The Influence of Monofunctional Silanes on the Mechanical and Rheological Properties of Hot Melt Butyl Rubber Sealants

Abstract

The influence of organosilicon compounds on butyl sealant blends’ mechanical and processing properties was investigated, particularly under increased humidity conditions. The addition of (3-mercaptopropyl)trimethoxysilane (MPTES), (3-aminopropyl)triethoxysilane (APTES), vinyltrimethoxysilane (VTMOS), and (3-glycidoxypropyl)trimethoxysilane (GLYMO) to elastomeric blends containing butyl rubber (IIR) and polyisobutylene (PIB) was studied. Key rheological parameters, including Mooney viscosity and melt volume rate (MVR), along with mechanical attributes such as peel resistance and cone penetration, were evaluated. Results indicated that functionalized silanes enhance sealant cohesion when their functional groups interact with the matrix and form cross-links under humid conditions. The presence of unreacted silanes acts as a plasticizer, increasing MVR and reducing viscosity. A notable MVR increase, up to 109 mL/10 min, was observed for the APTES-10 system. The most significant mechanical property enhancements were observed in blends containing MPTES and APTES, resulting in increased cohesion and peel resistance. The findings of this research are of considerable practical relevance, demonstrating that the modification of rubber sealants with monofunctional silanes improves their cohesion, delamination resistance, and processability, thereby making these materials suitable for the production of more durable sealants.

1. Introduction

Thermoplastic Sealants

Elastomeric sealing compounds, classified under the broad category of pressure-sensitive materials, exhibit three key properties: tackiness, peel resistance, and shear strength. These properties are directly influenced by the material’s ability to form bonds with a surface and the mechanisms occurring during their separation. Each of these parameters depends on the material’s capacity to establish a bond with the surface as well as the processes involved in detachment. The bonding process requires the material to flow freely and wet the surface under the application of minimal force. In contrast, separation is associated with the deformation of the adhesive layer and the dissipation of energy during peeling [1,2,3]. Both the bonding and detachment processes depend on the rheological properties of the material. Essentially, the performance characteristics of the compounds are determined by their viscoelastic properties. The functional performance of the compound is closely related to the storage modulus and the loss modulus [2,4]. The value of the storage modulus at elevated temperatures is crucial for the mechanical properties of the material. In the case of amorphous materials, the storage modulus remains relatively stable; however, exceeding a certain critical temperature results in its decline, leading to material softening. These properties can be improved by increasing the molecular weight of the elastomer and crosslinking the elastomeric base. Conversely, at low temperatures, sealing compounds harden and lose their tackiness.

Elastomeric compounds available on the market are typically offered in the form of strips and cords [5]. Due to the thermoplastic nature of the sealant, its cohesion decreases as the temperature rises. Standard sealants perform effectively at temperatures up to approximately 100 °C; however, at higher temperatures, the compounds lose their elastic properties and become prone to flow [6]. Reduced cohesion at elevated temperatures can lead to the failure of seals, especially under typical operating conditions for roofing accessories (80–100 °C). Market research highlights the need to improve the mechanical properties of these sealants. Efforts to enhance their performance have so far focused on partial crosslinking or the use of oligomers that subsequently crosslink under application conditions [7].

Raw Material Composition

Elastomeric Base

Due to its paraffinic nature, butyl rubber (IIR) is characterized by high resistance to weathering and relatively low production costs [8]. Additionally, its natural tackiness makes it an attractive raw material for the production of adhesives and sealing materials [6]. The first sealants utilizing IIR were developed in the mid-20th century and initially employed as solvent-based systems [9]. Dissolution allowed for easier application of elastomeric compounds containing rubber with a high molecular weight [10].

This type of sealant consists of polybutene, IIR, and polyisobutylene (PIB). It is characterized by high elongation at break (>1000%) but is susceptible to atmospheric oxidation and ozone degradation. These systems are available in the form of solvent-based sealants, extruded tapes and profiles, as well as hot-applied binders. Today, due to growing environmental concerns, solutions based on organic solvents are losing their significance. As a result, the next stage in the development of sealing compounds was the introduction of solvent-free thermoplastic sealants [11].

