface polarity of WPU films, so the interaction between WPU films and substrates remains almost unchanged. That is why the T-peeling strength for SBS and EVA merely increases slightly. As for the polar substrate PVC, the wettability and T-peeling strength of the WPU adhesive were both excellent even without wetting agents, which could be ascribed to the adequate contacting-interaction on their polar interface. However, with the addition of wetting agents, the T-peeling strength on PVC shows a slight decreased tendency. This should be attributed to the increased interfacial free energy (γ_SL) between the WPU adhesives and PVC, as illustrated in Table 4, which weakens the interface adhesion. Moreover, WPU adhesives with different kinds of wetting agents also exhibit a little disparity in adhesion property. Fluorocarbon surfactant (FC4430) and polyether siloxane (KL245) with lower γ_L show more adverse effects on the adhesion strength compared with the hydrocarbon surfactant (W77).

On the basis of the above considerations, it can be concluded that the surface polarity and the interfacial free energy play a decisive role in determining the adhesion strength of WPU-substrates joints, while the wetting agents can drastically increase the wettability of WPU adhesives, but cannot effectively enhance the adhesion strength.

3.4 Surface treatments of non-polar SBS-n

3.4.1. Analysis of ATR-FTIR. As mentioned above, SBS presents a poor wettability and lower T-peeling strength due to its low γ_S. To address this issue, one way is to enhance its γ_S by surface modifications, including halogenation and plasma treatments. Here, SBS samples treated with halogenation and plasma were marked as SBS-h and SBS-p, respectively, and SBS without treatment was denoted as SBS-n. The ATR-FTIR spectra of the SBS samples before and after the surface treatments are shown in Fig. 3.

Fig. 3 ATR-FTIR spectra of SBS-n, SBS-p, and SBS-h.

The ATR-FTIR spectrum of SBS-n shows the typical absorption of styrene (aromatic C–H stretching at 3080 cm⁻¹, aromatic C–C stretching at 1600 cm⁻¹, and C–H out-of-plane deformation of a phenyl group at 906, 791, and 696 cm⁻¹) and butadiene (C=CH stretching at 3225 cm⁻¹, –CH₂ stretching at 2926 and 2850 cm⁻¹, –CH₂ scissoring at 1457 cm⁻¹, and out-of-plane deformation of trans-1,4-C=C at 967 cm⁻¹).

In the ATR-FTIR spectrum of SBS-p, several new absorption peaks of polar groups can be observed. In detail, the absorption peak around 3375 cm⁻¹ is attributable to the O–H and N–H stretching vibrations, while C=O and C–O stretching vibrations are clearly evident at 1630 cm⁻¹ and 1137 cm⁻¹, respectively. Meanwhile, the intensity of the –CH₃ and –CH₂– absorption peaks at 2926 cm⁻¹ and 2850 cm⁻¹ are weakened, which reveals that the corrosion of plasma has introduced some polar groups to the surface of SBS. As for the ATR-FTIR spectrum of SBS-h, new absorption peaks ascribed to C–Cl stretching vibration at 760 cm⁻¹ and C=O stretching vibration at 1700 cm⁻¹ appear in comparison with SBS-n, unambiguously indicating the successful introduction of some –Cl and C=O polar groups onto the surface of SBS after halogenation.

3.4.2. Effects of the surface treatments on the surface free energy and wettability. The effects of the surface treatments on the γ_S and wettability of SBS were investigated. As shown in Table 5, the γ_S of SBS increases from 31.9 mJ m⁻² to 40.33 mJ m⁻² and 41.08 mJ m⁻², respectively after plasma treatment and halogenation treatment. Importantly, according to the data of θ and S_C, the wettability on SBS-p and SBS-h gains a tremendous improvement over SBS-n. Meanwhile, the surface free energy parameters (γ^LW_S, γ^AB_S, γ_S⁺, γ_S⁻) and W_A are also increased. All the data provide sufficient evidence to verify that the surface treatment by halogenation or plasma can efficiently increase the surface free energy and improve the wettability for non-polar substrates.

