Xu Yongxian, a native of Sixian County, Anhui Province, now lives in Jiangmen City, Guangdong Province. Jinan University manages EMBA
President of Guangdong silicon fluorine new material and Equipment Industry Research Institute
Secretary General of Dawan district society of silicon fluorine organic materials engineering
General manager of Shenzhen kejunchi Technology Co., Ltd
General manager of Jiangmen kejunchi new materials Co., Ltd
General manager of Guangdong Guolian Chemical Technology Information Center
Kejunchi company specializes in producing platinum catalyst for 25 years
1. Platinum catalyst is suitable for: silicone, ink, liquid silica gel platinum water (catalyst) 2000ppm-20000ppm special content can be fixed production!
2. Silicone rubber medical grade: General platinum extrusion; a / B group curing agent (catalyst) pl-10a / tm-10b is suitable for silicone rubber mix produced by any manufacturer!
3. Silicone rubber extrusion / molding inhibitor HR series.
4. Silicone rubber environmental protection water boiling does not whiten and transparent internal release agent kep-18.
5. Binder and primer series for addition silicone rubber
Tel: 13076919700 (wechat synchronous keyunchi platinum catalyst)
Definition of thermal conductive silicone rubber:
Filling and sealing is to fill the liquid compound into the device with electronic components and circuits by mechanical or manual means, and then solidify it into polymer insulation material with excellent performance under normal temperature or heating conditions. Potting is divided into local potting and overall potting. Local potting refers to the potting of high-power devices, so that they are connected with the radiator through heat conduction potting silicone rubber material, so that the generated heat can be quickly transmitted to the radiator. The whole potting is to encapsulate all the components inside the electronic products, so that the heat generated can spread rapidly and directly to the external environment. The heat conduction potting silicone rubber is generally two-component. After the two components are mixed and defoaming, it is potted to the required position and vulcanized into elastomer. This kind of product not only has the function of heat dissipation and insulation, but also has the function of shock absorption and three proofs. At present, it is widely used in LED power supply, bulb lamp built-in power supply, electric vehicle battery components and charging pile power supply module, as well as the bonding and sealing of some electronic devices, which plays the role of heat dissipation, sealing and shock absorption.
Potting is to inject the liquid compound into the device with electronic components and circuits by mechanical or manual means, and then solidify it into thermosetting polymer insulation material with excellent performance under normal temperature or heating conditions. It can strengthen the integrity of electronic devices, improve the resistance to external shock and vibration; improve the insulation between internal components and lines, which is conducive to miniaturization and lightweight of devices; avoid direct exposure of components and lines, improve the waterproof and moisture-proof performance of devices, and improve the use performance.
Potting process
The quality of potting products is closely related to product design, component selection, assembly and potting materials used. The potting process is also a factor that can not be ignored.
At present, there are two common ways: manual vacuum potting and mechanical vacuum potting. Mechanical vacuum potting can be divided into two situations: A and B components are mixed and defoaming first and then potting, and they are separated and then potting. The process flow is as follows:
(1) manual vacuum filling and sealing process
(2) mechanical vacuum filling and sealing process
The technology of mixing and defoaming first and then filling and sealing
A and B: separate defoaming and then mixed filling
In contrast, mechanical vacuum potting has the advantages of large equipment investment and high maintenance cost, but it is obviously superior to manual vacuum potting in terms of product consistency and reliability. No matter what kind of potting method, we should strictly abide by the given process conditions, otherwise it is difficult to get satisfactory products.
Analysis on the problems and causes of potting products
(1) The starting voltage of partial discharge is low, and line to line ignition or breakdown occurs High voltage electronic products such as TV, display line output transformer, automobile, motorcycle igniter and so on, often occur partial discharge (Corona), line to line spark or breakdown phenomenon due to improper filling and sealing process, because the wire diameter of high voltage coil of such products is very small, generally only 0.02-0.04mm, filling and sealing materials can not fully penetrate the turn to turn, so that there is a gap between the turns of the coil. Because the dielectric constant of the gap is much smaller than that of the epoxy potting material, under the condition of alternating high voltage, the uneven electric field will be generated, which will cause partial discharge at the interface, aging and decomposition of the material, and damage the insulation.
From the perspective of technology, there are two reasons for the gap between lines:
1) when filling and sealing, the vacuum degree is not high enough, and the air between the lines is not completely removed, so that the materials cannot be completely infiltrated.
2) before filling, the preheating temperature of the test piece is not enough, and the viscosity of the test piece can not be reduced rapidly, which affects the infiltration.
For manual filling or vacuum filling after mixing and defoaming, high temperature of material mixing and defoaming, long operation time or beyond the applicable period of material, and the product does not enter the heating and curing procedure in time after filling, the viscosity of material will increase, which will affect the infiltration of coil. The higher the starting temperature, the smaller the viscosity, and the faster the viscosity increases with time. Therefore, in order to make the material have good infiltration to the coil, the following points should be paid attention to in operation:
1) the potting compound shall be kept within the given temperature range and used up within the applicable period.
2) before filling and sealing, the test piece shall be heated to the specified temperature. After filling and sealing, it shall enter the heating and curing procedure in time.
3) the filling vacuum degree shall meet the requirements of technical specifications.
