3.3. Dynamic Mechanical Analysis
PSA is a viscoelastic material. To further characterize the viscoelasticity properties of the modified PSAs under dynamic external stress, DMA tests were conducted on the PSAs before and after modification in the temperature range of −60–160 °C. Plots of the storage modulus (G’) and loss factor (tanδ) with respect to temperature were obtained, as well as their glass transition temperatures (Tg), as shown in
Table 3 and
Figure 5.
The main component of all five groups of PSAs is the soft monomer 2-EHA, which has a low Tg at about −70 °C. Therefore, the overall Tg of PSA is low, below −20 °C, as shown in
Table 3. The long chain has a tangling effect, and its addition restricts the free movement of the acrylate molecular chain. The movement of the molecular chain segments requires higher energy and temperature, leading to an increase in Tg [
30]. Due to the same formulation, only changing the molecular weight of the modified HHTPB macromonomer and the amount of addition, there is little difference between the Tg of the four modified PSAs.
As shown in
Figure 5a, the storage modulus (G’) of the different PSAs all decrease sharply in the transition state region near Tg, and the decreasing trend slows when the temperature exceeds the transition state region. Compared with the low-temperature section, the storage modulus in the high-temperature section is very low, and in combination with the tanδ curve in
Figure 5b, it can be assumed that this phenomenon occurs because of the viscous flow of the adhesive sample in the high-temperature section. The storage modulus of the modified PSA is significantly larger than that of the unmodified PSA above 0 °C, and the higher the content of the HHTPB macromonomer, the relatively higher the storage modulus. Above 100 °C, the tanδ curves of the unmodified PSA are obviously jittery compared with those of the modified four groups of PSA curves, and it is no longer possible to maintain the conventional morphology of the PSA, which suggests that the modified PSA has a better temperature resistance performance. The modified PSA exhibits a lower tanδ above 0 °C due to the restricted freedom of movement of the molecular chains, indicating that its energy dissipation is reduced and it is more inclined to the elastomer form [
31].
Figure 6 shows the curves of the complex viscosity of the five groups of PSA with temperature, and
Table 4 lists the complex viscosity of PSA at different temperatures. As shown in
Figure 6, the overall trend of the complex viscosity of the five groups of PSA is approximately the same, which is due to the fact that the main body formulation is the same, and only changing the content of modified macromonomers has little effect on the viscosity. At temperatures below 60 °C, the viscosities of the five sample groups are similar. The two groups of PSAs with the highest content of HHTPB long chains exhibit relatively low viscosity, while P
Null exhibits the highest viscosity. This can be attributed to the fact that long-chain molecules are able to slide more easily, and, compared to shorter molecules, they are less likely to form strong network structures or intermolecular cross-links, which results in a reduction in the overall viscosity of the fluid. The decreasing trend of the P
Null viscosity is more obvious than that of the remaining four groups of PSA above 60 °C. At high temperatures, the polymer movement becomes more active; however, the long-chain molecules, due to their larger spatial structure, tend to be more difficult to completely unravel or “curl”, thus maintaining a certain degree of intermolecular forces and structural stability, which results in a slower decrease in viscosity.
3.6. Influencing Factors of Peel Strength
Peeling behavior is related to the modulus of the PSA and wetting area, while the wetting area is related to the surface energy of the adhesive, viscosity of the adhesive and surface energy and roughness of the substrate.
Wetting in adhesives refers to the phenomenon of a liquid adhering to a solid surface when the PSA comes into contact with the substrate. Since adhesion involves contact of the whole surface, the wetting at this time is different from the wetting phenomenon of an isolated droplet on the surface, which refers to the wetting within the pores of the rough surface. Different wetting abilities can change the wetting area, which will have an effect on the subsequent peeling behavior. The final degree of wetting is thermodynamically controlled, while the process of reaching the final degree of wetting is controlled by kinetic factors [
35,
36].
T. Young proposed Young’s equation [
37] through the mechanical analysis method; based on Young’s equation, Neumann proposed the ES second state equation [
38], which can be used for the explanation of bonding behavior; the equation is as follows:
In Equation (1),
γSL,
γS and
γL represent the interfacial tension, surface energy of the substrate, and surface energy of the PSA, respectively. Conversely, the basic prerequisite for the formation of PSA bonding is that the PSA can wet the substrate surface. Wetting is divided into two processes: (1) spreading on the surface of the substrate and (2) penetration into the substrate surface cavities and formation of a specific adhesive surface, i.e., the PSA must be able to form a bond on the surface of the fully unfolded adhesive substrate so that the molecules of the PSAsurface and substrate are in close contact to form an intermolecular adsorption force [
39].
