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Discrimination of thermosetting resin temperature index

with the development of national economy, resin matrix composites are more and more widely used, but people are still confused about many performance concepts of resin as the main material of resin matrix composites, and they can't make good use of the characteristics of various resins to serve people, especially the understanding of the characteristics of various temperature indexes. There are many temperature indicators of thermosetting resin, such as thermal deformation temperature, martin heat resistance, glass transition temperature, insulation heat resistance grade, thermal torsion temperature, embrittlement temperature, strength loss temperature, etc. in this paper, we will focus on the discrimination of five temperature concepts of resin: thermal deformation temperature, martin heat resistance, glass transition temperature, insulation heat resistance grade and corrosion resistance service temperature, and will not elaborate on other concepts one by one, Help people clear their minds in the process of use, correctly select resins, and effectively apply them to actual production

1. Glass transition temperature

the cured thermosetting resin is a linear amorphous polymer. Due to the change of temperature (under a certain stress), the linear amorphous polymer can present three mechanical states, namely, glassy state, high elastic state and viscous flow state. When the temperature is high, both macromolecules and segments can move thermally. At this time, the polymer becomes a viscous flow state, and when subjected to external forces, the molecules slide each other to produce deformation; After removing the external force, it cannot return to the original state, so the deformation is irreversible. This deformation is called viscous flow deformation or plastic deformation. The temperature at which this deformation occurs is called flow temperature TF, and this state becomes viscous flow state (also known as plastic state). If the polymer in viscous flow state is gradually reduced in temperature. The viscosity will gradually increase, and finally it will be in an elastic state. When stress is applied, it will produce slow deformation. After the external force is removed, it can slowly return to the original state. This state is called high elastic state. When the temperature continues to drop, the polymer becomes harder and harder. Under the action of external forces, it is imperative to explore a new development path of plastic granulator technology. This state of small deformation is called glassy state. Thermosetting resin curing products are used in the glassy state, so the higher the TG, the better. It is also an index to measure the heat resistance of the resin. For example, when the Tg of 898 high cross-linked epoxy vinyl resin is 190 ℃, it has high heat resistance and can withstand a high temperature of 200 ℃ in the flue gas desulfurization industry

there are three common methods to measure the glass transition temperature: thermomechanical analysis (TMA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Their test methods and principles are different, so the test results are quite different and cannot be compared

in addition, the glass transition temperature of resin products after annealing (i.e. after heating) will increase, because the internal stress of the products has been eliminated after annealing and heating

2. Thermal deformation temperature and martin heat resistance

2.1 thermal deformation temperature

thermal deformation temperature (full name: load thermal deformation temperature, English abbreviation: HDT) refers to applying a specified load (1.81n/mm2 or 0.45 n/mm2) to a certain size of rectangular resin sample immersed in a heat-conducting liquid medium heated at a heating rate of 120 ℃/h, The temperature at which the deformation of the midpoint reaches the specified value corresponding to the height of the specimen under the support of the national large aircraft major science and technology special material development project. It should be noted that the thermal deformation temperature determined by different load values is different, and there is no comparability, so the specified load value (i.e. the adopted standard) must be pointed out when determining the thermal deformation temperature value. Thermal deformation temperature is one of the main indicators to measure the heat resistance of plastic (resin). Now, in the standards of most plastic (resin) products all over the world, thermal deformation temperature is used as a product quality indicator, but it is not the maximum service temperature. The maximum service temperature should be determined according to the comprehensive factors such as the stress condition and service requirements of the products

there are many standards for measuring the thermal deformation temperature, and the common ones in China are: Chinese national standard (GB), American Society for testing and materials standard (ASTM), international organization for Standardization standard (ISO), European community standard, etc. because the test methods and unit systems specified in each standard are different, the test results are also different. For example, the thermal deformation temperature of a well-known foreign brand phenolic epoxy vinyl ester resin product, typical ASTM Test Value: ℃, GB measured value: 137 ℃; GB measured value of 898 resin: 155 ℃

2.2 martin heat resistance

martin heat resistance test method is one of the methods to test the heat resistance of plastics (resins). It was proposed by Martin in 1924 and officially used in the inspection of phenolic plastics in Germany in 1928. Later, some other hard plastics also used this test method. It is widely used in Europe and the former Soviet Union. In 1970, China also issued the national standard for this test method, which became one of the national standards for plastic (resin) test methods established in China in the early stage, so it has been used in China for a long time

Martin's heat-resistant temperature refers to the temperature at which the specimen bends and deforms under the action of a certain bending moment and in a constant heating environment, when the specified deformation is reached

2.3 discrimination between thermal deformation temperature and martin heat resistance

thermal deformation temperature and martin heat resistance are both one of the methods to test the heat resistance of plastics (resins), but due to the essential difference of test methods, there is no comparability and no transformation formula

because the measurement of Martin's heat-resistant temperature is to apply cantilever bending moment, the operation is not convenient; And the applied bending moment is large, so that the initial deflection of many plastics after loading is very considerable, so the scope of application is limited, and it is generally used for hard plastics. In addition, it uses air as the heat transfer medium, the temperature distribution of the box is uneven, and the heat transfer to the sample is slow, so the temperature rise speed should not be too fast. All this and so on, so that this method has not been adopted in many countries, and has been gradually eliminated in our country

therefore, when testing the heat resistance of plastics (resins), martin heat resistance and thermal deformation temperature cannot be compared. At the same time, it should be noted that they are not the maximum service temperature of plastic (resin). The maximum service temperature of plastic (resin) should be determined according to the stress condition of the product and the use requirements and other factors. In addition, the thermosetting resin after annealing treatment, that is, what we call the post heating treatment, will increase the thermal deformation temperature and martin heat resistance. Generally, the annealing treatment can increase the thermal deformation temperature by 10 ℃, which shows that the post heating treatment is still necessary in the daily use of thermosetting resin