Butyl tapes contain 100% solid components, which eliminates the issue of shrinkage after application. Butyl sealants can also be applied in a thermoplastic form at temperatures ranging from 100 to 160 °C using an appropriately heated applicator. The heated sealant is extruded into the joint, forming a smooth and airtight bond after cooling, characterized by excellent adhesion, even to oily and untreated surfaces. Butyl sealants are widely used in the glazing industry, particularly between glass and aluminum. These tapes can be compressed by 20–30% and have a service life of 10–20 years [12,13].

Polyisobutylene (PIB) as a homopolymer is permanently tacky. Due to its natural tackiness, it is self-healing. PIB is a transparent polymer with elastomeric properties, although it exhibits a much greater tendency to creep than most unvulcanized rubbers [4]. It is most commonly used as an additive to other binders or tapes, mainly of the butyl type. One possible solution to the problem of sealant softening at high temperatures, such as those reached by roofs during summer (90–100 °C), is crosslinking by the addition of a cross-linkable rubber component [7], crosslinking during compounding [14,15], or a combination of the aforementioned approaches [15,16]. Additionally, the use of siloxane compounds in the blend could improve adhesion to ceramic substrates. By using vinyltrimethoxysilane (VTMS) in compositions based on IIR and ethylene-vinyl acetate (EVA) copolymer, it has been shown that the improvement in binder properties results from the chemical interaction between EVAC and VTMS, forming semi-permeable network structures [7].

Thermoplastic binders based on IIR, containing polyethylene (PE) and ethylene-vinyl acetate (EVA) copolymer, are also used. These binders offer higher strength and a higher working temperature compared to non-vulcanized binders based on IIR [17]. In contrast to crosslinked binders based on reactive oligomers, where the required strength and adhesive properties are achieved only after full curing (typically after 24 h or more), these binders can be used immediately after application and cooling [7].

Additives

A typical thermoplastic sealant consists of an elastomer [18], fillers [19,20], plasticizers [6,21], resins [22], technological additives (antioxidants), and functional additives that modify its properties [4,7,11] and behavior in different pH conditions [23]. The role of technological additives in binders is to control viscosity during processing, but their inclusion often leads to increased strength and adhesive properties, as well as expanding the working temperature range. Hydrocarbon resins, rosin derivatives, and bituminous substances are commonly used as additives in unvulcanized sealants. These compounds exhibit multifunctional properties: they enhance strength and adhesion to substrates while effectively reducing viscosity during processing and raising the working temperatures of the binders (80–100 °C) [14,15,16].

Organosilicon compounds present a promising approach for enhancing both the adhesion of sealants to common building substrates and the internal cohesion of the sealant matrix. While the application of organosilicon compounds as crosslinking coagents is well-established in static vulcanization systems, this study explores their potential for creating thermoplastic semi-interpenetrating networks (semi-IPNs) composed of mutually soluble polymers as well as determining the impact of adding organosilicon compounds on the change in the mechanical and processing properties of butyl-based sealing compounds, particularly under increased humidity conditions. This work presents, for the first time, the application of silanes with various functional groups, such as -SH, -NH₂, -CH=CH₂, and CH₂(O)CHCH₂-, to elastomeric sealant blends containing butyl rubber (IIR) and polyisobutylene (PIB) and evaluates their influence on the rheological and mechanical properties.

The conducted research holds significant practical importance as it demonstrates that the modification of rubber sealants with functionalized silanes enables the development of materials with optimized properties, such as higher cohesion, improved delamination resistance, and enhanced processability. These characteristics make the findings directly applicable to the production of modern, more durable sealants. The developed approach to controlled crosslinking and property modification allows manufacturers to precisely tailor formulations to specific application requirements, thereby opening new possibilities for the implementation of these solutions in industry.