(2) In the process of heating and curing, there are two kinds of shrinkage, that is, chemical shrinkage in the process of liquid to solid phase transformation and physical shrinkage in the process of cooling. Further analysis shows that there are two processes of chemical shrinkage in the curing process, namely, the shrinkage from the beginning of the chemical crosslinking reaction after the encapsulation to the initial formation stage of the micro network structure, which we call gel precuring shrinkage. Shrinkage from gel to complete curing is called post cure shrinkage. The shrinkage of the two processes is different. In the process of the former changing from liquid state to network structure, the physical state changes suddenly, the consumption of reaction group is greater than that of the latter, and the volume shrinkage is also higher than that of the latter. At the gel precuring stage (75 /3h), the epoxy group disappeared more than the post curing stage (110 C /3h), and the results of DTA analysis also proved that the curing degree of the sample was 53% after 750 /3h treatment.
If we adopt a high temperature curing for the encapsulated specimen, the two stages in the curing process are too close, and the gel pre curing and post curing will be completed at the same time. This will not only cause exothermic peaks, damage elements, but also cause huge internal stress of the sealing parts, resulting in defects in the internal and external appearance of the products. In order to obtain good parts, we must focus on the matching of curing speed (A, B composite gel time) and curing conditions when designing the filling material formulation and curing process. The commonly used method is: according to the properties and uses of the potting materials, the technology of sectional curing in different temperature areas is adopted. According to experts, color TV output transformer potting according to different temperature zone subsection curing procedures and internal heat release curve of parts. The curing reaction of the encapsulated material in the gel precuring zone was slow, the reaction heat gradually released, the viscosity increased and the volume contracted slowly. At this stage, when the material is in the flowing state, the volume shrinkage shows that the liquid level drops until the gel can completely eliminate the shrinkage internal stress at this stage. From the gel pre curing to the post curing stage, the temperature rise should also be gentle. After the curing is completed, the sealing parts should be cooled slowly with the heating equipment at the same time, and the internal stress distribution of the parts can be reduced and adjusted in many ways, so as to avoid shrinkage holes, sunken or even cracking on the surface of the parts.
For the formulation of the curing conditions of the potting materials, the arrangement, fullness, size, shape and single potting amount of the sealing elements in the potting parts shall be referred to. It is necessary to reduce the gel precuring temperature and prolong the long time when the single encapsulating quantity is larger and the sealing elements are less.
(3) The poor surface or local non curing of solidified products are also related to curing process. The main reasons are:
1) Failure of metering or mixing device and operation error of production personnel.
2) A component precipitated after long-term storage, and was not stirred evenly before use, resulting in the imbalance of the actual proportion of resin and curing agent.
3) Component B is stored open for a long time, and moisture absorption fails.
4) During the high tide and wet season, the potting parts did not enter the curing process in time, and the surface of the articles was hygroscopic.
In short, in order to obtain a good potting product, potting and curing process is indeed a problem worthy of great attention.
1： How to avoid bubbles?
1. It is recommended to use products with low viscosity and long operation time, so that the air can be discharged before the material solidifies.
2. Keep the curing temperature below 45 ℃ to avoid the air hole can not be discharged due to too high temperature.
3. How to determine the ratio?
The amount of curing agent and the change of ambient temperature and humidity have great influence on the curing rate. When the amount of curing agent is increased, the curing reaction will be accelerated. Therefore, the curing rate can be adjusted by increasing or decreasing the amount of curing agent. Generally, the recommended amount is 10:0.8-10:1.2 (mass ratio). If it is necessary to change the proportion, it shall be applied after simple experiment.
2： How to remove residual silica gel?
Before curing, use a sharp knife (or clean cotton cloth) to scrape off the uncured gel, and then use isopropanol and other solvents to clean the residue. After the adhesive solidifies, scrape as much silica gel as possible with a knife, then use solvent (120 × solvent oil, alcohol, xylene and acetone, etc.) to remove various residues, soak them and decompose them.
3： What about the main agent settlement?
If it is placed for a long time (more than half a month), the filler may settle, such as group a separate barrel, a layer of colorless transparent silicone oil on the surface, the surface color may be gray if it is not stirred evenly, the surface will be sticky if it is operated for a long time, the color of the solidified product may be gray, and the solidified product may be brittle. Therefore, it is necessary to mix component a before use, especially up and down to fully mix.
4： Why do some of them solidify and some of them do not?
Uneven mixing will result in slow curing or non curing. For example, the curing time of the adhesive prepared at the same time is not the same, some of them are fast curing, some of them take a long time to cure; after curing, there are grooves on the surface of the adhesive, and the adhesive in the groove is not cured; some of them are not cured at all. It is necessary to strengthen the mixing, and the mixing time is about 2-5min, so that the glue is completely mixed evenly. When using the mixing, it is necessary to pay attention to whether the mixing can drive all the glue materials.
5： What about curing agent turning yellow?
In order to ensure the good adhesion of LED glue to PC shell, LED lamp and PCB board, ammonia coupling agent is generally used to improve the adhesion. Therefore, the color of curing agent will turn yellow when it is placed for about 2 months, without affecting the curing time and product performance.
Synthesis and preparation of sulfurizer
Different packing forms of two-component RTV silicone rubber will result in corresponding changes in the composition and ratio of vulcanizing agent
According to different use requirements, cross-linking agent, catalyst, accelerator or inhibitor can be used to prepare curing agent in proper proportion.
Typical curing agent compatibility:
① General curing agent: TEOS / dibutyltin dilaurate = 4 / 1;
② Fast curing agent: crosslinker + catalyst + accelerant;
③ Delayed curing agent: crosslinker + catalyst + inhibitor;
④ Curing agent of adhesive silicone rubber: crosslinker + catalyst + tackifier
6： Can the curing be accelerated by heating?