Influence of Acid-Modified Clinoptilolite on the Self-Adhesive Properties of Silicone Pressure-Sensitive Adhesives

The preparation of a new “eternally alive adhesive” based on silicone pressure-sensitive adhesives with clinoptilolite is presented. Neat and acid-modified (i.e., treated with sulfuric acid (VI)) clinoptilolite was used. The effect of clinoptilolite acid treatment on the adhesive properties of pressure-sensitive adhesive tapes was tested. The obtained tapes exhibited increased thermal resistance when compared to the reference tapes. Despite introducing the filler, the pressure-sensitive adhesive tapes maintained good functional properties. The new self-adhesive materials show promising implementation potential where increased thermal resistance is required.


Introduction
A primitive concept of adhesive tape, based on a starch-based paste applied on strips of cloth, was used by the ancient Egyptians [1][2][3]. However, the long history of adhesive materials, self-adhesive tape technology is a fairly new concept, originating in the mainstream history of adhesives as one of its youngest branches.
The pressure-sensitive adhesives (PSAs) are defined as a group of adhesives exhibiting significant adhesive strength and tack at room temperature. Importantly, good adhesive parameters do not result from the chemical reaction between the adhesive and the substrate [2][3][4]. The most important performance properties defining PSAs are: adhesion, tack, and cohesion. These determine the adhesive-cohesive balance between the internal cohesion of the adhesive film and its interaction with the substrate [5][6][7][8]. PSAs play an important role in everyday life and have been intensively developed over the last decade. The most common are solvent-borne PSAs [9][10][11]. These PSAs are used for connecting various materials, such as metal, paper, plastics, glass, wood, or leather. They are applied for use in labels, protective films, mounting tapes, masking tapes, advertising banners, and medical products (including patches, bandages, surgical tapes, and biomedical electrodes). These high-quality self-adhesives should exhibit a constant level of shear strength, as well as excellent resistance to aging (at room and elevated temperatures). Moreover, they should be resistant to light, oxygen, moisture, and other environmental factors, such as the high daily amplitude of temperature [2,[12][13][14][15][16]. The adhesive layer must be able to be applied on a flexible carrier (fabric, foil, paper), while maintaining a very long tack life (however, protection against dirt is required). The polymer liquid is applied on the carrier by rollers and subsequently, within a few seconds, the solvent evaporation in a drying channel and the cross-linking of the polymer chains are performed. The silicone resin was mixed with the filler and a crosslinker (1.5 wt% based on the resin weight) in toluene to obtain a homogenous composition containing 50 wt% polymer. The filler content was: 0.1, 0.5, 1, and 3 wt% (based on the resin). The composition was coated onto a polyester film (50 g/m 2 ) using a semi-automatic coater and was subsequently placed in a drying duct for 10 min at 110 • C for cross-linking. The obtained adhesive film was secured with a polyester film.

X-ray Diffraction (XRD)
X-ray diffraction (XRD) analysis was carried out to determine the crystal structure of the modified clinoptilolite. The XRD patterns were recorded by an Empyrean PANalytical X-ray diffractometer (Malvern Panalytical Ltd, Malvern, UK), with a Cu lamp used as the radiation source in the 2θ 9-34 • range with a step size of 0.026.

Fourier Transform Infrared Spectroscopy (FT-IR)
For each sample, FT-IR spectra were obtained with a Thermo Nicolet 380 (Waltham, MA, USA) spectrometer with ATR unit in the range of wavenumbers from 400 to 4000 cm −1 .

Pot Life
The pot life was defined as the time after which the viscosity increased (two or four times) when compared to the viscosity of the fresh mixture. The tests were carried out at room temperature immediately after mixing the adhesive components [31].

Adhesion
Adhesion is defined as the interaction of the surfaces of two bodies or phases. It is closely related to the forces of interfacial tension on the contact surfaces of both materials. The work test was performed according to the FINAT FTM 1 method [33].

Cohesion
Cohesion refers to the strength of the adhesive joint. Next to adhesion, it is one of the most important properties of adhesives. Temperature, type of cross-linking compounds, or thickness of the adhesive film affect the cohesion value. Cohesion was determined by the FINAT FTM 8 method. The measurement was carried out at room and elevated temperature (70 • C) [34,35].

Shrinkage
Shrinkage is defined as the reduction in the surface area of the adhesive when compared to its initial size. It is an important mechanical and functional property, especially when it can result in surface deformations. In the case of pressure-sensitive adhesives, the change in surface area is given in millimeters or percentages. The value above 0.5 mm or 0.5% exceeds the acceptable shrinkage in the technology of self-adhesive products [36].

SAFT Test
The resistance to elevated temperature was tested using the SAFT test. The measurements were performed in a similar way to those for cohesion; however, the temperature during the test increased up to 230 • C (2 • C/min) [37].
The detailed descriptions of the research methods regarding these properties (adhesion, cohesion, and tack) have been reported in previous papers [38,39].

