Review of the first issue: silicone adhesive Dr. Zhu Yuduo special column vol.001 (click the title to enter the review)
Summary of the second phase
Structure and properties of organosilicon
Silicone elastomers and sealants
Columnist Dr. Zhu Yuduo
Huameilian company
Chapter III
Structure and properties of organosilicon
If people carefully observe the microstructure of polydimethylsiloxane, it will be found that the main chain of polydimethylsiloxane is Si-O-Si linear structure; each silane atom has two methane.
Fig. 17 microstructure of polydimethylsiloxane.
According to the specific scientific description between atoms, it is also found that organosilicon materials have unique structure:
(1) The main chain of polysiloxane with high energy is shielded by sufficient methyl group on Si atom;
(2) C-H has no polarity, which makes the intermolecular interaction very weak;
(3) The length of Si-O bond is longer and the angle of Si-O-Si bond is larger.
(4) Si-O bond is a covalent bond with 50% ionic bond (covalent bond has directionality, ionic bond has no directionality).
Because the methane on the silicon atom is symmetrical, and there is no other shielding element on the oxygen atom, the silicon atom and the oxygen atom on the polydimethylsiloxane can rotate freely on the plane perpendicular to the main chain, which results in the glass transition temperature of polydimethylsiloxane being particularly low, - 120 ℃.
Because of the unique structure of organosilicon and the properties of inorganic and organic materials, it has the basic properties of low surface tension, small viscosity temperature coefficient, high compressibility, high gas permeability, and has the excellent properties of high and low temperature resistance, electrical insulation, oxidation stability, weather resistance, fire resistance, hydrophobicity, corrosion resistance, non-toxic and tasteless, and physiological inertia, which are widely used in aviation Aerospace, electronic and electrical, construction, transportation, chemical industry, textile, food, light industry, medical and other industries, in which organosilicon is mainly used for sealing, bonding, lubrication, coating, surface activity, demoulding, defoaming, foam suppression, water-proof, moisture-proof, inert filling, etc. With the continuous growth of the number and variety of organosilicon, the application field is expanding, forming a unique and important product system in the field of new chemical materials. Many varieties are irreplaceable and essential for other chemicals.
The metabolism of organism also needs the participation of organosilicon, which is usually represented by CH3 (SiOH) 3. Organosilicon plays an important role in various functions of the body and has a direct relationship with mineral absorption. The human body has an average of about seven grams of silicon, far more than other important minerals, such as iron. Iron and silicon are necessary for human body and play an important role in maintaining normal metabolism.
Organosilicon materials can be classified into silane coupling agents (organosilicon chemical reagents), bioactive organosilicon, silicone oil (silicone grease, silicone emulsion, silicon surfactant), high temperature vulcanized silicone rubber, liquid silicone rubber, silicone resin, composite material and so on.
Characteristic
The basic structural unit of silicone product is composed of silicon oxygen chain link, and the side chain is connected with other organic groups through silicon atom. Therefore, the structure of organosilicon products contains both "organic group" and "inorganic structure". This special composition and molecular structure make it integrate the characteristics of organic matter and the functions of inorganic matter. Compared with other polymer materials, the most outstanding properties of silicone products are:
1. Temperature resistance: silicone products take silicon-oxygen (Si-O) bond as the main chain structure, the bond energy of C-C bond is 82.6 kcal / g, and the bond energy of Si-O bond is 121 kcal / g in silicone, so the thermal stability of silicone products is high, and the chemical bond of molecules under high temperature (or radiation) does not break or decompose. Organosilicon is not only resistant to high temperature, but also to low temperature. It can be used in a wide temperature range, - 118? C - 316? C. Both chemical and physical mechanical properties have little change with temperature.
2. Weather resistance: the main chain of organosilicon product is - Si-O -, without double bond, so it is not easy to be decomposed by ultraviolet light and ozone. Organosilicon has better thermal stability, radiation resistance and weather resistance than other polymer materials. The service life of organosilicon in natural environment can be up to several decades.
3. Electrical insulation performance: all organosilicon products have good electrical insulation performance, and their dielectric loss, voltage resistance, arc resistance, corona resistance, volume resistance coefficient and surface resistance coefficient are among the top insulating materials, and their electrical performance is little affected by temperature and frequency. Therefore, they are a kind of stable electrical insulation materials, which are widely used in the electronic and electrical industries. In addition to excellent heat resistance, organosilicon also has excellent water repellency, which is the guarantee of high reliability of electrical equipment in wet condition.
4. Biological characteristics: bioactive organosilicon is a necessary nutrient for human body. Organosilicon is an important element of human tissue and metabolism. It exists in every cell of human body as the support of cell construction and helps other important substances such as magnesium, phosphorus, calcium, etc. The human body can only obtain organosilicon through food.
