Natural rubber has many valuable properties. Before synthetic rubber appeared in large quantities, natural rubber was a universal raw material for the rubber industry and its products, and was called "brown gold" . Simply put, natural rubber is actually formed by the condensation and coagulation of natural rubber latex. Among them, the chemical structure of rubber hydrocarbon is mainly cis 1,4 -polyisoprene (about 98 %), and its molecular structure is as follows:
Basic characteristics of natural rubber
1. Physical characteristics
1) Natural rubber does not have a certain melting point. It softens slowly after heating, and is completely softened to a molten state at 130-140 ℃; it begins to decompose at about 200 ℃, and rapidly decomposes at 270 ℃.
2) The glass transition temperature of natural rubber is -74～-69 ℃. It is slightly plastic at normal temperature, and gradually becomes harder when the temperature decreases . The elasticity drops greatly at 0 ℃, and becomes brittle when it cools to about -70 ℃. If the frozen raw rubber is heated to room temperature, it can still be restored to its original state.
3) Natural rubber has good elasticity, the modulus of elasticity is 2~4MPa , which is about 1/30000 of steel . Maximum elastic elongation up to 1000 % rebound at 0 to 100 up to the range deg.] C 70 ~ 85 %.
4) Natural rubber is a kind of crystalline rubber, which is self-reinforcing and has very good mechanical strength. The tensile strength of pure rubber vulcanized rubber is 17~25 MPa , while the vulcanized rubber reinforced by carbon black can be as high as 25~35 MPa . The strength retention rate at high temperature (93 °C ) is about 65 %.
5) gum vulcanizate better flex resistance, flex 20 is million to appear above this gap, the small hysteresis loss due to the natural rubber, low heat deformation results in multiple times.
6 ) Natural rubber is a good electrical insulation material, and the electrical insulation performance is better after removing the protein, and the volume resistance can reach 10 17 Ω . cm , and does not change much under wet or immersed conditions.
7) Natural rubber has good air tightness.
2. Chemical characteristics
1) Natural rubber is a highly reactive substance due to unsaturated double bonds. Each double bond forms a reactive point. They are distributed throughout the long chain of rubber molecules and dominate the chemical changes of rubber. The reaction can be divided into addition, substitution, cyclization, cracking, etc., which can become vulcanized rubber and other modified natural rubber or natural rubber derivatives. Natural rubber is uniformly mixed with the vulcanization system, and reacted for a certain time under a certain temperature and pressure, it will transform from a linear structure of raw rubber to a network structure of vulcanized rubber.
2) Natural rubber is prone to autocatalytic oxidation chain reaction with oxygen in the air, molecular chain breaks or excessive cross-linking, rubber sticks and cracks, and the physical and mechanical properties of rubber decrease, which is rubber aging. Light, heat, flexural deformation, copper, manganese, iron and other metals can promote rubber aging. Natural rubber without antioxidants will crack after being exposed to strong sunlight for 4 to 7 days. If it comes in contact with a certain concentration of ozone, cracks will even occur within a few seconds. Inability to age is the Achilles heel of natural rubber. However, by adding antioxidants, the aging performance of natural rubber can be greatly improved, so that it will not change much after two months of exposure to the sun, and it can still be used as usual after three years of storage in the warehouse . The aging rate of rubber accelerates at high temperature, generally according to the regular change that the aging rate increases by 1 time for every 10 ℃ increase in temperature . If the use temperature exceeds 130 ℃, the service life of natural rubber will be greatly shortened.
3) When stored for a long time at low temperature, natural rubber is easy to crystallize and become hard; at about 0 ℃, it takes about 37 days for vulcanized rubber to crystallize to about 50 % , while raw rubber only takes 6 days. However, this crystallization is reversible, and the rubber will return to its original elastic state once the temperature is raised.
4) Natural rubber is non-polar rubber. Therefore, it can withstand some polar solvents, but not oil and other non-polar solvents.
In addition, natural rubber has good alkali resistance, but it is not resistant to high concentrations of strong acids (such as 10% nitric acid, 37 % hydrochloric acid, 50 % sulfuric acid, etc.).
3 , with the natural rubber
Natural rubber raw rubber generally cannot be used directly in the production of products, and various compounding agents must be added to make a rubber compound, and after vulcanization, it can meet the performance requirements of various actual use conditions. According to the application requirements of natural rubber products, various compounding agents should be selected scientifically and reasonably (determining the type and amount, etc.) so that the natural rubber material has physical and mechanical properties that meet the application requirements, good processing performance and lower product costs. This process is called formula design of natural rubber.
The main coordination systems of natural rubber include: vulcanization system, accelerator system, protection system, reinforcing filling system, etc.
Curing system
The applicable vulcanizing agents for natural rubber are: a. sulfur, selenium, tellurium; b. sulfur donors; c. organic peroxides; d. esters; e. quinones.
Generally, different curing systems are used according to different performance requirements.
1 ) Sulfur vulcanization system
Sulfur vulcanization is the most traditional method in natural rubber processing. According to the different proportions of sulfur and accelerator in the vulcanization system, the sulfur vulcanization system can be divided into three types: ordinary vulcanization system, semi-effective vulcanization system and effective vulcanization system.
Ordinary vulcanization system, also known as traditional vulcanization system, is a vulcanization system that uses a high amount of sulfur and a small amount of accelerator. The standard amount of sulfur is 2.75 parts ( per 100 parts NR) . This vulcanizate has fewer single sulfur bonds, while polysulfide bonds are as high as 70 % or more. Due to the low bond energy and poor stability of polysulfide bonds, the heat resistance and aging resistance of the vulcanizate are poor, even if added A lot of antioxidants are not helpful, but the comprehensive physical and mechanical properties of this vulcanizate are better
2 ) Sulfur donor curing system
Commonly used sulfur donors for natural rubber are thiurams and dithiomorpholine (DTDM).