3. Corrosion resistant service temperature:

compared with metal materials, resin FRP has light weight, high specific strength, good corrosion resistance, good transient ultra-high temperature resistance and lower price than metal materials, so it has been applied in related fields. For example, 8mm ordinary carbon steel can be corroded thoroughly in only months in a humid environment with a concentration of 0.1% sulfur dioxide, while FRP products with 6mm 890 resin anti-corrosion layer can maintain a service life of 10 years. Therefore, various resin matrix composites are widely used in various anti-corrosion occasions, especially in heavy-duty anti-corrosion occasions. This involves an important concept: corrosion resistant service temperature

corrosion resistant service temperature generally refers to the maximum temperature that the resin product can withstand in a specific environment (specific corrosive medium, specific corrosive medium concentration), and necessary measures should be taken during the exemption period. This temperature is different from the thermal deformation temperature, glass transition temperature and insulation heat-resistant grade, such as: 898 vinyl resin thermal deformation temperature 155 ℃, glass transition temperature 190 ℃, insulation heat-resistant grade C (Chinese standard). In the wet desulfurization process, the temperature of the mixed gas at the inlet is about ℃, and the components in the system have to withstand instantaneous temperature alternation, potential thermal damage and strong corrosive by-products. Table 2.1 is an excerpt from the 898 resin corrosion resistance thermometer

it is not difficult to see from the above table that the corrosion-resistant service temperature always has a specific medium service condition. Without the medium service condition, the corrosion-resistant service temperature is not tenable. In different medium conditions, the corrosion resistance temperature of the same resin is usually different. This also requires that attention be paid to the conditions of corrosive medium when selecting anti-corrosion resin. However, there are some unscientific statements in the market at present, which are even directly written in the instructions of resin products, such as: "what is the operating temperature of the resin; what is the corrosion-resistant operating temperature of the resin; what is the thermal deformation temperature; what is the corrosion-resistant operating temperature." This kind of statement has no scientific basis. It is a misunderstanding of the corrosion-resistant use temperature of resin and a misunderstanding of the use of resin. In the process of using resin, we should eliminate these misleading and correctly use the characteristics of resin

4. Insulation heat resistance grade

4.1 overview

resin polymers as insulating materials should have good heat resistance in addition to good mechanical and dielectric properties. For example, plastic installation lines used in aviation and rockets generally work at 350 ℃, and some even require to withstand a high temperature of 500 ℃. However, after flying into the stratosphere, the temperature suddenly drops to about -70 ℃. At this time, the impact of temperature is a severe test for materials. Therefore, good heat resistance requires not only high temperature resistance, but also temperature resistance. The so-called heat resistance refers to the ability of materials to maintain their basic properties and use normally under high temperature and rapid temperature changes for a short time or a long time

heat resistance can be divided into short-term heat resistance (referred to as heat resistance) and long-term heat resistance (also known as thermal aging performance) according to the length of time the material is subjected to high temperature. Short term heat resistance and thermal aging performance are two completely different concepts, which cannot be confused. Short term heat resistance refers to whether the material softens, deforms, decomposes and other phenomena under high temperature or the change of performance index of the material under hot state, which is usually expressed in TG, TF, TM, TD, etc. Long term heat resistance refers to whether the resin polymer can obtain the expected life under a certain working temperature, which is usually expressed by the heat resistance grade and temperature index of insulating materials

generally, the temperature index is calculated according to the life value specified in the standard aging test. Therefore, the meaning of temperature index and softening point are different. Whether the material can be used at a certain temperature, not only can there be no significant performance changes in a short time (such as constant softness, no ignition, no significant decline in dielectric properties, etc.), but also there will be no undue performance changes in a long time. Therefore, if the service temperature of the material is to be determined, the short-term heat resistance and thermal aging performance must be measured at the same time Generally, the short-term heat resistance shall be measured first. When the short-term heat resistance meets the service conditions, the thermal aging test shall be further carried out to evaluate its temperature index, but the long-term heat resistance is emphasized in the insulation technology

4.2 long term heat resistance - insulation heat resistance grade

the thermal aging performance of insulating materials has long been expressed in terms of heat resistance grade. The current heat resistance grades in China are as follows:

the maximum allowable working temperature here is not equal to the short-term heat resistance index, for example: 902 resin thermal deformation temperature 82 Li Jianbo, Institute of building fire protection, Chinese Academy of Building Sciences, Sichuan Fire Protection Institute, Ministry of public security The fire protection test results of the external wall external insulation system carried out by the three authoritative institutions of the Tianjin Fire Protection Institute of the Ministry of public security are ℃, and the heat resistance grade is f; The heat deformation temperature of 890 resin is 135 ℃, and the heat resistance grade is h

due to high temperature resistant materials