, Zhao JingqiOil bleed is a serious problem in elastomeric thermal silicone conductive pads. The components of the oil bleed and the effect of the silicone chemical parameters on the amount of oil bleed have been determined. The main components of oil bleeds are the uncrosslinked silicones in the cured resins, which include the unreacted silicone materials and the macromolecular substances produced by the hydrosilylation reaction. Cured resins with a high crosslinking density and a high molecular weight of vinyl silicone residues had a lower amount of oil bleed. In addition, a low Si-H content also reduced the amount of oil bleed.
Elastomeric thermal silicone conductive pads [ 1, 2] are currently used as gap fillers for device cooling owing to their good thermal performance [ 3, 4], natural tackiness [ 5], deflection property [ 6], fire retardant insulation [ 7] and good flexibility [ 8]. They are commonly used in communication, information technology, medical, automobile, electronic and mobile storage devices. These thermally conductive materials are produced by many different companies including Dow Corning, Shin Etsu and Laird. In addition to elastomeric thermal silicone conductive pads, other kinds of thermally conductive silicone rubber gap fillers like thermal silicone greases and thermal silicone rubbers have also been developed in order to meet various application requirements [ 9, 10]. These materials are all used as thermal interface materials to quickly transfer heat in order to control devices at operating temperatures that are beneficial for maintaining long lifespans [ 11, 12].
All of these thermal conductive silicone rubber gap fillers are composite materials. They usually consist of an inorganic filler and an organic silicone matrix [ 13]. Aluminum oxide, aluminum nitride, boron nitride and silicon carbide are usually used as inorganic fillers since these materials impart improved thermal conductivities [ 14‒ 19]. The surface treatment, type and size distribution of the inorganic filler affect the thermal conductivity of the material [ 20‒ 25]. Al2O3 fillers with different particle sizes have been demonstrated to be advantageous over inorganic fillers with a single particle size [ 22‒ 23]. It has also been reported that the thermal conductivity of pure silicone can be increased by about two times when a carbon nanotube array silicone composite is used [ 21]. This material has shown great promise as a thermal interfacial material.
The organic silicone matrix affects the elastic properties of the gap filler materials [ 26‒ 28]. The silicone matrix acts as an elastomeric component in the thermally conductive silicone rubber gap fillers. The silicone resins are prepared by the hydrosilylation of vinyl silicones and hydrosilicones with a Pt complex catalyst [ 29‒ 34]. A schematic diagram of the crosslinking reaction between Si-CH=CH2 (Si-Vi) and Si-H is shown in Fig. 1. The raw materials needed to produce thermal conductive silicone rubber gap fillers consist of an inorganic filler, a vinyl silicone, a hydrosilicone, a catalyst and an inhibitor. The inhibitor is generally added to control the crosslinking reaction and to produce silicone rubber gap fillers with longer storage lifetimes.
Some problems have been encountered with elastomeric thermal silicone conductive pads. Elastomeric thermal silicone conductive pads would bleed oils which can cause contamination of device surfaces as is shown in Fig. 2. These oils can then adhere to other substances in the surrounding area which would affect the conductivity of the devices and may cause many other serious problems [ 35].
In this paper, the components of the oil bleeds from elastomeric thermal silicone conductive pads have been identified. In addition, the effect of the chemical parameters of the silicones on the oil bleeds have been investigated. A simple silicone matrix model system composed of vinyl silicones, hydrosilicones, a catalyst and an inhibitor was designed. In order to simplify the oil bleeds, an inorganic filler was not used in this model system. The chemical components of the bleeding oils from these model vinyl silicone/hydrosilicone systems were determined using gel permeation chromatography (GPC) and 1H nuclear magnetic resonance (1H NMR). In addition the mechanism and process of oil bleeding was determined from the data.
