Silicone Fluids

Silicone Fluids

Silicone oils, or polydimethylsiloxanes, are basic constituents in most silicone-based applications. The most common PDMS fluid is a non-reactive, trimethyl terminated linear polymer with a repeating, easy-to-rotate dimethyl siloxane backbone. These polymers are commercially available in relatively short chains as highly volatile, VOC-exempt cosmetic carriers all the way up to barely flowable silicone gums of ~20,000,000 cSt viscosity, and everything in between. Alternative linear silicone polymers may have reactive endgroups such as silanol fluids (also called OH fluids, or SFR fluids for &#;reactive&#;), which are the most common, or other reactive endgroups such as vinyl, hydride, carboxyl or myriad other options in order to bring polymer chain extension and elastomeric crosslinking options into play. Silicone fluids are one of the most important classes of polymeric materials in the world today, touching nearly every market segment including the limited representations listed below:

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• Cosmetics
• Medicine and pharmaceutical applications
• Slip aids
• Water repellents
• Personal intimate lubricants
• Mold release
• Textile finishing
• Car care
• Anti-mar overcoats
• Dry cleaning
• Lubricants
• Food grade release and de-nesting
• Glossy print topcoats

To see more about our silicone fluids see the following pages Silicone Quats & Aminosiloxanes 

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The compatibility of three types of silicone oil with polydimethylsiloxane, the phase separation of their mixture and the microstructure and properties of their composite coatings were investigated. The existing form of silicone oil in the coating and the precipitation behavior were also studied. The compatibility observed experimentally of the three silicone oils with PDMS is consistent with the results of the thermodynamic calculation. The silicone oil droplet produced by phase separation in the mixture solution can keep its shape in the cured coating, also affecting the microstructure and mechanical properties of the coating. It was found that methyl silicone oil and methyl fluoro silicone oil do not precipitate on the surface, and they have no effect on the surface properties of the coating. In contrast, phenyl silicone oil has obvious effect on the surface, which makes the water contact angle and diiodomethane contact angle of the coating decrease significantly.

During recent years, some research progress has been made on adding silicone oil to improve antifouling performance [ 28 , 29 , 30 ]. Research on mixing silicone oil into silicone coatings mainly focuses on the antifouling performance of some specific fouling organisms [ 31 , 32 ]. Ba et al. further expounded the influence of the type, viscosity and addition amount of silicone oil on the precipitation property of silicone oil on the surface [ 33 ]. Related studies were mainly focused on the antifouling performance of silicone coatings. However, the influence of the compatibility of silicone oil and silicone blend solutions on the structure of the blend coating is still unknown. This paper will focus on comparing the compatibility of three types of silicone oil with PDMS through thermodynamic calculation and contrast observations of the blend solution, and the influence of the compatibility on the mechanical properties and surface properties of the composite coatings containing solid and liquid phases.

Ships that stay in the port for a long time tend to collect and deface organisms on the surface [ 21 ]. The antifouling properties are often improved by adding silicone oil to silicone coatings to mimic the properties of silicone oil deposited on the skins of large animals in the ocean [ 22 , 23 ]. Moreover, the low toxicity of silicone oil poses little risk to marine life [ 24 ]. The earliest study on silicone oil-modified silicone coatings was in ; Milne tried to add inert silicone oil into silicone coatings to improve the antifouling performance [ 25 ]. Hoipkemeier-Wilson et al. added methyl silicone oil to silicone coatings and proved that adding silicone oil improved the antifouling performance through a spore adhesion experiment [ 26 ]. Truby et al. added phenyl silicone oil to silicone coatings and conducted long-term panel immersion tests in Hawaii; the results showed that the addition of silicone oil effectively reduced the adhesion strength of barnacles and oysters to the silicone coating [ 27 ].

Organosilicon was first used in marine antifouling coatings in , developed by Mueller [ 16 ]. However, it was not widely used due to the shortcomings of its poor mechanical properties, construction and recoating property. After , Yonehara and colleagues produced a series of organosilicon coatings with different compositions and additives. Through panel immersion tests, they proved that the antifouling effect of silicone antifouling coatings was improved compared with traditional antifouling coatings [ 17 ]. After that, Masato, Takafumi and colleagues made different modifications to silicone antifouling coatings and achieved a good antifouling effect [ 18 , 19 ]. Ba et al. improved the mechanical properties of silicone coatings by adding different nano-powders [ 20 ].