Table 5 Effects of the surface treatments on the surface free energy and wettability of SBS

Parameters	SBS-n	SBS-p	SBS-h
γS (mJ m^−2)	31.9	40.33	41.08
γ^LWS (mJ m^−2)	31.9	34.44	36.31
γ^ABS (mJ m^−2)	0	5.89	4.77
γS^+ (mJ m^−2)	0	0.81	0.45
γS^− (mJ m^−2)	0.01	10.74	12.69
Surface polarity (%)	0	14.6	11.6
θ (°)	86.8	41.4	40.5
WA (mN m^−1)	52.47	86.98	87.49
SC (mN m^−1)	−46.92	−12.42	−11.91
γSL (mN m^−1)	29.13	3.05	3.29

---

3.4.3. Effects of the surface treatments on the adhesion property. For the purpose of studying the interfacial interaction between the adhesives and surface-treated SBS, the isocyanate-terminated hardener DN was used to compare the differences in adhesion property between WPU-n and WPU-DN (WPU mixed with DN), and the corresponding results are presented in Fig. 4. For untreated SBS-n, the T-peeling strength values of WPU-n and WPU-DN were both quite poor. After the surface treatments, the polar groups in SBS-p and SBS-h can offer more interactions, such as polarization forces (orientation forces) and hydrogen bonds, with the WPU adhesive, which is beneficial for enhancing the T-peeling strength. Although different modification treatments bring about a similar increase in γ_S and wettability, the T-peeling strength values of the WPU adhesives to the substrates show some disparities. The improvement in T-peeling strength for SBS-h (7.21 N mm⁻¹) is greater than that for SBS-p (6.32 N mm⁻¹), while in the case of using the hardener DN, the opposite result is obtained (7.73 N mm⁻¹ for SBS-h and 8.13 N mm⁻¹ for SBS-p). This should be attributed to the difference in interaction mechanism between the substrates and WPU adhesives. That is, the polarity of the –Cl groups in the halogenation-treated SBS-h is stronger than that of the –NH– and –OH groups in the plasma-treated SBS-p, accordingly, concerning to WPU-n, the interaction derived from the polarization force for SBS-h is stronger than that for SBS-p; thereby, the T-peeling strength for SBS-h is higher than for SBS-p. In the presence of the hardener DN, the terminal isocyanate groups can react with the reactive hydrogen (–NH–, –OH groups) of SBS-p as well as WPU to form chemical bonds, which offers additional covalent interaction between the WPU-DN adhesive and SBS-p, whereas such interaction is relatively weak due to the lack of active hydrogen in SBS-h. Hence, without exaggeration, SBS-p shows a larger increase in the T-peeling strength with the application of the hardener DN.

Fig. 4 Effect of surface treatments on T-peeling strength of WPU adhesives.

4. Conclusion

The key adhesion parameters of the WPU adhesives and various substrates, such as the surface free energy (γ_S, γ_L), interfacial free energy (γ_SL), wettability, and adhesion property, were analyzed, and were found to be quite different from the solvent-base adhesion system due to the great disparity in γ_L between waterborne and solvent-base adhesives. The relationship among these parameters was summarized and the interfacial interaction mechanism concluded. The surface free energy γ_S of the substrate is the dominant parameter to influence the wetting and adhesion property of WPU adhesives. A non-polar substrate (γ_S < 35 mJ m⁻²) is hard to be wetted and bound together by WPU adhesives, whereas a polar substrate (γ_S > 40 mJ m⁻²) displayed excellent wettability and superior adhesion strength. The wetting and spreading of a non-polar substrate (SBS) can be improved by the addition of wetting agents, whereby generally, S_C > −20 mJ m⁻² or γ_SL < 18 mJ m⁻² is necessary for wetting in the WPU adhesive system. For non-polar substrates, the increase in wettability and the decrease in γ_SL is beneficial for adhesion, but cannot significantly enhance the adhesion strength. On the contrary, for polar substrates (PVC), the increase in γ_SL leads to a slight decrease in adhesion strength. Surface modification of the non-polar substrate can enhance the γ_S and improve the wettability and adhesion strength. In the presence of an isocyanate-terminated hardener, the plasma treatment for SBS shows a more enhanced effect than that from halogenation treatment due to the formation of covalent bonds.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51273128 and 21206096).

