Preparation of one-component addition-cure liquid silicone rubber coating with enhanced storage stability and bond strength

It is of great interest and challenge to simultaneously improve the storage stability and bond strength of one-component addition-cure liquid silicone rubber (OLSR) coating for electronic circuit board. In this work, we proposed an efficient approach to address this issue by incorporating both cyano-modification platinum catalyst (CN–PT) and urethane-containing poly(hydromethylsiloxane) (N-PHMS) into OLSR coating using a mechanical mixer. The CN–PT and N-PHMS were synthesized through thermal polymerization reaction and hydrosilylation reaction, respectively. The OLSR coating containing 0.3 phr CN–PT and 3 phr N-PHMS showed the best properties, with a tensile bond strength of 1.63 MPa, a storage period > 180 d at 25 °C, and a curing rate < 2 h at 80 °C. The salt-spray resistance and continuous power-on performance of OLSR coating were also improved, compared with pure OLSR coating. The synergism of CN–Pt and N-PHMS on the enhancement of both the storage stability and bond strength of OLSR coating was also explored. Our findings exhibited great potentials for fabricating OLSR coating with excellent long-term stable service performance including high storage stability and protective ability.


Introduction
Printed circuit boards (PCBs) are the heart of the communication and electronics industry. Most of time, PCBs are often exposed to humidity leading to the corrosion of printed circuit boards and subsequent failure of the electronic devices [1][2][3][4][5]. Currently, electrically conductive adhesives [6], nanosized Ag paste [7], and carbon nanotubes [8] are the most commonly used packaging materials in the electronics industry. However, traditional packaging materials are not suitable for modern electronic packaging of printed circuit boards because of high conductivity and cost. In general, silicone rubber coating is widely used in protection of PCBs because of its good dielectric properties, water repellency, ultraviolet durability, and excellent chemical and thermal degradation resistance [9][10][11]. Nowadays, solvent-based condensation cure silicone rubber coating [12] for the protection of PCBs is one of the few commercialized coatings. During the curing process of solvent-based condensation cure silicone coating, the solvents and small molecules will be released leading to the pollution of the environment and the PCB [13], even the reduction of the reliability and the service life of electronic products.
One-component addition-cure silicone rubber (OLSR) coating has the advantages of low shrinkage, the ability to adjust reaction speed and conditions over a wide range, and the fact that no byproducts or degradation products are formed [14][15][16][17][18]. It can obviously solve the problem caused by the released solvents and small molecules for solvent-based condensation cure silicone rubber. Although the utility of OLSR coating was accepted, it was not often well satisfied with their applications in the fields of the protection of PCBs due to its storage instability and poor bond strength. Therefore, how to improve the storage stability and bond strength of OLSR coating has become an important direction for the researchers in the field of protection of PCBs.
Thermal-latent hydrosilylation catalyst is crucial for the enhancement of the storage stability of OLSR coating. A typical series of thermal-latent hydrosilylation catalysts consisting of H 2 PtCl 6 and special polymers was developed by Katsuhiko research group [19]. These catalysts were successfully applied in the curing process of silicone resin with high thermal-latent characteristics. Many functional groups including propargyl [20], amine [21], 1-phenylethylisocyanide [22], and isocyanide [23] often caused the poison of the platinum catalysts. However, most of these thermal-latent hydrosilylation catalysts are used at a high curing temperature for addition-cure liquid silicone rubber (the curing temperature [ 100°C) and a low storage temperature (\ 10°C). They are not suitable for OLSR coating obviously.
For enhancing the bond strength of OLSR coating, an effective way is to incorporate an adhesion promoter on OLSR coating to make it self-adhering when cured on the surface of the substrate. There are a lot of reports devoted to the studies of adhesion promoters. Examples include organosilanes with silylene group as adhesion promoters to adhere silicone elastomers on polyethylene [24]; organosilanes with epoxy group enhancing the adhesion on polyethylene terephthalate [25][26][27]; organosilanes with phenyl group and epoxy group enhancing the adhesion on electrolyte membrane [28]; organosilanes with ester groups and vinyl groups enhancing the adhesion on polycarbonate [29,30]; organosilanes with tetra-tertbutyl titanate enhancing the adhesion on copper, polybutylene terephthalate, polyphenylene sulfide, and polystyrene [31]; organosilanes with allyl group enhancing the adhesion to nylon [32]; organosilanes with isocyanurate group enhancing the adhesion on epoxy resin and phenolic resin [33]. However, there are few reports on the addition of adhesion promoter on the addition-cure silicone rubber to enhance adhesion to PCBs.
In this work, cyano groups will be introduced to platinum catalyst (CN-PT) for enhancing the storage stability, and urethane-containing poly(hydromethylsiloxane) (N-PHMS) will be synthesized through hydrosilylation as an adhesion promoter for enhancing the bond strength. Fourier transform infrared spectroscopy (FT-IR) will be used to characterize the chemical structure of CN-PT and N-PHMS. The mechanical properties and curing rate of OLSR coating will be tested in order to obtain optimum content of N-PHMS and CN-PT. The properties including storage stability, bond strength, surface morphology, and protective performance of the OLSR coating will be tested to evaluate the advantages of this combination-designed OLSR coating for PCBs.
2.2 Synthesis of the cyano-containing platinum catalyst (CN-PT) CN-PT was synthesized based on a one-step thermal polymerization reaction [21], as shown in Fig. 1. This reaction was achieved by mixing 100 g of the commercially bought platinum catalyst (Pt) with 200 g of ethyl acetate in a 500-mL four-necked round-bottomed flask equipped with a mechanical stirrer. The mixture was stirred for 0.5 h at 60°C, and then acrylonitrile (AN, 2 g, 4 g, 6 g, 8 g, 10 g) were added dropwise. Thereafter, the reaction mixture was stirred for another 6 h at 80°C before being distilled under vacuum in order to obtain the cyano-containing platinum catalyst (CN-PT). Figure 2 shows the FT-IR spectra of CN, and CN-PT.

