In Figure 2 the effect of benzoyl peroxide content on the peel adhesion of self-adhesive compositions based on 288 and 922 resins were presented. Despite the crosslinking agent content the peel adhesion of PSA 288 was substantially than PSA 922. Interestingly, for both adhesives, the highest values of this parameter were noted for 1.5 wt. % and 2.5 wt. % DClBPO content, indicating the compositions with the most promising potential for forming robust and durable bonds. The synergy between benzoyl peroxide concentration and adhesive adhesion indicates the careful balance required to optimize adhesive performance.
Thus, the performance of adhesive essentially depends on the PSA composition. The practical consequences of these results are manifold. PSA 288, with its superior adhesion at specific DClBPO content could find applications where strong and reliable bonding is required. Whereas, PSA 922 could be applied where moderate adhesion values are desirable.
Figure 2 presents results of the tack performance for pressure-sensitive adhesives 288 and 922. Much like the pattern observed in adhesion, adhesive 288 once again emerges as the frontrunner in terms of tack. The data points to higher tack values exhibited by adhesive 288 compared to adhesive 922. This consistency across adhesive properties suggests a fundamental divergence in the response of these formulations to benzoyl peroxide concentrations.
In the case of adhesive 288, tack values demonstrate an intriguing trend. Initially, as the concentration of benzoyl peroxide increases, the tack values steadily ascend, reaching a critical threshold at around 2 wt. %. Beyond this point, however, an unexpected and significant drop in tack values is observed. This phenomenon raises questions about the underlying mechanisms that govern tack and how they interact with the presence of benzoyl peroxide. The subsequent decrease in tack values beyond the critical threshold could potentially be attributed to complex interactions between the adhesive matrix, crosslinking agent, and the substrate surface.
On the other hand, adhesive 922 showcases a distinct behavior in relation to tack. Here, the tack values exhibit a consistent decline as the concentration of benzoyl peroxide rises up to 2 wt. %. This unexpected reduction in tack could be linked to the intricate interplay between adhesive components and the evolving chemical environment created by increasing benzoyl peroxide content. However, intriguingly, the trend reverses beyond the 2 wt. % mark, leading to an unexpected resurgence in tack values, peaking even at 8 N for 3 wt. % of benzoyl peroxide. This phenomenon opens up avenues for speculation about the influence of crosslinking dynamics, chemical interactions, and the delicate balance between adhesive and substrate.
These results underscore the complexity inherent in adhesive behavior and the multifaceted influence of crosslinking agents such as benzoyl peroxide. The interplay between formulation components, curing reactions, and substrate interactions creates a rich tapestry of adhesive properties that is both challenging and exhilarating to decipher.
From an application perspective, these insights carry immense value. The ability to predict and manipulate tack behavior based on benzoyl peroxide concentration offers engineers and manufacturers a powerful tool in tailoring adhesive solutions to specific needs. Adhesive 288's peak tack values within a controlled range could potentially make it an excellent choice for scenarios where quick, robust initial bonding is essential. Conversely, the tack resurgence observed in adhesive 922 at higher benzoyl peroxide concentrations could find applications where repositionability and tack recovery are desired.
Figures 4 and 5 provide cohesion to varying concentrations of crosslinking agent at two distinct temperatures: 20°C and 70°C. In the case of adhesive 922, cohesion values exhibit remarkable cohesion, showcasing maximum values across the entire range of crosslinking agent concentrations. This robust and consistent underscores the inherent strength of cohesion within adhesive 922, rendering it resilient and reliable under different crosslinking conditions. The steady increase in cohesion as the concentration of the crosslinking agent rises signifies a direct relationship between the strength of the adhesive's internal bonds and the presence of benzoyl peroxide.
Particularly noteworthy adhesive 288 is the observation of lower cohesion values at the lowest concentration of the crosslinking agent. This phenomenon calls for a nuanced understanding of the interplay between crosslinking dynamics and cohesion. It raises intriguing questions about the threshold at which adhesive 288's internal bonds attain optimal strength, suggesting that a certain concentration of benzoyl peroxide may be necessary to trigger cohesive forces effectively.
The temperature dimension adds an extra layer of complexity to the cohesive behaviors exhibited by these adhesive formulations. At 20°C, both adhesives demonstrate intriguingly similar trends in response to varying crosslinking agent concentrations. The values follow an ascending trajectory with increasing benzoyl peroxide content, underscoring the role of crosslinking in enhancing cohesion. The differences in maximum cohesion values, however, highlight the distinct inherent properties of adhesives 288 and 922. The consistently higher cohesion values of adhesive 922 suggest a robustness that is unaffected by temperature fluctuations.