Scientists believe that organosilicon exists in human body mainly in three forms:
（1） Soluble organosilicon, 10% by weight
（2） 30% in various cell matrices
（3） 60% is used to synthesize protein, which shows that the silicone we need every day is quite high.
If you want to stay young for 5 years, 10 years or even 30 years, it is particularly important to take 20-30 mg of organosilicon every day.
This is why polydimethylsiloxane is widely used in the pharmaceutical and health industry!
5. Low surface tension and low surface energy: only 20-22 dyne / cm. The main chain of organosilicon is very flexible, and its intermolecular force is much weaker than that of hydrocarbons. Therefore, compared with hydrocarbons of the same molecular weight, organosilicon has low viscosity, weak surface tension, small surface energy and strong film-forming ability. This low surface tension and low surface energy are the main reasons for its application in many ways: hydrophobic, defoaming, foam stabilization, anti sticking, lubrication, polishing and other excellent properties.
Because organosilicon has these excellent properties, it has a wide range of applications. It is not only used as a special material in aviation, cutting-edge technology and military technology sectors, but also used in various sectors of the national economy. Its application range has been expanded to: construction, electronics and electricity, textile, automobile, machinery, leather and paper making, chemical light industry, metal and paint, medicine and medical treatment, etc.
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Chapter IV
Silicone elastomers and sealants
When the deformation of material is caused by external force, when the deformation is proportional to the external force (in accordance with Hooke's law), the material is considered as an ideal elastomer or Hooke's elastomer.
An ideal elastomer is an object that can be completely restored to its original state after removing the external force.
In fact, any kind of material has inertia effect and does not obey Hooke's law. However, in a certain range, the relationship between stress and strain can meet the requirements of Hooke's law and can be treated as an ideal elastomer. The deformation and recovery deformation of an ideal elastomer are instantaneous, independent of the action time of the external force.
According to elastic mechanics, the ideal elastomer should meet four conditions:
(1) Suppose the object is continuous
(2) Assume that the object is completely elastic
(3) Suppose the object is isotropic
(4) Suppose the object is uniform
After these conditions are satisfied, it can be said that the object is an ideal elastic body. But in fact, there is no ideal elastomer, only relative ideal elastomer.
So what kind of material has the characteristics of elastomer. The author thinks that there are two conditions for elastomer
First, the glass transition temperature of the material is low;
Second, the material is mainly thermosetting.
The so-called thermosetting material refers to the formation of three-dimensional network structure on the molecular structure of the material after curing. And this change is irreversible!
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Figure 18 microstructure of thermoplastic materials, elastomers, and thermosetting materials
It is known that the glass transition temperature of PDMS is very low, close to - 120 ℃. So, will polydimethylsiloxane form a three-dimensional cross bonding network during curing? In order to understand the whole process of polydimethylsiloxane curing, people must discuss it from the production of silicone elastomer, curing in the process of use, or called curing machine.
In the production process of silicone elastomer, the hydroxyl terminated polydimethylsiloxane is mixed with silane cross adhesive. When the cross adhesive encounters the siloxane on the polydimethylsiloxane, one of the siloxanes on the cross adhesive will react with the active hydrogen on the polydimethylsiloxane. In this way, polydimethylsiloxane becomes the prepolymer of siloxane sealing.
Fig. 19 the hydroxyl terminated polydimethylsiloxane changes into the silane terminated polydimethylsiloxane
In the process of using silicone elastomer, especially silicone sealant, silicone sealant will be cured or vulcanized because it absorbs moisture.
FIG. 20 moisture curing principle of silicone sealant
In fact, the curing of silane terminated polydimethylsiloxane can be divided into two steps. The first step is hydrolysis. When the silane capped polydimethylsiloxane encounters water, it will react and generate the hydroxyl capped polydimethylsiloxane again. When two hydroxyl terminated silane terminated polydimethylsiloxanes meet, the second step reaction, namely polycondensation, will occur. Two hydroxyl terminated silane terminated polydimethylsiloxane releases a water molecule, and the main part becomes an irreversible three-dimensional network!
From the above analysis, it is not difficult to conclude that silicone sealant is a very authentic elastomer! Because it is an elastomer, because silicone elastomer has so many characteristics and advantages. This is why silicone sealant is widely used in various fields.
To be continued
Coming soon
About the author
Dr. Zhu Yuduo
Huameilian company
Technical consultation
Dr. Zhu Yuduo graduated from the Department of polymer physics, Lehigh University in 1994. After graduation, he worked in fuller, Ge, schnee Morehead, Bayer and other multinational companies, successively served as research chemist, technology market manager, R & D Manager, chief scientist and technical director, etc., obtained a number of international technical patents in the field of adhesives, and published more than 20 papers in well-known academic journals at home and abroad. Dr. Zhu has made unique achievements in product research and development, industrial production, market development, sales, technical service and management. In 2015, he joined huameilian company of the United States to provide technical and business consulting services for small and medium-sized adhesive enterprises in China and the United States.
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