Synthesis of urethane-containing poly(hydromethylsiloxane) (N-PHMS)
The two-step hydrosilylation reaction developed herein was used to synthesize the N-PHMS adhesion promoter. First, hydroxyethyl acrylate (HEA, 10 g), ethyl acetate (200 g), and the platinum catalyst (0.2 g) were mixed in a 500-mL four-necked round-bottomed flask for 0.5 h at 60°C. Subsequently, poly (hydromethylsiloxane) (PHMS-1, 100 g) was added dropwise to the mixture, followed by stirring for 3 h at 60°C and then heating to 90°C (3 h). Afterward, 3-isocyanatopropyltriethoxysilane (ICPTES, 21.3 g) was added to the flask in a dropwise manner, and the reaction was continued for another 3 h. Finally, reduced pressure distillation was applied to remove ethyl acetate and isolate the synthesized N-PHMS product. The synthetic scheme is shown in Fig. 3, and Fig. 4 presents the FT-IR spectra of HEA, PHMS and N-PHMS.

Preparation of the OLSR samples
VPDMS, PHMS-2, CN-PT, N-PHMS, 1-ethynylcyclohexanol, and Karstedt's catalyst were uniformly mixed using an electric mixer, and then they were vulcanized at the appropriate temperature in order to obtain the OLSR coating. CN-PT and N-PHMS were uniformly dispersed in the OLSR coating. The proportions of materials used to prepare different samples of OLSR coating are listed in Table 1.

Fourier transform infrared (FT-IR) spectrometry
Liquid OLSR samples were coated on the surface of KBr slice for IR absorption measurement using an Avatar 370 FT-IR spectrometer (Thermo Nicolet, American). The wavelength range, resolution, and number of scans were set to 4000-400 cm -1 , 4 cm -1 , and 16 cm -1 , respectively.

Mechanical properties
The tensile strengths of the OLSR coatings were measured using a universal testing machine (HD-WDW-100A, Xiamen HaiDa Inc., China), according to ISO 37-2005. The Shore A hardness of the samples was assessed using a Shore A durometer (LX-A, Shanghai Liuling Instrument Factory, China), in accordance with ISO 7619-1-2004.

Tensile bond strength testing
The tensile bond strengths of the OLSR coatings were measured using a universal testing machine (HD-WDW-100A, Xiamen HaiDa Inc., China), according to ASTM D 3164-03 protocols, respectively.

Time-viscosity testing
The viscosity of different OLSR coating samples was monitored throughout the curing process in order to examine the curing rate. The measurements were performed using a rotating viscometer (NDJ-8S, Shanghai Jitai Inc., China), in accordance with ASTM D 1084-16.

Surface morphology
After tensile shear failure, the morphology of the failure surface was analyzed by field emission-scanning electron microscopy (FE-SEM, EVO-18, Carl Zeiss AG, Germany) and energy-dispersive spectrometer (EDS, Oxford Inca 250, Oxford Instruments Co., Britain). The surface tension of the OLSR samples was measured using a Spinning drop interface tensiometer (TX500, Kino, China).