Silicone coatings can effectively prevent fouling organisms from adhering to the coating surface [ 10 ]. Due to its low surface energy and low elastic modulus, most of the mucus secreted by fouling organisms cannot wet the coating surface, meaning it is difficult for it to be fixed on the coating surface [ 11 , 12 ]. The lower elastic modulus enables the fouling organisms on the coating surface to peel off from the surface with lower energy under the scouring of seawater [ 13 ], which can effectively remove the attached organisms by the scouring effect of the ship and seawater during navigation [ 14 ], which is an efficient &#;fouling release&#; coating [ 15 ].

Marine biological fouling is one of the most common fouling forms in the ocean [ 1 , 2 ], which has caused great harm and potential safety hazard to ships, bridges, offshore drilling platforms and other offshore industrial facilities [ 3 , 4 ]. It is an effective and long-standing method to an apply antifouling coating on the surface of the metal immersed in seawater that needs to be protected [ 5 ]. It can not only inhibit the attachment of marine pollutants such as algae, oysters and even tiny bacteria and reduce the harm of marine biological pollution and but also effectively improve the life of ships and industrial facilities [ 3 , 6 ]. Organotin self-polishing antifouling coatings were widely used because of their excellent properties [ 7 ]. However, research found that the organotin antifouling coatings in use caused pollution to the ocean, damaging the marine ecological environment. With the promulgation and implementation of the &#;International convention for the control of harmful antifouling systems on ships&#; (AFC), many countries have turned to study environmentally friendly marine antifouling coatings [ 8 , 9 ].

The contact angles of the coatings were measured by the angle measuring method. A JCC contact angle measuring instrument (China Zhongchen Co., Ltd., Shanghai, China) was used to extrude about 3 μL of distilled H 2 O and CH 2 I 2 , and the camera of the equipment was used to save the image and record the outline of the contact angle between the droplet and the surface. For every coating, five positions were measured, and the average value was taken as the contact angle. Based on the measured water and diiodomethane contact angle (CA), the surface energy of the sample was calculated by the Owens double liquid method.

The sample was placed on the surface of the operating table, and the TH shore hardness tester (Beijing Shidai Zhifeng Technology Co., Ltd. Beijing, China) was held to make the presser foot parallel to the surface of the sample. The presser needle was pressed into the sample vertically without vibration. When the hardness tester made contact with the sample surface, it read and recorded the data within 1 s, selecting 6 points for each sample, taking the average value as the hardness. To meet the test requirements, the thickness of the sample should be more than 5 mm.

The tensile curve of the coating was measured using a UTM computer-controlled electronic universal testing machine (Jinan Wance Testing Electric Equipment Co., Ltd., Jinan, China), with a tensile rate of 50 mm/min. The dumbbell-shaped sample was prepared according to the national standard, GB/T 528- (ISO 37: [ 34 ]); the length of the sample was 75 mm, the gauge length was 25 mm and the width was 4 mm. The tensile data less than 0.1 mm/mm were selected to fit the elastic modulus of the coating. Three samples were prepared and tested for each coating. The stress&#;strain curve was plotted.

The surface of the coatings with different silicone oils was observed by a WT-GM stereomicroscope (Shanghai micro path photoelectric technology, Shanghai, China) and by CLSM, photographed. The photos were processed by Photoshop software (Adobe Systems, San Francisco, CA, USA) to obtain the ratio of the silicone oil coverage area and coating coverage area.

The coating was cut into 40 × 10 mm 2 samples. They were immersed in liquid nitrogen and cooled for 20 min before breaking. The fracture morphology was observed by an Olympus OLS confocal laser scanning microscope (CLSM) (Olympus (China) Co, Ltd, Beijing, China).