At 70°C, the dynamics of cohesive behavior take on a more pronounced form. Adhesive 922 maintains its trend of increasing cohesion with higher crosslinking agent concentrations, indicative of the crosslinking-induced enhancement in internal bonding. In contrast, adhesive 288 showcases an interesting reversal in behavior. Here, the cohesion values increase more markedly with the inclusion of benzoyl peroxide, highlighting the temperature-sensitive interaction between cohesion and crosslinking dynamics. This observation hints at the complex interplay between crosslinking agent, temperature, and the adhesive matrix's propensity to form strong cohesive bonds.
Figure 6 presents the results of the SAFT (Shear Adhesion Failure Temperature) test for both analyzed adhesives, PSA 922 and adhesive 288. The values depicted in the graph provide insights into the behavior of both adhesives in terms of thermal resistance, which is a crucial aspect in the use of pressure-sensitive adhesives across various applications. When examining these results, an interesting trend can be observed. At lower concentrations of the cross-linking compound, the SAFT test results are more favorable for samples made from PSA 922. This implies that at lower quantities of DCLBPO used, this adhesive demonstrates better thermal resistance. This is significant information, as it may indicate the ability of PSA 922 to maintain its cohesion and durability even at higher temperatures.
In the case of adhesive 288, we observe that at higher concentrations of DCLBPO, the tapes made from this adhesive achieve higher SAFT test results. This suggests that, in the case of this adhesive, a higher concentration of the cross-linking compound is more advantageous in terms of thermal resistance. Thermal resistance is a key parameter, especially in applications where adhesives are subjected to extreme temperature conditions. Adhesives that retain their properties at high temperatures can find utility in various industries, such as the aerospace and automotive sectors.
Therefore, the selection of the appropriate composition, both in terms of the type of adhesive and the concentration of the cross-linking compound, is crucial in tailoring the adhesive to specific applications. These results are important as they provide a better understanding of the factors influencing the thermal properties of pressure-sensitive adhesives, which, in turn, can lead to the development of more optimized adhesive products in the future.
Figures 7 and 8 present the results regarding the shrinkage of the tested materials in the case of pressure-sensitive adhesives. It is worth noting that the shrinkage is significantly lower for adhesive 922 compared to adhesive 288. Both of these results are relevant in the context of the adhesives' applications and performance.
For adhesive 922, the shrinkage values remain relatively low across the entire range of concentrations of the tested substance. This means that even with varying concentrations of the cross-linking compound, the shrinkage does not exceed the critical threshold of 0.5%. This is important because the shrinkage of an adhesive can impact its practical use. Exceeding this level of shrinkage is unacceptable for pressure-sensitive adhesives and tapes, as it can lead to a loss of bond durability. Adhesives characterized by low shrinkage are more desirable, especially in applications where bond stability and durability are critical.
In the case of adhesive 288, the shrinkage values show some dependency on the concentration of the cross-linking compound. Initially, the shrinkage values are the highest for the pure adhesive, without any modifications. However, as the concentration of the cross-linking compound increases, the shrinkage values decrease. This may suggest that the addition of a higher concentration of the cross-linking substance aids in controlling and reducing shrinkage. This is a significant finding, as it implies that the formulation of adhesive 288 can be optimized to achieve the desired shrinkage characteristics, depending on the specific application.
To sum up, the results regarding the shrinkage of these pressure-sensitive adhesives are important as they affect their practical utility. Adhesives with low shrinkage are more desirable, and the ability to control this parameter can lead to more optimized adhesive products for various applications.
Based on previous results, the most effective concentration values of the cross-linking compound were selected and these samples were modified with a filler called citrine at concentrations of 0.1 to 3.0 wt. %. Taking into account the above-obtained tack, adhesion, cohesion and shrinkage values, it was decided that the best results were achieved by samples containing 1.5 wt. % of the cross-linking compound.
Table 2 shows the adhesion, tack, cohesion and SAFT test results for both tested adhesives with different filler concentrations for 922 adhesive.
The analysis of adhesion, tack, cohesion, and SAFT test results provides valuable information about the properties of samples depending on the filler concentration. It is worth considering the reasons for the observed trends and their implications for the practical application of materials.