2. Materials and Methods

2.1. Materials

The following materials were used for this study: Polyisobutylene Oppanol N100 (N100) (density 0.92 g/cm³, molecular weight Mw ~1,100,000), supplied by BASF (Poznań, Poland); butyl rubber IIR1675 (density 0.92 g/cm³, Mooney viscosity 46–56 ML (1 + 8) at 125 °C, degree of unsaturation 1.4–1.8% mol), supplied by Konimpex Sp. z o.o. (Konin, Poland); polybutene plasticizer PIB32, with a density of 0.92 g/cm³ and a number average molecular weight Mn of 1300, supplied by DMR sp. z o.o. (Warsaw, Poland); surface-modified mineral filler Omyacarb 2t (CaCO₃) with a density of 2.70 g/cm³, average particle size of 2 µm, and oil absorption of 11 g/100 g of filler, supplied by Omya sp. z o.o. (Ołdrzychowice Kłodzkie, Poland); silanes: (3-Mercaptopropyl)trimethoxysilane (MPTES), (3-Aminopropyl)triethoxysilane (APTES), Vinyltrimethoxysilane (VTMOS), and (3-Glycidoxypropyl)trimethoxysilane (GLYMO), supplied by Brenntag sp. z o.o. (Kędzierzyn-Koźle, Poland); and Norperox BIPB-40 (BIPB), Di(tert-butylperoxyisopropyl)benzene, supplied by Nordmann-Rassmann sp. z o.o. (Warsaw, Poland).

2.2. Methods

Cone penetration (further referred to simply as “penetration”) measurements were performed according to ISO 2137 using a Haida HD-R811 cone penetrometer (Haida International Equipment Co., Ltd., Dongguan, China) (cone weighing 150 g). Samples with a thickness of 25 mm were conditioned for 24 h at 20 °C. The results represent the average of 5 measurements.

Peel resistance was tested using a Shimadzu EZ-LX testing machine (Shim-Pol sp. z o.o., Izabelin, Poland), according to Finat FTM1. Stainless steel and 50 µm PET film were used as substrates. The tape thickness used was 1 mm. The peel rate was 300 mm/min, unless otherwise specified. The result is the average of 3 measurements. Resistance to peel at 10 mm/min will be referred to as cohesion.

The volumetric melt flow rate was determined according to ISO 1133 using a Haida HD-R801 (Haida International Equipment Co., Ltd., Dongguan, China) plastometer. The test was conducted at a temperature of 140 °C with a 5 kg load.

Density was determined according to ISO 1183 using the displacement method at 20 °C.

Mooney viscosity and relaxation testing were performed according to ASTM D1646 using a Haida HD-R812 Mooney viscometer (Haida International Equipment Co., Ltd., Dongguan, China) with a test temperature of 25 °C, a heating time of 1 min, and a reading time of 4 min. The relaxation rate coefficient was calculated as the slope of the curve between 1 and 10 s after the rotor stopped.

2.3. Preparation of Blends

The blends were prepared using a laboratory Sigma mixer produced by Almara sp. z o.o. sp.k. (Warsaw, Poland). Using a mixer, four series of blends were prepared with the composition provided in

Table 1. The composition of the blends, expressed in weight parts per hundred rubber (PHR). Rubber base was 80 PHR of N100 + 20 PHR of IIR1675. The letter X represents the modifier (MPTES, APTES, VTMOS, GLYEOS).

Ingredient/Sample ID	X-0	X-1	X-2.5	X-5	X-10	X-10B
N100	80	80	80	80	80	80
IIR1675	20	20	20	20	20	20
PIB32	250	250	250	250	250	250
CaCO3	450	450	450	450	450	450
X	-	1	2.5	5	10	10
BIPB	-	-	-	-	-	0.2

. For each series, an identical starting formulation was prepared without modifiers as a reference system (MPTES-0, APTES-0, VTMOS-0, GLYEOS-0). The raw materials were mixed at 110 °C, with 37 rpm fast mixing blade and 21 rpm slow mixing blade, for 60 min, until a homogeneous mass was obtained. The appropriate amount of modifier was then added to the prepared starting formulation and mixing was continued until a homogeneous mass was achieved. Samples of the blends were formed with the appropriate thickness for each test and then conditioned for 24 h at 20 °C and 50% relative humidity.