A small amount of the prepared mixed solution absorbed by the pipette was dripped on the glass slide with about 0.2 g solution, which was allowed to stand for 48 h to make the mixed solution fully spread on the surface of the glass slide. Then, the sample was observed with an optical microscope (OM), and photos were taken. The mixture samples were set as XY, in which the letter X is the type of silicone oil, where MSO represents methyl silicone oil (MSO), PSO represents phenyl silicone oil (PSO) and FSO represents fluoro silicone oil (FSO); the letter Y is the mass of silicone oil per 100 g of PDMS.

PDMS was blended with silicone oil at a mass ratio of 10%, 20%, 30% or 40% to 90%. After dispersing for 10 min with a multi-purpose mixer at the speed of rpm, the mixture was poured into the test tube and kept for 48 h. The phase separation state was observed and recorded by taking a photo. The blend solution samples were set as X%Y/PDMS, in which the letter X is the mass of silicone oil per 100 g of PDMS, and the letter Y is the type of silicone oil, where MSO represents methyl silicone oil (MSO), PSO represents phenyl silicone oil (PSO) and FSO represents fluoro silicone oil (FSO).

The blank control sample without PSO was set as P0. The coating samples were set as X-ZY, in which the letter X is the type of silicone oil, where MSO represents methyl silicone oil (MSO), PSO represents phenyl silicone oil (PSO) and FSO represents fluoro silicone oil (FSO); the letter Z is the ratio (O/B) of silicone oil to base material, and the letter Y represents the mass of silicone oil per 100 g of PDMS.

Silicone paint with silicone oil was prepared by a two-step method: First, 200 g PDMS, and silicone oil with the corresponding mass ratio were added into a 500 mL metal stirring tank. The mixture was dispersed for 30 min at rpm by using a sand mill dispersion mixer. After the dispersion, the mixture was poured into a clean tinplate tank for standing and defoaming to form component A. TEOS and xylene were mixed into a solution according to the mass ratio of 3:7, poured into a clean white plastic bottle and recorded as component B. BIND and ethyl acetate were mixed into a solution according to the mass ratio of 3:7 and poured into a clean white plastic bottle, which was marked as component C. The three components, A, B and C, were mixed according to the mass ratio of 20 (PDMS): 4:1 and stirred with a paint mixing knife for 3 min, followed by painting and curing to obtain the composite coating.

Generally, silicone paint consists of three parts: film former, crosslinking agent and catalyst. The film former used in the experiment is hydroxyl-terminated PDMS, which needs to be modified by adding silicone oil (silicone oil does not participate in the film-forming reaction).

Hydroxy-terminated polydimethylsiloxane (PDMS) was purchased from Shandong Dayi Chemical Industry Co., Ltd. (Yantai, China), with a viscosity of 10,000 Pa·s and molecular weight of 60,000. Methyl silicone oil (MSO) was purchased from Shandong Dayi Chemical Industry Co., Ltd. (Yantai, China), with a viscosity of 15 Pa·s and a molecular weight of . Phenyl silicone oil (PSO) was purchased from Shanghai Hualing Resin Co., Ltd. (Shanghai, China), with a viscosity of 30 Pa·s and a molecular weight of 450. Methyl fluoro silicone oil (FSO) was purchased from Wuhan Huaxiang Kejie Biotechnology Co., Ltd. (Wuhan, China), with a viscosity of Pa·s and a molecular weight of 26,000. Ethyl orthosilicate (TEOS) was obtained from Tianjin Chemical Co., Ltd. (Tianjin, China). New bismuth decanoate (BIND) was obtained from Shanghai Deyin Chemical Co., Ltd. (Shanghai, China). Xylene and ethyl acetate were also analytical grade and provided by Yongda Chemical Reagents Co., Ltd. (Tianjin, China).

3. Results

3.1. Phase Separation Thermodynamic of Silicone Oil/PDMS Mixture

The chemical structures of three types of silicone oils, MSO, PSO and FSO, are shown in . The different chemical structures determine their difference with PDMS, and the different compatibilities of each silicone oil and PDMS mixture.

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The Flory&#;Huggins lattice model is most commonly used to describe the mixing phenomenon of polymer solutions. Based on this model [35,36], therefore, the thermodynamics of the silicone oil/PDMS blend solutions can be analyzed as follows.