Four kinds of vinyl silicones (Vi-1, Vi-2, Vi-3 and Vi-4) and three kinds of hydrosilicones (H-1, H-2 and H-3) were purchased from Zhejiang Runhe Organosilicon New Material Co., Ltd. The weight percent of the Si-CH=CH2 (Si-Vi) in the vinyl silicones and the Si-H in the hydrosilicones were supplied by Zhejiang Runhe Organosilicon New Material Co., Ltd. (Table 1). The compositions were verified by 1H NMR (Viscotek TDA 305 ) and GPC (AVANCE III 400 HZ ). The solvent for 1H NMR was CDCl3 and the solvent for GPC was tetrahydrofuran (THF).
Tab.1 The weight content of functional groups in the silicones
A Pt Karstedt catalyst (3000 ppm Pt) was purchased from Zhengzhou Alfa Chemical Co., Ltd. The 1-ethynylcyclohexanol was purchased from Tianjin Guangfu Fine Chemical Research Institute and was used as the inhibitor.
The simplified silicone matrix model system was prepared according to following steps: first the desired molar ratios of vinyl silicone and hydrosilicone were mixed. The Si-Vi : Si-H ratios are shown in Table 2. Next the inhibitor and catalyst were added and then the mixture was defoamed and cured in an oven at 120 °C for 2 h. The resulting product were the model silicone matrix pads.
Tab.2 Molar ratio of Si-Vi : Si-H for the simplified silicone matrix model system
A silicone matrix pad was weighed (denoted a), and one layer of fiberglass mat (with a total weight of b) were used to cover the pad. Then put three filter papers on each side of the fiberglass mats. Finally, the sandwich-like structure was placed into the test mold and compressed to 50% of its original thickness. The test molds containing the samples were then placed in an oven at 120 °C for 24 h to complete the oil bleeding test.
During the test, the oils bled out of the pads through the fiberglass mat and were then absorbed by the filter papers. After 24 h, the weight of the silicone pad and the fiberglass mats was recorded (denoted as c). Since the density of each silicone matrix pad was different, the amount of oil bleed was calculated by volume percent instead of weight percent using the equation:
where a is the original mass (g) of the silicone matrix pad; (a + b – c) is the mass (g) loss of the silicone matrix pad after the test; ρ1 is the density of mixed vinyl silicone and hydrosilicone system before curing and was considered to be a constant, 0.98 g/cm3 and ρ2 is the density (g/cm3) of the silicone matrix pad.
The bleeding oils were obtained from silicone matrix pads by the filter paper method. First the bleeding oils absorbed by the filter papers were extracted with CCl4 and then the solvent was evaporated to obtain the oils. The obtained oils were then analyzed by GPC using THF as the solvent and by 1H NMR (CDCl3).
The uncrosslinked components in the silicone matrix pads were extracted by CCl4 and analyzed by 1H NMR and GPC.
The vinyl silicones and hydrosilicones were analyzed by 1H NMR and GPC. Table 3 shows the Si-Vi content of the vinyl silicones and the Si-H content of the hydrosilicones. When multiplied by the molecular weight, this data is in good agreement with the data supplied by the manufacturer. The GPC results are summarized in Table 3. The total number of Si-Vi units is lowest in Vi-1 has the fewest Si-Vi content. The average molecular weight of the silicones increases in the order Vi-1>Vi-2>Vi-3>Vi-4. As the average molecular weight increased, the density of the Si-Vi groups (Table 3) decreased (Vi-1<Vi-2<Vi-3<Vi-4). For the hydrosilicones, the Si-H increased in the order H-1<H-2<H-3. H-1 with the lowest Si-H content of 10‒3 mol/g serves as only a weak crosslinker. That is why the vinyl silicones were reacted with H-1 at a Si-Vi : Si-H ratio of 1 : 1.14, but a ratio of 3 : 1 was used for H-2 and H-3.