3. Results and Discussion

3.1. Cone Penetration

Determining the tackiness of adhesive sealant samples is more challenging than testing the tackiness of pressure-sensitive adhesives. Typical methods for testing the tackiness of pressure-sensitive adhesives, such as loop tack, quick stick, rolling ball tack, or probe tack [24,25], are not suitable for adhesive sealants. In the case of sealants, due to their relatively low cohesion compared to pressure-sensitive adhesives, testing tackiness often leads to cohesive failure, which prevents reliable results from being obtained. One of the simple, indirect methods for assessing the tackiness of a material is the hardness measurement using a cone penetration method. It is assumed that butyl sealants with a penetration below 40 [1/10 mm] lose tack, while a penetration above 80 [1/10 mm] indicates high tack and a tendency toward cohesive failure during delamination. The temperature of testing and application significantly influences the results due to the viscoelastic nature of pressure-sensitive sealants. For most construction applications, the useful penetration range for general-purpose butyl sealants is between 50 and 80 [1/10 mm]. Figure 1 presents penetration changes depending on modifier content.

Based on the conducted studies, it can be stated that the introduction of BIPB into the VTMOS-10B had a significant impact on increasing the penetration value. In the case of the MPTES-10B system, a decrease in this parameter was observed, reaching a value of 75 [1/10 mm], which may suggest structural changes in the material. Both IIR and PIB tended to degrade in the presence of radicals by β-scission (Figure 2), which was associated with the presence of a tertiary carbon atom in their chains [26,27].

The introduction of MPTES at concentrations of 1, 2.5, and 5 PHR into the systems had a minimal effect on the change in penetration values. For the system containing the highest silane content with thiol groups (MPTES-10), an increase in penetration to 79 [1/10 mm] was observed. In the case of the MPTES-10B blend, with the inclusion of a radical source, penetration decreased (75 [1/10 mm]) compared to MPTES-10 (79 [1/10 mm]). In the presence of radicals, the IIR chain may have been grafted with MPTES, forming a network under high-humidity conditions [28]. In this scenario, competing grafting and degradation processes may have occurred.

The addition of VTMOS did not have a significant impact on the properties of the blends. Apart from the sample VTMOS-10B containing BIPB peroxide, the penetration values remained in the range of 75–78 [1/10 mm] in the series. VTMOS carries a vinyl group that is capable of radical or addition reactions under the right conditions. However, the butyl rubber matrix is predominantly composed of saturated hydrocarbon chains derived from isobutylene. The low level of unsaturation and the lack of readily available radical initiators or other reactive sites at standard processing conditions limit the possibility of grafting or crosslinking. The vinyl group on VTMOS often requires either strong radical initiation (e.g., peroxides under carefully controlled conditions) or specific coagents that facilitate bond formation [29]. Without these conditions, VTMOS may remain largely unreacted and simply act as a low-viscosity additive, slightly altering rheology but not significantly enhancing mechanical properties. The inclusion of BIPB peroxide in VTMOS-10B as a radical source led to blend degradation, softening the blend and increasing its penetration value compared to VTMOS-10.

The introduction of APTES and GLYEOS silanes into the systems resulted in a significant increase in penetration. This effect was attributed to the unreacted silanes, which acted as plasticizers in the systems, leading to their softening [30,31,32,33]. This effect was evidenced by an increase in penetration from 76 to 79 [1/10 mm] for the APTES-containing systems and from 75 to 80 [1/10 mm] for the GLYEOS-containing system.

3.2. Peel Resistance

Peel resistance at a 180° angle is a typical test used to determine the mechanical properties of an adhesive layer. In the case of sealants used for moisture or corrosion protection, cohesive failure in the sealant layer is desirable. The results of the tests are significantly influenced by both the temperature and the peel rate due to the viscoelastic nature of pressure-sensitive materials. Under standard conditions, a peel rate of 300 mm/min allows for the evaluation of the material’s adhesion to the surface, while a peel rate of 10 mm/min typically leads to cohesive failure. The adhesive failure modes recognized in conducted test specimens were “adhesive”, where a clean separation of the adhesive from the adherend was observed; “cohesive”, where the failure was in the adhesive layer; or “mixed”, where both adhesive and cohesive failure was observed (

Table 2. Results of failure mode.