Taking silicone oil as polymer A and PDMS as polymer B, we state that the molecular chain of polymers A and B contains &#;chain segments&#; xA and xB, respectively. The amount of each substance in the silicone oil/PDMS mixture is nA and nB, and the volume fraction is φA and φB, respectively. The molar volume of both &#;chain segments&#; A and B is Vu. The total volume of the mixture system is V. χ1 is the interaction parameter between the silicone oil and PDMS. The unit mixed Gibbs free energy &#;GM of the mixture can be calculated by formula (1) [35].

&#;GM = R T V (φA lnφA/xA + φB lnφB/xB + χ1 φA φB)/Vu

(1)

For a blend solution, we usually observe the phase separation point of the blend solution by an observation method and then substitute the data into Equation (1) to obtain the interaction parameters of the blend solution. However, by combining thermodynamics with the Flory&#;Huggins lattice model, we obtain the relationship between the interaction parameter χ1 and the solubility parameter of silicone oil δ1 and PDMS δ2; VM1 is the molar volume of silicone oil [36].

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Χ1 = VM1(δ1 &#; δ2)2/RT

(2)

The solubility parameter of PDMS δ2 was determined by the intrinsic viscosity method in this investigation, and it is equal to 15 (J/cm3)1/2. Therefore, the solubility parameter of the silicone oil δ1 obtained by the group contribution method was substituted into Equation (2), and the interaction parameter χ1 between silicone oil and PDMS was obtained. As the main chain structure of silicone oil and PDMS is the same, their repeating unit is a Si-O bond, the repeating unit can be regarded as a &#;chain segment&#; and the &#;chain segment&#; number of the polymer can be estimated by the relative molecular weight of the polymer. The molecular weight of PDMS resin is 60,000, and the number of chain segments is 810. The thermodynamic parameters calculated are listed in .

Table 1

Types of Silicone OilMSOPSOFSOχ10...δ1/(J/cm3)1/215...83xAVM1/(mL/mol)..,894.00Open in a separate window

Substituting the data in into Equation (1), the curves of the concentration φA and &#;GM of the mixture system of silicone oil and PDMS were obtained, as shown in . According to thermodynamics, when &#;GM < 0, the blend solution can spontaneously fuse, and when &#;GM > 0, phase separation will inevitably occur. As it is shown in a, for the mixed solution of methyl fluoro silicone oil and PDMS at any concentration, &#;GM > 0, and this means that methyl fluoro silicone oil and PDMS cannot be miscible, and phase separation between methyl fluoro silicone oil and PDMS is inevitable. On the contrary, as shown in a,b, when methyl silicone oil and PDMS are blended, &#;GM is always less than 0 at any ratio, that is to say, methyl silicone oil and PDMS are miscible, and phase separation cannot occur. As it is shown in b, when the concentration of phenyl silicone oil and PDMS is low, &#;GM < 0, and then it is more than 1.9%, &#;GM > 0, which means that when the volume concentration of phenyl silicone oil reaches 1.9%, phase separation begins to appear. In other words, the larger the interaction parameter χ1 between silicone oil and polydimethylsiloxane, the lower the compatibility between silicone oil and polydimethylsiloxane, and the easier it is for phase separation to occur.

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3.3. Fracture Microstructure of the Cured Coatings

The compatibility of silicone oil and polydimethylsiloxane and the degree of phase separation in the mixed solution will inevitably affect the microstructure of the cured coating. As it is shown in a,d,g,j, the MSO droplets produced by phase separation cannot be found in the fracture of samples MSO-Z5, MSO-Z10, MSO-Z15 and MSO-Z20. The solubility of MSO and PDMS is excellent, and they can dissolve each other in any proportion. In the liquid state, the phase separation of MSO dissolved in PDMS still did not occur during the film-forming process.

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The trace of PSO droplets on the fracture surface of the coating can be found in b,e,h,k.

The compatibility between FSO and PDMS is very bad. The phase-separated FSO droplets can be seen clearly in c,f,i,l. Compared with PSO, FSO has poor miscibility with PDMS and high viscosity, meaning the volume of FSO droplets formed under the same stirring action was large. Therefore, the size of FSO droplets on the fracture is bigger than that of PSO.