Notes and references

1. A. Baldan, Int. J. Adhes. Adhes., 2012, 38, 95–116.
2. D. E. Packham, Handbook of polyurethanes, Springer Press, Heidelberg, 2011, pp. 12–36.
3. D. Y. Yang, L. Han, H. Q. Zhang and F. X. Qiu, J. Macromol. Sci., Pure Appl. Chem., 2011, 48, 277–283.
4. D. Wasan, A. Nikolow and K. Kondiparty, Curr. Opin. Colloid Interface Sci., 2011, 16(4), 344–349.
5. M. R. Chashmejahanbin, A. Salimi and A. E. Langroudi, Int. J. Adhes. Adhes., 2014, 49, 44–50.
6. W. X. Chen, J. S. Yu, G. L. Chen, X. P. Qiu, W. Hu, H. Y. Bai and J. Z. Shao, RSC Adv., 2015, 5, 87963–87970.
7. J. Tyczkowski, I. Krawczyk, S. Kuberski and P. Makowski, Eur. Polym. J., 2010, 46, 767–773.
8. M. D. Romero-Sánchez, M. M. Pastor-Blas and J. M. Martín-Martínez, Compos. Interfaces, 2003, 10(1), 77–94.
9. V. Fombuena, J. Balart, T. Boronat, L. Sánchez-Nácher and D. Garcia-Sanoguera, Mater. Des., 2013, 47, 49–56.
10. M. R. Chashmejahanbin, H. Daemi, M. Barikani and A. Salimi, Appl. Surf. Sci., 2014, 317, 688–695.
11. KRÜSS GmbH Website, https://www.kruss.de/fileadmin/user_upload/website/literature/kruss-ar260-en.pdf, accessed July 2007.
12. N. Bhosale, A. Shaik and S. K. Mandal, RSC Adv., 2015, 5, 103625–103635.
13. F. Yu, X. Xu, N. Lin and X. Y. Liu, RSC Adv., 2015, 5(89), 72544–72552.
14. C. J. van Oss, L. Ju, M. K. Chaudhury and R. J. Good, J. Colloid Interface Sci., 1989, 128(2), 313–319.
15. L. H. Lee, Langmuir, 1996, 12, 1681–1687.
16. R. J. Good, M. K. Chaudhury and C. J. van Oss, Fundamentals of Adhesion, Plenum Press, New York, 1991, pp. 153–172.
17. B. B. Lee, E. S. Chan, P. Ravindra and T. A. Khan, Polym. Bull., 2012, 69, 471–489.
18. N. Wu, J. L. Dai and F. J. Micale, J. Colloid Interface Sci., 1999, 215, 258–269.
19. A. B. Ortíz-Magán and M. M. Pastor-Blas, Plasma Processes Polym., 2008, 5, 681–694.
20. R. J. Good and C. J. van Oss, J. Macromol. Sci., Chem., 1989, 26(8), 1183–1203.
21. D. E. Packham, Int. J. Adhes. Adhes., 1996, 16, 121–128.
22. K. L. Mittal, Polym. Eng. Sci., 1977, 17(7), 467–473.
23. R. J. Good, J. Adhes. Sci. Technol., 1992, 6(12), 1269–1302.
24. D. E. Packham, Int. J. Adhes. Adhes., 2003, 23, 437–448.
25. J. H. Clint, Curr. Opin. Colloid Interface Sci., 2001, 6, 28–33.

---