	Failure Mode *
Modifier/Content [%]	0	1	2.5	5	10	10 + 0.1 B
MPTES	A	A	A	A	A/K	A/K
GLYEOS	A	A/K	A/K	A/K	A/K	A/K
VTMOS	A/K	A/K	A/K	A/K	A	A
APTES	A/K	A	A	A	A/K	-

* Adhesive (A); cohesive (K).

).

Figure 3 and Figure 4 illustrate cohesion changes based on modifier content. The addition of MPTES in the MPTES-1, MPTES-2.5, and MPTES-5 systems resulted in a decrease in peel adhesion values by 10 N/24 mm, 5 N/24 mm, and 2 N/24 mm, respectively, compared to MPTES-0. However, the introduction of the silane did not significantly affect the adhesive properties of the blends, which still exhibited adhesive failure. In contrast, cohesion, measured at a separation rate of 10 mm/min, increased by 27% for the MPTES-1 system and 38% for the MPTES-2.5 system. Water immersion tests, however, did not show an improvement in cohesion compared to the reference sample, as adhesive failure at the steel interface resulted in a low value of 0.68 N/24 mm for the MPTES-1 sample. The addition of 10 PHR MPTES to the MPTES-10 and BIPB samples changed the fracture mode to a mixed failure mode. In the MPTES-10B blend, which contained a radical source (BIPB), cohesion increased significantly, particularly after water immersion, suggesting that the presence of BIPB may have promoted the formation of stronger internal bonds and improved the material’s properties under wet conditions.

The addition of VTMOS did not have a significant impact on the properties of the blends. However, comparing the values of cohesion for VTMOS-10 and VTMOS-10B, the matrix degradation caused by peroxide led to a 50% decrease in cohesion, as evident in Figure 4B. The addition of GLYEOS at concentrations of 0–5 PHR did not significantly alter the cohesion of the blends; however, the 10 PHR addition resulted in a decrease in cohesion. Epoxides generally react with nucleophiles (such as amines or carboxylic acids) or under strongly acidic or basic conditions to open the epoxide ring and form covalent bonds [34,35]. In a primarily hydrocarbon-based butyl rubber matrix, suitable reactive species are scarce. The matrix lacks polar functional groups or strong nucleophiles under tested conditions. Without an appropriate curing agent or elevated temperatures specifically designed to promote epoxide ring-opening reactions, the glycidyl group remains dormant, preventing GLYEOS from integrating into the polymer network. Both VTMOS and GLYEOS, under this study’s given conditions, lacked the necessary chemical reactivity pathways to effectively bond with the butyl rubber matrix. VTMOS’s vinyl group and GLYEOS’s epoxy group were not readily reactive with the saturated IIR/PIB backbone, especially in the absence of a strong nucleophile, suitable catalyst, or radical initiator designed for their particular functionalities. As a result, these silanes remained mostly unreacted within the matrix, functioning as mild plasticizers rather than crosslinking agents and providing no substantial improvement in mechanical or cohesive properties.

3.3. Melt Volume Rate and Density

The Melt Volume Rate (MVR) is a simple measure used to assess the processing properties of butyl sealants. Due to the sealant’s inherent tackiness, determining the mass flow rate can be challenging. The MVR measurement helps determine whether a sealant has the appropriate viscosity for processing by a given method. Butyl sealants intended for processing using screw extruders typically have an MVR in the range of 40–150 mL/10 min (5 kg/140 °C), whereas processing with barrel extruders is feasible when the MVR is greater than 50 mL/10 min (2.16 kg/140 °C). The typical viscosity of sealants extruded from barrels is in the range of 50,000–150,000 mPas. The MVR can be converted to viscosity [36,37].