When comparing with and , it can be found that when the phase-separated silicone oil can be observed in the blending solution, the phase-separated silicone oil can also be observed at the fracture. When the volume of phase-separated silicone oil is large, the large and irregular silicone oil droplets can also be observed at the fracture; this shows that the free silicone oil droplets in the coating are related to the silicone oil droplets in the blend solution, and the silicone oil droplets in the blend solution will continue to exist in the silicone coating in a free form during the curing process.

3.4. Leaching Behavior of Silicone Oil on the Coating

The morphologies of the three types of coatings exposed for 1 month to ambient conditions were observed by OM and CLSM, shown in . As it is shown in a,c, both MSO and FSO cannot be observed even by CLSM on the coating surface. There are no free silicone oil droplets in the coating, meaning there is no silicone oil precipitation on the coating surface. As MSO is miscible with PDMS in any proportion, phase separation cannot occur between MSO and PDMS. On the contrary, the surface of the FSO/PDMS coating has no leached trace of silicone oil due to the poor compatibility of FSO with PDMS, and to the higher density of FSO. As it is shown in b, phase-separated PSO droplets can be observed. The reason is that PSO is partially compatible with PDMS, which can diffuse through capillary action and move to the coating surface. With the increase in the PSO content in PDMS, as shown in , not only the coverage area of leached PSO on the coating surface increased, where it increased from 28.09% for coating PSO-Z5 to 37.23% for PSO-Z10, 43.36% for PSO-Z15 and 46.04% for PSO-Z20, but also the size of the leached PSO droplets increased.

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3.5. Mechanical Properties of the Composite Coatings

The tensile stress&#;strain curves of the coatings are shown in . For the same silicone oil, the stress&#;strain curves and tensile behavior of the coatings hardly changed when increasing the silicone oil concentration. Compared to the pure PDMS coating, as shown in d, the tensile curve of MSO-Z10 is highly coincident with PDMS because methyl silicone oil is dissolved in the silicone coating, and there is no phase separation. However, under the same strain, the tensile stress of PSO-Z10 and FSO-Z10 decreased to some degree due to phase separation, which resulted in silicone oil droplets forming in the coating. The droplets formed in FSO-Z10 are larger and more severe than those in PSO-Z10. Therefore, the decline in the mechanical properties is more serious. In addition, by introducing silicone oil, as shown in , the elastic modulus and hardness of the coating decreased to a certain extent respectively with the increase in the silicone oil addition. In conclusion, the lower the compatibility of silicone oil with PDMS, the more silicone oil droplets produced by phase separation during the curing process of the coating, the larger the size of the droplets and the lower the elastic modulus and hardness of the composite coating.

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3.6. Surface Energy of the Composite Coatings

For the coating with MSO, there is no MSO precipitation on the surface. The structure and group of MSO and PDMS are similar. Therefore, it can be seen from and that the surface energy increases slightly with the addition of methyl silicone oil. However, the water and diiodomethane contact angle do not change obviously with increasing MSO, which indicates that the addition of MSO has little effect on the surface properties of the coating.

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Table 2

SampleContact Angle (°)Surface Free Energy (mJ/m2)WaterDiiodomethane P.05 ± 1..65 ± 1..49 ± 0.86MSO-Z.15 ± 1..15 ± 1..89 ± 0.86PSO-Z.10 ± 0..85 ± 1..39 ± 1.02FSO-Z.75 ± 1..85 ± 2..79 ± 1.41Open in a separate window

However, a large amount of silicone oil was precipitated on the surface of the PDMS coating with the added PSO. As shown in , the higher the amount of PSO, the higher the amount of PSO that is precipitated on the coating. Compared with PDMS, PSO has many phenyls in the side chain, and phenyl has polarity. Therefore, the water contact angle and diiodomethane contact angle of the coating decreased significantly with the increase in PSO, but the surface energy of the coating increased.

The side chain of FSO has a perfluorinated group, which has strong polarity and an amphiphilic property. However, no FSO precipitates on the surface of the coating. Therefore, both the contact angle and surface energy hardly changed with the increase in FSO.