Tab.3 Si-Vi and Si-H contents for the silicones as determined by 1H NMR and molecular weight for the vinyl silicones as determined by GPC
The oils bled form the simplified silicone matrix models were analyzed by 1H NMR and GPC. By comparing the components of the vinyl silicone and hydrosilicone raw materials with the oil bleed components, the source of the oil bleeds can be determined. In addition, the silicone matrix pads were extracted with CCl4 in order to determine the uncrosslinked components in the pads. Finally the relationship between the bleeding oils and the uncrosslinked components was explored by comparing these components.
H-1 is a hydrosilicone with a low Si-H content. When vinyl silicones are reacted with H-1 at a Si-Vi : Si-H ratio of 1 : 1.14, the Si-H amount is theoretically enough to hydrosilyate all the Si-Vi bonds in the vinyl silicone. This means that any unreacted Si-H functional groups in the cured resin should be detected by 1H NMR. The NMR spectrum of the oils bled from the cured resin formed by reacting H-1 with Vi-2 had no peaks for either Si-Vi or Si-H (Fig. 3). Since there are no vinyl groups in the bleeding oils, all the Si-Vi bonds in Vi-2 were hydrosilylated completely. The absence of Si-H peaks in the spectrum is probably because side reactions between Si-H and O2 or H2O in the atmosphere readily occur in the oils.
Fig.3 Partial 1H NMR spectra of the bleeding oils from the cured resin of Vi-2 reacted with H-1, the original vinyl silicone Vi-2, and H-1
Fig.4 GPC data of vinyl silicone Vi-2 (black line), uncrosslinked components in the cured resin (red line), and bleeding oils (blue line) from the cured resin when Vi-2 reacted with H-1
The GPC results for Vi-2, the uncrosslinked components in the cured resin, and the bleeding oils are shown in Fig. 4. The retention volume curve of bleeding oil is partly similar with that of the uncrosslinked components. Comparing with the original vinyl silicone Vi-2 (black line), the bleeding oil showed a lower retention volume corresponding to a higher molecular weight component. This component in the bleeding oil is probable owing to some macromolecular substances produced during the hydrosilylation process. While the components of low molecular weight (high retention volume) may be impurities in original vinyl silicones of Vi-2 because no Si-Vi bonds were found in the bleeding oils (Fig. 3). It is common for vinyl silicones that some impurities without Si-Vi groups are formed during their synthesis. Furthermore, the bleeding oils from H-1 with Vi-3 or Vi-4 model systems were also analyzed, and the results were similar with H-1 and Vi-2 model system.
Vi-1 has the largest moleculer weight and lowest Si-Vi content among vinyl silicones. When Vi-1 reacted with H-1 to form resin, some unreacted Si-Vi groups were detected in the bleeding oil from the cured resin (Fig. 5). From the view point of equivalent ratio, the vinyl functional groups of silicones should be fully crosslinked by Si-H in hydrosilicone. However, the vinyl functional groups of high molecular weight vinyl silicone Vi-1 (30900) could not fully react with Si-H in 2 h. Even Si-H content is enough, the vinyl functional groups of Vi-1 could not be hydrosilylated completely.
Fig.5 Partial 1H NMR spectra of bleeding oils from Vi-1 reacted with H-1, and original silicone materials
Fig.6 GPC data of vinyl silicone (black line), uncrosslinked components in cured resin (red line), and bleeding oils (blue line) from the cured resin of Vi-1 reacted with H-1
The GPC results of the bleeding oil from the cured resin of Vi-1 and H-1 model system demonstrated a similar tendency with that of Vi-2 and H-1 model system (Fig. 6). Importantly, the bleeding oil showed a higher molecular weight peak than Vi-1. Therefore, the components of bleeding oils include the macromolecules formed from H-1 and Vi-1 in hydrosilylation process. Besides, the low molecular weight components in bleeding oils and vinyl silicone Vi-1 should be the unreacted original vinyl silicones as mentioned in other model systems.