Butyl-based pressure-sensitive sealants typically contain fillers in the range of 30–70% by weight. Fillers generally serve as thickeners or reinforce the blends [15]. Density testing is a simple and quick method that allows for estimating the degree of filler content in a blend. Due to the high tackiness, an easy method is the displacement density measurement. By knowing the density of the individual components of the blend, the theoretical density of the blend can be estimated [38]. Figure 5 illustrates MVR changes depending on modifier content.

The addition of MPTES up to 2.5 PHR to the investigated systems did not significantly affect their processing properties, as evidenced by the stability of the MVR, which ranged from 76 to 78 cm³/10 min for samples MPTES-0 to MPTES-2.5. Only when the MPTES content was increased to 5 and 10 PHR was a noticeable rise in MVR observed, which indicated the plasticizing effect of silane. A similar trend was observed for the GLYEOS and APTES silanes: up to a concentration of 2.5 PHR, changes in MVR were negligible, but, at higher contents of functional silanes, this parameter increased. In particular, for APTES-10, a significant jump in the melt flow rate was recorded, reaching 109 cm³/10 min, which represented a 56% increase compared to APTES-0. In contrast, the addition of VTMOS did not cause significant changes in the MVR, which remained constant across all samples in this series. However, the introduction of BIPB peroxide as a radical source led to polymer blend degradation, as evidenced by an increase in the melt flow rate. This effect was observed in every tested system: for MPTES-10B, the MVR increased by 23% compared to MPTES-0; for GLYEOS-10B, the MVR rose by 69% compared to GLYEOS-0; and for VTMOS-10B, the increase was as high as 80% relative to VTMOS-0.

Table 3. Density changes depending on modifier content. Sample compositions in Table 1.

	Density [g/cm3]
Modifier/Content [%]	0	1	2.5	5	10	10 + 0.1 B
MPTES	1.44	1.44	1.44	1.44	1.43	1.44
GLYEOS	1.43	1.43	1.44	1.43	1.43	1.43
VTMOS	1.43	1.43	1.43	1.43	1.43	1.43
APTES	1.43	1.44	1.44	1.44	1.43	-

summarizes the density changes depending on the modifier content. Density measurements were performed using Archimedes’ method, which offers remarkable accuracy despite its simplicity. The sample’s weight was first measured using an analytical balance, followed by measuring the weight of the displaced liquid while the sample was fully submerged. This allowed for the precise calculation of the sample’s volume. The density was determined with an accuracy of 0.01 g/cm³. Based on the obtained results, it can be concluded that the density values for the mixtures fell within the range of 1.43–1.44 g/cm³.

3.4. Mooney Viscosity and Relaxation Testing

A related method for studying the rheology of blends is the measurement of viscosity, elasticity, and relaxation rate using a Mooney viscometer. This test, traditionally used to evaluate the processing properties of rubber compounds, can, however, be easily adapted to assess thermoplastic sealant blends. Although values exceeding 100 MU are generally considered high, no rotor or wall slippage was observed during the tests. The elasticity ratio (R′) and relaxation rate (k) were obtained from Mooney relaxation measurements (

Table 4. Elasticity R′ and relaxation rate k depending on modifier content.

	R′
Modifier/Content [%]	0	1	2.5	5	10	10 + 0.1 B
MPTES	1.40	1.42	1.46	1.43	1.38	1.40
GLYEOS	1.35	1.32	1.36	1.37	1.41	1.43
VTMOS	1.39	1.33	1.26	1.3	1.32	1.27
APTES	1.46	1.33	1.37	1.39	1.51	-
	k (1–10 s)
MPTES	0.1971	0.1989	0.2004	0.1943	0.2053	0.2036
GLYEOS	0.1967	0.1982	0.1942	0.1982	0.2059	0.2180
VTMOS	0.1957	0.1939	0.1982	0.1997	0.1985	0.2238
APTES	0.1912	0.1893	0.2036	0.2025	0.2067	-