Clinoptilolite Characterization
To enhance the compatibility between the hydrophilic filler and the hydrophobic polymer matrix, clinoptilolite was modified with sulfuric acid. This process affects the chemical composition of the filler, its roughness, and its surface energy [40] The FTIR spectra of neat and acid-modified clinoptilolite were shown in Figure 1. The broad bands between 2900 and 3745 cm −1 could be attributed to the O-H stretching vibration mode of water adsorbed in the zeolite (water molecules bound to Na and Ca in the channels and cages of the zeolite structure), intermolecular hydrogen bonds, and Si-OH-Al bridges [30]. The characteristic band at 1630 cm −1 was ascribed to the H 2 O bending vibration. The band detected at 600 cm −1 was attributed to bending vibrations between tetrahedra, specifically, double ring vibrations [41]. The band at 787 cm −1 belonged to the Si-O-Si bonds [42]. The band at 445 cm  [41]. The shifting of this band to a higher wave number was observed in the spectra of modified clinoptilolite and could be associated with an increase in the Si/Al ratio in the clinoptilolite backbone after acid modification. Moreover, for clinoptilolite etched with 0.5 M, 1 M, and 2 M acid solutions, the increased intensity of this band indicated the Si/Al ratio increase. The bands at 725, 669, 600, and 520 cm −1 were attributed to the off-frame cations in the clinoptilolite matrix [42,43]. The off-framework cations were completely removed using concentrated sulfuric acid solutions (1 M and higher). 600, and 520 cm −1 were attributed to the off-frame cations in the clinoptilolite matrix [42,43]. The off-framework cations were completely removed using concentrated sulfuric acid solutions (1 M and higher).  The XRD diffractograms of neat and modified clinoptilolite are presented in Figure 2. The peaks characteristic for clinoptilolite, according to JCPDS sheet 25

Silicone Pressure-Sensitive Adhesives
In the next step, acid-modified clinoptilolite was used as a component of silicone pressure-sensitive adhesives. Two commercial silicone resins were selected: Q2-7358 and

Silicone Pressure-Sensitive Adhesives
In the next step, acid-modified clinoptilolite was used as a component of silicone pressure-sensitive adhesives. Two commercial silicone resins were selected: Q2-7358 and Q2-7355. The physical properties (adhesion, cohesion, and tack) of the systems without the filler are shown in Table 2. The adhesive films exhibited good functional properties, with notably excellent cohesion. The effect of clinoptilolite content on the viscosity of silicone pressure-sensitive adhesive compositions is presented in Tables 3 and 4. The lowest viscosity values were noted for the neat systems. The viscosity values increased with time and filler content. Moreover, the adhesives containing filler modified with a higher concentration of acid solution exhibited higher viscosity when compared to the systems with the same filler load. This is likely caused by the increase in the number of silicon groups. In the case of the Q2-7358 resin-based compositions, all the systems exhibited gelation after 7 days; therefore, coating them was no longer possible. For the Q2-7355 resin-based systems, some of the samples exhibited gelation on the second day. Thus, such adhesive compositions could not be stored on a long-term basis, requiring real-time coating upon receipt. This tendency was reported for many adhesives containing mineral fillers [44,45].  The effect of clinoptilolite content on peel adhesion and tack is presented in Figures 3  and 4 and Figures 5 and 6, respectively. Generally, the adhesion of the systems based on the Q2-7358 resin decreased after the filler addition. In the case of the adhesives based on Q2-7355 resin, the adhesion was improved in the presence of a modified filler, and generally, the higher the acid concentration for filler modification, the higher the adhesion. This was probably due to the better ordering and tighter structure of the adhesive film [46,47]. The highest values of adhesion, about 13 and 11.5 N/25mm for Q2-7358 and Q2-7355, respectively, were noted for the systems containing clinoptilolite modified with the highest concentration of the acid solution. The maximum tack value (ca. 11.5 N) was noted for the Q2-7358 adhesives containing clinoptilolite etched with the 0.5 M acid solution, whereas for the Q2-7355 adhesive, the maximum tack value (ca. 9.5 N) was noted for the system with filler etched with the 2 M acid solution.
In Tables 5 and 6, the cohesion values at room and elevated temperature are given. For the Q2-7355 resin-based systems, nearly every composition exhibited a higher cohesion value than that required by the self-adhesive tapes production industry (above 72 h). Specifically, only in the case of the Q2-7358 resin-based adhesives did the filler content caused a decrease in cohesion value below the required standard (0.1 and 3.0 wt%).

Content of clinoptilolite [wt%]
K 0.00 K 0.01 K 0.10 K 0.50 K 1.00 K 2.00 Figure 6. Effect of clinoptilolite content on the tack of silicone pressure-sensitive adhesives based on Q2-7355 resin. The SAFT (shear adhesion failure temperature) test results are shown in Table 7. In most cases, the adhesives with the fillers exhibited a very good thermal resistance (exceeding those of the reference systems). For both resins, the test limit (>225 • C) was reached by three samples, i.e., those containing 0.1 wt% of neat filler and 0.1 wt% clinoptilolite etched with 0.01 to 0.5 M acid solutions.  The shrinkage values of the obtained adhesives are presented in Tables 8 and 9. Generally, the value of this parameter decreased with clinoptilolite content. This is due to the better alignment of the polymer net and the more compact internal structure of the adhesive film [48][49][50]. For the Q2-7358 resin-based systems, the 0.1 wt% filler content appeared to be too low to achieve the required shrinkage, i.e., below 0.5%. In the case of the Q2-7355 resin-based adhesives, a similar phenomenon was noted for 0.1 and 0.5 wt% filler content. The systems with a higher filler load exhibited a lower shrinkage than required. Interestingly, the acid-modified clinoptilolite slightly modified the shrinkage value.

Conclusions
New self-adhesive silicone adhesives including clinoptilolite were obtained. The filler was chemically modified by acid etching with sulfuric acid (VI). The prepared one-side self-adhesive tapes exhibited good functional properties, such as adhesion, cohesion, and tack. More importantly, the thermal resistance was improved when compared to that of the reference tapes.
Such materials could be used for covering installations and fireplaces, for connecting elements exposed to high temperatures in households, in hot-stamping technology, and in heavy and automotive industries. These materials also have applications in aeronautics, i.e., as a binder for solar batteries on satellite decks, and could be used in masking tapes in powder coating processes.