Salt-spray testing
Salt-spray tests were performed on PCBs coated with OLSR coating (300 h), according to the ASTMB 117 protocol.

Continuous power-on testing
Continuous power-on tests were carried out on samples of bluetooth headset circuit boards coated with OLSR coating, as shown in Fig. 5. Brine was used as liquid medium, and the time of continuous power-on of the PCBs was recorded.
3 Results and discussion 3.1 ATR-FTIR analysis

CN-PT
The cyanide group in acrylonitrile was grafted onto the platinum catalyst in order to improve the storage stability of the OLSR coating material. To ensure that the grafting process was successful, the CN-modified catalyst was characterized by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy in the range of 400-4000 cm -1 . The CN-PT spectrum illustrated in Fig. 2 shows absorption peaks at 1715 cm -1 and 1625 cm -1 corresponding to the vibrations of carbonyl and C-N (primary amide) groups, respectively. The 3074 cm -1 and 2232 cm -1 peaks observed in the spectrum of acrylonitrile are characteristic of the vinyl and nitrile groups, respectively, and they are not evident in the CN-PT spectrum. These results indicate that nitrile was successfully transformed to a primary amide by hydrolysis reaction, and that the primary amide is well grafted onto the platinum catalyst by vinyl addition reaction. Thus, the storage stability of the OLSR produced using the CN-PT catalyst is expected to be improved.

N-PHMS
Addition-cure silicone is nonpolar in nature. Therefore, to enhance the adhesion of this material on PCBs, the hydromethylsiloxane used in the curing process was grafted with ICPTES. The modified (N-PHMS) and non-modified (PHMS) hydromethylsiloxane samples were characterized by ATR-FTIR in the range of 400-4000 cm -1 . N-PHMS presents peaks at 1706 cm -1 , 1524 cm -1 , and 1363 cm -1 corresponding to carbonyl, N-H bond, and C-N bond vibrations in a secondary amide, respectively. These peaks are not evident in the PHMS spectrum. Meanwhile, the characteristic peaks of HEA (3600-3200 cm -1 ) was not found. Thus, we may conclude that HEA was successfully grafted with PHMS by hydrosilylation reaction. And besides, the hydroxyl was successfully transformed to secondary amide by reaction.

Effect of N-PHMS on the mechanical properties of OLSR
The effect of N-PHMS on the mechanical properties of OLSR coating, namely tensile strength and Shore A hardness, was analyzed. The obtained results illustrated in Fig. 6 (Fig. 7). This results in the formation of an interpenetrating network structure that is closely packed and thus, has a larger cross-linking density, compared with the original material. Within a certain range of cross-linking density values, tensile strength increases strongly with increasing N-PHMS content. However, when the density exceeds a certain limit, the tensile strength begins to decrease. But the Shore A hardness continues to decrease with increasing N-PHMS content, due to the introduction of more branches. Overall, the results demonstrate that the optimum N-PHMS content is 3%.

Effect of CN-PT on the curing rate of OLSR coating
To evaluate the activities of CN-PT catalysts synthesized using different concentrations of acrylonitrile, the viscosity of the OLSR coatings prepared using these catalysts was monitored throughout the hydrosilylation process in order to assess the efficiency of VPDMS and PHMS conversion. The initial (0% conversion) and final (nearly 100% conversion) viscosity values of the reaction mixture are 1000 cps and 100,000 cps, respectively. Since the conventional storage and curing temperatures of OLSR coating are 25°C and 80°C, the evolution of viscosity as a function of time was monitored at these temperatures. The results depicted in Figs. 8 and 9 demonstrate that the curing rate of OLSR coating decreases with increasing acrylonitrile concentration (2-10%), irrespective of temperature. At 25°C and acrylonitrile concentration C 6%, OLSR coating could not be cured, even after 180 days (Fig. 8). However, at 80°C, the OLSR coating is cured within only 2 h, using CN-PT catalysts prepared with acrylonitrile concentrations B 6% (Fig. 9). These results can be explained based on the electron adsorption theory. Briefly, when the electrons in the p bond of the amide group enter the d orbital of Pt, a strong adsorption bond is formed between acrylonitrile and the platinum catalyst. This bond inhibits the efficiency of the hydrosilylation process, thereby suppressing the curing rate of OLSR. At elevated temperatures (80°C), the adsorption bond dissociates, and the catalytic rate of hydrosilylation increases. Overall, our results demonstrate that the activity of CN-PT catalysts may be controlled simply by varying the concentration of acrylonitrile (higher concentrations lead to lower curing rates), which is consistent with previously reported data [18]. Considering that the concentration of 6% affords both, a long storage period ([ 180 days at 25°C) and a high curing rate (cured within 2 h at 80°C) of OLSR coating, it is taken to be the optimum concentration of acrylonitrile.