The sample containing the highest amount of filler exhibited the highest adhesion value, while these values are relatively close (ranging from 6.5 to 8.8 N/25mm). In the case of adhesion, where the sample with the highest filler content achieved the highest value, it may suggest that adding a larger amount of filler positively influences the material's ability to adhere to surfaces. There are several potential reasons for this phenomenon, such as increased contact surface (a higher filler content may increase the contact surface between the material and the substrate, promoting better adhesion), improved internal structure (the addition of more filler may affect the material's internal structure, contributing to better adhesion through more complex interactions between particles), or better-matched mechanical properties (filler can improve the material's mechanical properties, which, in turn, can affect its ability to adhere). On the other hand, tack values were inversely correlated, with the highest value obtained for the lowest filler concentration. There are several possible explanations for this phenomenon: the sticky characteristics of lower concentrations (lower filler concentrations may favor a more sticky material characteristic, leading to better adhesion to the substrate during tack testing), a balance between elasticity and adhesion (lower filler concentrations may influence the material's elasticity, which is crucial for tack testing where the balance between elasticity and adhesion is key), environmental conditions (tack results may strongly depend on environmental conditions, such as temperature and humidity, and lower filler concentrations may be more resistant to variable conditions), filler type (tack values may depend on the specific type of filler, as different fillers can have different adhesive and tack properties depending on their structure and interaction with the substrate), and complex interparticle interactions (tack may result from complex interparticle interactions between the filler and the polymer matrix, and this influence may be nonlinear and dependent on various factors).
Regarding cohesion, all samples exhibited maximum values at 20°C across all concentrations, but only the highest filler concentration achieved a lower result at 70°C (56.4 h). The results suggest that a higher amount of filler may influence the stability of the material under elevated temperatures. Potential consequences of this phenomenon can be significant, especially in industries where temperature plays a crucial role, such as automotive, electronics, or construction. Several aspects are worth considering: structural stability (a higher filler content may improve the structural stability of the material under high temperatures, essential for structural components exposed to high temperatures), resistance to deformations (increased filler content may reduce the susceptibility to material deformations under elevated temperatures, crucial in many applications), and phase transformations (higher filler concentrations may decrease the material's tendency to undergo phase transformations due to temperature, crucial for materials used in variable temperature conditions).
In the case of the SAFT test, high results were obtained for lower filler concentrations. Regarding the SAFT test results, where lower filler concentrations achieved high concentrations, this may indicate better flexibility of the samples. Lower filler concentrations may contribute to better flexibility of the material, which is beneficial in conditions of dynamic loads and variable temperatures.
Table 2. The adhesive properties measured for PSA based on 922 resin with various citrine content
Citrine content
[wt. %]
|
Adhesion
[N/25 mm]
|
Tack
[N]
|
Cohesion [h]
|
SAFT test
[°C]
|
at 20 °C
|
at 70 °C
|
0.1
|
6.445
|
4.11
|
>72
|
>72
|
>225
|
0.5
|
6.055
|
3.89
|
>72
|
>72
|
>225
|
1.0
|
7.895
|
3.26
|
>72
|
>72
|
197
|
3.0
|
8.835
|
2.11
|
>72
|
56
|
116
|
The analysis of the adhesive properties of adhesive 288 modified with different concentrations of citrine sheds new light on the impact of this filler on the adhesive’s characteristics (Tab. 3). In comparison to the previous adhesive, a clear trend of increased adhesion and tack is evident, suggesting that the addition of citrine positively affects the adhesive’s ability to effectively bond to various surfaces. This phenomenon may be particularly significant in diverse applications, where high adhesion is crucial, such as in the industrial or construction sectors.
Simultaneously, the values of cohesion, indicating the internal consistency of the adhesive, show a decrease. This implies that the citrine modification influences the internal structure of the material, which can be perceived as a compromise between improved adhesion and internal consistency. Such changes may find application in situations where there is a need to achieve a balance between the adhesive nature and the internal cohesion of the material.
Furthermore, the citrine-modified adhesive demonstrates increased thermal resistance, as confirmed by the results of the SAFT test. This discovery suggests that this type of adhesive may be more effective in conditions where higher temperatures are present, applicable in industries such as automotive, electronics, or the production of components exposed to elevated temperatures.
Therefore, the modification of the adhesive with citrine opens up prospects for improving adhesive properties, and understanding these changes can contribute to better adapting such materials to specific applications. It is also worthwhile to continue research to delve into the detailed mechanisms of interactions between adhesive components and citrine, potentially leading to even more precise modifications and optimizations.
Table 3. The adhesive properties measured for PSA based on 288 resin
Citrine content
[wt.%]
|
Adhesion
[N/25 mm]
|
Tack
[N]
|
Cohesion
[h]
|
SAFT test
[°C]
|
at 20°C
|
at 70°C [h]
|
0.1
|
12.85
|
14.1
|
>72
|
>72
|
>225
|
0.5
|
12.8
|
10.7
|
>72
|
>72
|
>225
|
1.0
|
12.2
|
9.91
|
>72
|
>72
|
221
|
3.0
|
11.6
|
2.4
|
42
|
12.3
|
186
|
The analysis of adhesive shrinkage results depending on various concentrations of citrine, presented in tables 4 and 5, provides significant insights into the material behavior over time. Shrinkage values in the first 10 minutes are higher for adhesive 922 compared to adhesive 288, and this trend persists throughout the entire study period, up to 7 days.