When vinyl silicones Vi-2, Vi-3 or Vi-4 reacted with H-1 at the molar ratio of Si-Vi : Si-H= 1 : 1.14, the components of bleeding oils came from the macromolecular substances produced during the hydrosilylation process, and impurities in original vinyl silicones. Furthermore, when Vi-1 reacted with H-1 at the same molar ratio (Si-Vi : Si-H), bleeding oils consist of hydrosilylated macromolecules, and the residual unreacted vinyl silicones due to low reactivity of Vi-1. Thus, bleeding oils should be considered as uncrosslinked substances in the cured resins including already existing in original vinyl silicones and formed silicones during hydrosilylation process.
In this study, H-2 or H-3 reacted with vinyl silicones at a high molar ratio of Si-Vi : Si-H (3 : 1). The vinyl functional groups of vinyl silicones were significantly excessive compared with Si-H groups. Thus H-2 could react completely with vinyl groups. The Si-Vi functional groups of the bleeding oils were observed in 1H NMR. Their chemical shifts were similar with the original vinyl silicone Vi-1 (Fig. 7). Furthermore, the bleed oils and the uncrosslinked components are approximately similar with the original vinyl silicone (Fig. 8). The uncrosslinked vinyl silicones in the cured resins were the source of oil bleeds in this model system. When Vi-1 reacted with H-3 at the same ratio of Si-Vi : Si-H= 3 : 1, the analysis results are similar with the Vi-1 and H-2 model system. This demonstrated the probable source of bleeding oil components is the uncrosslinking vinyl silicones in the model systems with a Si-Vi : Si-H of 3 : 1.
Fig.7 Partial 1H NMR spectra of bleeding oils from cured resin of Vi-1 reacted with H-2, original silicone Vi-1 and H-2 (Si-Vi : Si-H of 3 : 1)
Fig.8 GPC data of vinyl silicone Vi-1 (black line), uncrosslinked components in cured resin (red line), and bleeding oils (blue line) of cured resin of when Vi-1 reacted with H-2
The crosslinking density of cured resins in elastomeric thermal silicone conductive pads has been generally considered to have a significant effect on the amount of oil bleed. The molar ratio of Si-Vi : Si-H= 1 : 1.14 was used to investigate the effect of crosslinking density of the silicone matrix on oil bleed. The amount of Si-H functional groups of H-1 was sufficient for the hydrosilylation of vinyl silicones, thus the vinyl silicones should be crosslinked to form crosslinked resins. The crosslinking density acts as a main factor to affect oil bleed. From average molecular weight per Si-Vi in vinyl silicones (Table 3), it could be concluded that the order of crosslinking density of cured resins is Vi-1<Vi-2<Vi-3<Vi-4. The corresponding results of oil bleed were shown in Fig. 9 in red color, suggesting the tendency order is Vi-1>Vi-2>Vi-3>Vi-4. It is clear that high crosslinking density of cured resins could lead to a low amount of oil bleeds.
Fig.9 Effect of crosslinking density on oil bleed when vinyl silicones reacted with H-1 at the ratio of Si-Vi : Si-H= 1 : 1.14
When vinyl silicones reacted with H-2, the molar ratio of Si-Vi : Si-H= 3 : 1 was used to remain parts of vinyl silicones unreacted in order to investigate the effect of number-average molecular weight on oil bleed. The results of oil bleed are shown in Fig. 10. It is obvious to see that the cured resins prepared from high molecular weight of vinyl silicones bled a low amount of oils.