Properties of different OLSR samples
Four samples of OLSR coating were prepared in this study using different proportions of starting materials, as shown in Table 1. The properties of these samples were analyzed and compared, as discussed below.

Curing rate analysis
As shown in Figs The effect is more important in OLSR-2 than in OLSR-3, due to the fact that the amine moiety in CN-PT attaches to Pt directly via a coordination bond, while N-PHMS does not. As a result, the curing rate of OLSR-2 is less than that of OLSR-3. OLSR-4 exhibits the lowest curing rate, possibly due to synergy between CN-PT and N-PHMS.

Surface tension analysis
As shown in Fig. 13

SEM-EDS analysis
Considering the similarity between OLSR-1 and OLSR-2 and between OLSR-3 and OLSR-4, the surface morphologies of PCB samples coated with either OLSR-1 or OLSR-4 were analyzed by SEM-EDS. The FE-SEM images presented in Fig. 14 show that the OLSR-1-coated sample exhibits an almost pure interface failure with no remains on the coating surface. This indicates that the bonding strength between OLSR-1 and the PCB boards is poor. Meanwhile, interface and cohesive mixing failures are observed for the OLSR-4-coated sample, with These results indicate that the PCB boards are more strongly bonded to OLSR-4 than to OLSR-1, which is consistent with the bonding strength data.

Salt-spray test analysis
The corrosion protection effects of OLSR samples were assessed in a static mode using salt-spray tests. The results presented in Fig. 16 demonstrate that after 300 h, the OLSR-1 and OLSR-2 coatings of PCBs become detached due to adhesion bond damage. However, OLSR-3 and OLSR-4 remain strongly bonded to the PCB surfaces. Therefore, we may conclude that compared with CN-PT, N-PHMS is much more effective in enhancing the corrosion protection properties of OLSR.

Continuous power-on test analysis
The dynamic of corrosion protection properties of OLSR coatings was investigated via continuous power-on testing. The obtained results are illustrated in Fig. 17, and they show that the duration of continuous power-on in brine decreases in the following order:

Possible mechanism of CN-PT/N-PHMS synergism
As seen above, the storage stability and bond strength of OLSR coating are most effectively enhanced by combining CN-PT and N-PHMS. Thus, it is expected that these two entities may interact synergistically. Figures 18 and 19 present the proposed mechanism of synergistic CN-PT/N-PHMS effects in improving storage stability and corrosion  resistance of OLSR coating, respectively. This mechanism suggests that unlike CN-PT (OLSR-2), the amide of N-PHMS does not undergo chemical bonding with the platinum catalyst, which leads to a weakened inhibition effect (OLSR-3). However, the hydrogen bonds formed between the secondary amide groups of different N-PHMS molecules allow for indirect bonding between N-PHMS and the platinum catalyst, resulting in the depression of the curing rate at higher N-PHMS content. When CN-PT and N-PHMS are added together, hydrogen bonds are formed between the secondary amide of N-PHMS and the primary amide of CN-PT, resulting in a significantly improved interface of OLSR adhesion on PCBs. This enhances the bond strength and corrosion resistance of the OLSR coating.

Conclusion
In summary, OLSR coating featuring outstanding storage stability and bond strength was successfully prepared by incorporating CN-PT and N-PHMS. Based on viscosity tests, the concentration of 6% was taken to be the optimum concentration of acrylonitrile, with the ability to store the material for more than 180 days at 25°C and cure it with PCB within 2 h at 80°C. As for N-PHMS, it enhances the bond strength of OLSR coating by increasing the surface tension of the material and promoting the interfacial reaction of reactive groups. Tensile strength and Shore A hardness assessments indicate that the optimum content of N-PHMS is 3%. The combination of CN-PT and N-PHMS synergistically reduces the curing rate and further enhances storage stability via hydrogen bonding on the PCB surface. It also could significantly improve the salt-spray resistance and continuous power-on performance of OLSR coating. Our findings exhibited great potentials for fabricating OLSR coating with excellent long-term stable service Fig. 18 Possible mechanism of CN-PT and N-PHMS synergistic effects in enhancing the storage stability of OLSR coating Fig. 19 Possible mechanism of CN-PT and N-PHMS synergistic effects in enhancing the corrosion resistance of OLSR coating performance including high storage stability and protective ability. But the application of OLSR coating will be limited in occasion when the heating is not allowed.