An intriguing phenomenon is the observation that shrinkage values decrease with an increase in the amount of citrine in the adhesive sample. These values can offer insights into the impact of the filler on the curing and shrinking processes of the adhesive. The substantial shrinkage of adhesive 922 in the initial minutes may indicate intense curing processes in the early stages, which could be crucial for rapid applications or processes where curing speed is essential.
Furthermore, the decreasing shrinkage values with an increase in citrine content may suggest that this filler influences the shrinking processes, perhaps by regulating chemical reactions or the internal structure of the material. This discovery may be crucial for adapting the adhesive to specific application conditions where shrinkage control is significant, such as in the production of precision components or applications where minimizing deformation is key.
It is also worth noting that differences in the behavior of adhesives 922 and 288 may arise from their distinct chemical compositions, which can affect chemical reactions, curing rates, and overall mechanical properties. Therefore, continuing the analysis of these results, along with further research on the influence of citrine on other adhesive properties, may yield even more detailed and precise outcomes.
Table 4. Shrinkage for PSA based on 922 in time
Citrine content [wt.%]
|
Shrinkage [%]
|
0.2 h
|
0.5 h
|
1 h
|
3 h
|
8 h
|
24 h
|
1 day
|
3 days
|
4 days
|
5 days
|
6 days
|
7 days
|
0.1
|
0.28
|
0.30
|
0.32
|
0.39
|
0.42
|
0.48
|
0.50
|
0.59
|
0.60
|
0.69
|
0.69
|
0.69
|
0.5
|
0.21
|
0.24
|
0.29
|
0.30
|
0.38
|
0.40
|
0.49
|
0.54
|
0.60
|
0.64
|
0.63
|
0.64
|
1
|
0.06
|
0.07
|
0.08
|
0.08
|
0.09
|
0.09
|
0.09
|
0.10
|
0.10
|
0.11
|
0.15
|
0.14
|
3
|
0.03
|
0.04
|
0.04
|
0.05
|
0.05
|
0.05
|
0.06
|
0.06
|
0.07
|
0.08
|
0.08
|
0.08
|
Table 5. Shrinkage for PSA based on 288 in time
Citrine content [wt.%]
|
Shrinkage [%]
|
0.2 h
|
0.5 h
|
1 h
|
3 h
|
8 h
|
24 h
|
1 day
|
3 days
|
4 days
|
5 days
|
6 days
|
7 days
|
0.1
|
0.15
|
0.22
|
0.27
|
0.33
|
0.35
|
0.38
|
0.42
|
0.48
|
0.50
|
0.51
|
0.55
|
0.55
|
0.5
|
0.14
|
0.20
|
0.24
|
0.28
|
0.31
|
0.35
|
0.40
|
0.43
|
0.47
|
0.51
|
0.51
|
0.51
|
1
|
0.09
|
0.10
|
0.11
|
0.12
|
0.13
|
0.14
|
0.17
|
0.24
|
0.27
|
0.30
|
0.34
|
0.34
|
3
|
0.08
|
0.12
|
0.15
|
0.17
|
0.19
|
0.22
|
0.25
|
0.26
|
0.28
|
0.30
|
0.30
|
0.30
|
The next stage of the research involved examining the viscosity of the adhesives, and the obtained results are presented in Table 6. Viscosity was assessed for the highest filler concentration, which amounted to 3% by weight. The symbol "-" was used to indicate that the viscosity was exceptionally high, preventing the completion of the full study.
It is worth noting that adhesive 288 exhibits a lower viscosity, allowing it to be coated even on the fifth day. In the case of adhesive 922, the viscosity reaches a very high level as early as the second day, making the coating process impossible by the third day.
This discovery could be crucial for the practical application of adhesives in various industries. The lower viscosity of adhesive 288 makes it more accessible for later-stage applications, which may be significant for manufacturing processes requiring longer preparation or assembly times. On the other hand, the high viscosity of adhesive 922 on the second day may suggest the necessity of using this adhesive promptly after preparation, crucial for processes requiring rapid application or bonding.
Table 5. Viscosity for PSA based on 922 and 288 after 1-7 days of PSA preparation
Adhesive resin
|
Viscosity [mPas]
|
after 1 day
|
2 days
|
3 days
|
5 days
|
7 days
|
288
|
32.2
|
33.1
|
39.4
|
51.8
|
-
|
922
|
61
|
72
|
-
|
-
|
-
|
“-“ - too high to be applied