The actual substrate is seldom completely smooth. If the pores on the sticky substrate are regarded as capillary tubes, the velocity of a liquid with viscosity (
η) at time (
t) deep into a certain capillary tube with radius (
R) and length (
L) can be calculated according to the following equation:
By integrating the above equation, we obtain:
In Equation (3),
η represents the viscosity of PSA, and
θ represents the contact angle. Combining Young’s equation:
with Equations (1) and (3) can obtain Equation (4):
Within a single capillary pore, the wetting area of the PSA and capillary surfaces (
A) is calculated using Equation (5):
In Equation (5), the main factors affecting the variation in the wetting area (
A) within a single capillary pore channel with time (
t) are
γL,
γS,
η, and
R. The two surface energies of the PSAs have been measured in the calculation, as shown in
Table 5 and
Table 7. The viscosity of solid PSA is difficult to measure; therefore, complex viscosity is used instead of the viscosity to simulate the viscosity of different PSA at different temperatures, as shown in
Table 4. R is determined to be 0.1 μm.
Combined with the PSA commonly used temperature range and test conditions, three testing temperatures: 20 °C, 40 °C, and 60 °C were selected. According to Equation (5) and the measured data mentioned above, the wetting areas of the PSAs on the capillary pore adhesive and capillary pore surface can be calculated. The time (
t) is 24 h. The results are shown in
Table 8. The calculated wetting area for the five groups of samples increases with temperature. This is attributed to the significant decrease in viscosity as the temperature increases, which enhances the fluidity of the PSA. As a result, the PSA spread more easily on the substrate surface, leading to a larger wetting area.
The wetting area (
A), energy storage modulus (
G’), loss modulus (
G”), and loss factor (
tanδ) are commonly used parameters to characterize the adhesion behavior of PSA. This work focuses on the results of the measured mechanical properties and explores the different mechanisms represented by the combinations of these four parameters. A data regression analysis was performed on the experimental data, and the corresponding peel strength (
P) model and correlation coefficient (
R) were obtained, as shown in
Table 9.
Model (a) corresponds to a mechanism in which the peeling force is entirely controlled by the contact area (A) of the adhesive within the wetting aperture. However, the regression results of the measured data in this study show a correlation coefficient of only 0.147, indicating a weak correlation between the measured data and this mechanism. This suggests that the peeling force is not significantly correlated with the wetting area as a single factor, and other factors likely influence the peeling force in conjunction with the wetting area.
Model (b), which incorporates both the wetting area and the modulus as influencing factors, yields a higher correlation coefficient, indicating that the model effectively describes the peeling behavior. This suggests that the peeling force can be regulated by three factors: the storage modulus (G’), loss modulus (G”), and single capillary channel wetting area.
In Model (b), the storage modulus (G’) and loss modulus (G”) are closely related to each other. Therefore, by replacing the G’ and G” terms with the loss factor (tanδ) in Model (c), a simplified model is obtained. The correlation coefficient of Model (c) remains greater than 0.9, demonstrating that it can effectively describe the peeling behavior.
Figure 5b shows that the loss factor (
tanδ) in Model (c) is close to 1. This parameter represents a strength property that remains unaffected by changes in the wetting area. In both Models (b) and (c), item A refers to the wetting area within a single capillary channel. While it does not significantly contribute to the peeling force by itself, it is a breadth property that influences the peeling force through the additive and cumulative effects of numerous capillary channels. Since the peel strength is measured linearly, and assuming that the plate cavities are uniformly distributed and closely spaced, with a cavity radius of 1 × 10
−7 m and a diameter of 2 × 10
−7 m, the number of cavities along a line is calculated as 25 mm/(2 × 10
−7 m), or 1.25 × 10
5 cavities. These capillaries collectively contribute to the total wetting area. If A in Model (c) is replaced by the total wetting area, Model (c) can be rewritten as Equation (6), as follows:
In Equation (7), represents the total contribution wetting area.
By analyzing Equation (7), below 0 °C, the tanδ values of the five adhesives were approximately the same; therefore, the contribution of this parameter to the peel strength was also similar. However, the viscosity and surface energy of the unmodified PSA are higher, while the modified PSA exhibits a larger wetting area, which makes a more significant contribution to the peel strength of the PSA on HDPE. Between 0 °C and 60 °C, the difference in viscosity is relatively small, but the difference in surface energy is more pronounced. The loss factor of unmodified PSA ranges from 1 to 2, while that of modified PSAs fluctuates around 1 and is lower than that of unmodified PSA. In this temperature range, according to Model (c), both the loss factor and wetting area terms influence the peel strength, with the contribution of the loss factor term to the peel strength of the modified PSA still being greater than that of the unmodified PSA. Above 60 °C, the unmodified PSA exhibits higher viscosity and surface energy, contributing more to the peel strength of PSAs on HDPE. In contrast, the modified PSAs have a greater wetting area. At temperatures above 60 °C, the loss factor of the unmodified PSA is much higher than that of the modified PSAs, at which point the effect of the wetted area on the peel strength is significantly reduced and is primarily determined by the loss factor. Therefore, the adhesive strength of PSAs is jointly influenced by the wetting area and loss factor, with the wetting area being more affected by viscosity than the loss factor. The factors that primarily govern the adhesive strength of PSAs vary across different temperature ranges.