). R′ indicates the degree of elastic recovery after deformation, while k represents the speed at which stored stress relaxed. Higher R′ values suggested more elastic, network-like structures, whereas higher k values indicated faster stress dissipation and more fluid-like behavior. A description of the measurements and calculations of elasticity ratio R′ and relaxation rate k can be found in [39,40]. Figure 6 shows the Mooney viscosity ML changes depending on the modifier content. The addition of MPTES up to 2.5 PHR did not significantly affect the processing properties of the blends as the Mooney viscosity for samples MPTES-0 to MPTES-2.5 varied between 146.5 and 153.5 MU. As the concentration of MPTES in the systems increased, a noticeable reduction in Mooney viscosity was observed compared to the reference sample (MPTES-0). The measured Mooney viscosity values for MPTES-5 and MPTES-10 were 141.1 and 134.8, respectively, indicating a progressive decrease with higher MPTES content. In the presence of radicals in sample MPTES-10B, the IIR chain may have been grafted with MPTES and partially crosslinked during processing, as the viscosity increased to 138.1 compared to 134.8 MU for sample MPTES-10 and the R′ elasticity increased from 1.38 to 1.4. At higher MPTES contents (5 and 10 PHR), a noticeable decrease in Mooney viscosity was accompanied by slight changes in R′ and k. When a radical source was introduced (MPTES-10B), R′ slightly increased again and relaxation slowed compared to the non-radical sample. In systems containing GLYEOS, APTES, and VTMOS, a similar decrease in viscosity was observed with increasing silane concentration. GLYEOS-10 exhibited a reduction of 21.4 compared to GLYEOS-0, APTES-10 showed a decrease of 21.24 relative to APTES-0, and VTMOS-10 demonstrated a decrease of 10.7 in comparison to VTMOS-0. The R′ rate of GLYEOS samples decreased at a 1 PHR concentration but gradually increased to a maximum of 1.43 for samples containing 10 PHR and a radical source. The relaxation rate remained stable at 0–5 PHR concentrations but increased at 10 PHR. The introduction of VTMOS reduced the R′ of the samples to a minimum of 1.26 at a 2.5 PHR concentration.

4. Conclusions

The addition of functionalized silanes can enhance the cohesion of sealants, provided that their functional groups react with the matrix and crosslink under the influence of moisture. Although butyl rubber exhibits low gas and moisture permeability, crosslinking effects became more prominent after extended immersion (24 h). This underscores the importance of water or humidity exposure for driving silane functionality, especially when targeting improved long-term properties such as peel resistance and cohesion. In the presence of radicals, silanes may act as coagents, facilitating the attachment of silane molecules to the butyl rubber chain, thereby increasing cohesion. While silane-induced crosslinking can raise cohesion and peel resistance, radical-mediated degradation processes occurring simultaneously tend to reduce molecular weight and compromise mechanical properties. The net effect depends on which reaction—crosslinking or degradation—dominates under specific formulation and processing parameters. The chain degradation process is associated with the reduction in molecular weight, which, in turn, is evident as reduced peel adhesion at a 10 mm/min peel rate. Unreacted silanes in the sealant matrix serve as plasticizers. This behavior is evidenced by higher melt flow rates (MVRs) and lower Mooney viscosity, highlighting how even low concentrations of silanes can alter a sealant’s processability. Systems without radical sources exhibited few significant changes, implying that the matrix itself (IIR/PIB) did not strongly interact with the silanes unless a radical pathway was present.

The findings suggest that partial silane crosslinking is a promising route to balance cohesive strength and processability in butyl-based systems. By judiciously controlling silane type and concentration—along with radical initiators—manufacturers can tailor sealant formulations for higher-performance end uses. However, excessive silane content can lead to over-plasticization or competing degradation, underscoring the need for formulation optimization. The absence of significant changes in the properties of blends without a radical source suggests a lack of reactivity between the blend components (IIR, PIB) and the modifiers used (MPTES, APTES, VTMOS, GLYEOS). Therefore, the PIB/IIR matrix is a convenient basis for the investigation of potential filler–modifier interactions in further studies. Overall, these findings position silane-modified butyl rubber as a viable pathway for achieving customizable levels of cohesion and tack, provided that careful consideration is given to crosslinking conditions, silane selection, and potential degradation pathways.