Fig.10 Effect of molecular weight of vinyl silicones on oil bleed when vinyl silicones reacted with H-2 at the ratio of Si-Vi : Si-H= 3 : 1
When vinyl silicones and hydrosilicones reacted in the molar ratio of Si-Vi : Si-H= 3 : 1, the reaction degree of vinyl functional groups of vinyl silicones should be same in each cured resin. Among the vinyl silicones of Vi-1, Vi-2, Vi-3 and Vi-4, the order of average molecular weight per Si-Vi functional group is Vi-1>Vi-2>Vi-3>Vi-4, so the crosslinking density in these model systems is Vi-1<Vi-2<Vi-3<Vi-4. As we mentioned in above discussion, high crosslinking density benefits for low oil bleed. Therefore, the oil bleed from the four cured resins should be in following order: Vi-1>Vi-2>Vi-3>Vi-4. However, the results were opposite (Fig. 10), which indicated that the crosslinking density is not the sole reason for oil bleed if the cured resins have uncrosslinked silicones. The movement of uncrosslinked silicones in the cured resins should also affect the oil bleed. If the silicone has a high molecular weight, its movement and diffusion abilities are relative low. Thus, it is probably difficult to bleed out of the cured resin if the molecular weight is higher than a critical value. This means that the cured resins prepared from high molecular weight silicones with an excessive content of Si-Vi will bleed low oil amount.
In conclusion, the molecular weight of original silicones also plays an important role on oil bleed. Furthermore, this effect is even stronger than the crosslinking density. High molecular weight of residue uncrosslinked silicones in the cured resins could reduce the amount of oil bleed.
Fig.11 Effect of molecular weight of polydimethylsiloxane (PDMS, 1%) on oil bleed
In addition, we investigated the effect of different molecular weight of polydimethylsiloxane (PDMS) as a model polymer to simulate unreacted silicones on oil bleed. In these tests, PDMS with three different molecular weights were added into the same model system, respectively. The results were shown in Fig. 11. The cured resins prepared from high molecular weight of silicone resins showed low oil bleed, on the other hand, the cured resin from low molecular weight of silicone resins should bleed high oil. But the trend was not obviously as Fig. 10. It is due to the low content of PDMS (1%) in the thermal silicone model system, so the effect of molecular weight is relatively low.
When vinyl silicones reacted with H-2 or H-3, the amount of oil bleed in H-2 and Vi-1 model system was approximately similar with H-3 and Vi-1 model system in a molar ratio of Si-Vi : Si-H= 3 : 1 (Fig. 12). H-2 or H-3 and Vi-2 model systems showed similar result. While, for H-2 or H-3 and Vi-3 model systems, the amount of oil bleed of H-2 and Vi-3 model system was obviously lower than H-3 and Vi-3 model system. Vi-4 model systems also showed significant difference in oil bleed. It can be concluded that when Vi-3 or Vi-4 reacted with different Si-H content silicones, H-3 with high Si-H content induced a larger amount of oil bleed from the cured silicones. Because this hydrosilicone has high steric hindrance in hydrosilylation. It is difficult for Si-H functional groups fully crosslinked with vinyl functional groups. Thus, some Si-H functional groups remained unreacted in cured resins. Meanwhile, more original vinyl silicones remained in cured resins, finally resulted in a higher amount of oil bleed. The different oil bleed between H-2 and H-3 when reacted with Vi-4 is also probable due to the steric hindrance. Vi-4 having high Si-Vi density should cause relatively high steric hindrance compared with Vi-1, Vi-2 and Vi-3 vinyl silicones.
Fig.12 Effect of hydrosilicones on oil bleed from cured resins when different vinyl silicones reacted with H-2 and H-3 at a molar ratio of Si-Vi : Si-H= 3 : 1
The components of bleeding oil were the uncrosslinked substances in the cured resins, such as macromolecular substances produced during the hydrosilylated process, unreacted vinyl silicones and some impurities without Si-Vi bonds in organic silicones. Furthermore, the amount of oil bleed is closely related to crosslinking density, number-average molecular weight of silicones and Si-H content of side Si-H hydrosilicones. High crosslinking density, high molecular weight of unreacted silicones in cured resins and low Si-H content of hydrosilicones could decrease the oil bleed.
The authors have declared that no competing interests exist.
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