Thiurams (such as tetramethylthiuram disulfide TMTD, tetraethylthiuram disulfide TETD , tetramethylthiuram tetrasulfide TMTT, etc.) can release active sulfur at the curing temperature, so it can be used as The vulcanizing agent can vulcanize the rubber without adding sulfur (so-called "sulfur-free vulcanization"). Among them, TMTD is used more frequently. However, the rubber compound using thiuram has the disadvantages of easy blooming and short scorch time.
Dimorpholine disulfide DTDM is commonly used in semi-effective and effective curing systems. In vulcanization, when DTDM completely replaces sulfur, an effective vulcanization system is formed; if part of sulfur is replaced, a semi-effective vulcanization system is formed. The natural rubber compound with DTDM has a long scorch time, no blooming, no pollution, and good physical and mechanical properties of the vulcanized rubber.
3 ) Other curing systems
The ester vulcanization system refers to the urethane crosslinking system, which is an adduct of diisocyanate (TDI, MDI ) and p-nitrosophenol (p-quinone monooxime carbamate), which can give natural rubber a good Reversion resistance, heat resistance and aging resistance. It can improve the adhesion of natural rubber to cords, fabrics, steel wires and other materials.
The imide vulcanization system belongs to the high temperature vulcanization system. The vulcanizate has good reversion resistance and thermal stability, and has small compression set and good adhesion to glass fibers. Maleimide that can be used as a vulcanizing agent mainly contains N ,N`-m-phenylene bismaleimide, 4 , 4` -methylene bismaleimide, 2 , 6- di-tert-butyl- 4- (maleimide methyl) Phenol and 4 , 4' -dithiobisphenylmaleimide, etc. Among them, dithiobisphenylmaleimide and m-phenylene bismaleimide have the best effect.
Natural rubber can be vulcanized with organic peroxides. The most commonly used organic peroxide is dicumyl peroxide (DCP). The amount of DCP is 2 to 4 parts, and the cross-linked bond formed by the vulcanized rubber is a carbon - carbon bond. Vulcanized rubber has good thermal stability and excellent high temperature aging resistance, small creep, small compression set, good dynamic performance, and good reversion resistance. The disadvantage is that the rubber has a fast vulcanization speed, high crosslinking efficiency, and good dynamic fatigue resistance of the vulcanizate.
Such varieties include poly (2-diethylamino bis- 4 , 6 -dithiol triazine), poly ( 2 -hexamethylene bis- 4 , 6 -dithiol triazine) and poly ( 2 -N- methyl-cyclic bis-ethylamino -4 , 6- dithiol triazines).
The sulfur vulcanization system of natural rubber is usually equipped with active agents such as 2 to 5 parts of zinc oxide and 1 to 3 parts of stearic acid .
4 ) Accelerator system
The combination of accelerator and vulcanizing agent can activate the vulcanizing agent, accelerate the vulcanization reaction, and shorten the vulcanization time. The accelerators commonly used in natural rubber mainly include dithiocarbamates (such as accelerators ZDC, PZ, etc.), thiuram accelerators (such as accelerators TMTD , TETD, etc.), thiazoles (such as accelerators D , DM , etc.) Etc.), sulfenamides (such as accelerators CZ , AZ , DZ, etc.).
When selecting accelerators for natural rubber, the following two points should be considered: One is the critical temperature of accelerator vulcanization, that is, the temperature at which the accelerator starts to accelerate during the vulcanization process (see Table 5-3). Generally, accelerators with a low critical temperature are easy to scorch, but the reaction speed is fast; accelerators with a high critical temperature are not easy to scorch, and only react at a higher temperature. For thick-walled products, in order to prevent early scorching of the rubber, accelerators with a higher critical temperature should be used.
Ordinary sulfur vulcanization system (conventional vulcanization system, also known as high-sulfur low-accelerating system) is a vulcanization system that uses a high amount of sulfur and a low amount of feed agent. Its crosslinking bonds are mainly polysulfide bonds. General physical and mechanical properties are good, manifested by high strength, good elasticity, high wear resistance, low cost, but poor heat resistance, aging resistance, and high reversion during vulcanization. Due to the high degree of unsaturation of natural rubber, the amount of sulfur can be more than that of synthetic rubber. The amount of sulfur in soft rubber products is about 2 to 3 parts, and usually 1.75 to 2.75 parts. When the amount of accelerator is less than 1 part and the amount of sulfur is more than 2.5 parts, the physical and mechanical properties such as tensile strength and elongation do not change much, but the permanent deformation, hardness and tensile stress increase. Use accelerators include M , DM , CZ , NOBS , D , TT, etc., which can be used singly or in combination. Commonly used forms are DM , D/TS , D/DM , M/H , M/TT , M/TS , CZ/TT , DM/D/H , DM/M/TT, DM/TT , CZ/DM , NOBS/DM, etc.
The semi-effective vulcanization system is between the ordinary sulfurization system and the effective sulfur vulcanization system. The semi-effective vulcanization system is composed of medium sulfur (1 to 1.7 parts) and accelerator. There are polysulfide bonds as well as single and disulfide bonds in crosslinking bonds. Its vulcanizate has multiple comprehensive functions such as heat resistance, fatigue resistance and resistance to reversion, so it is widely used. Typically equipped with S/NS (1.5/1.5) , S/NS/DTDM (1.5/0.6/0.6) .
5 ) Protection system
Natural rubber is a highly unsaturated rubber. During storage and use, raw rubber and its rubber products are degraded by oxygen, ozone, ultraviolet rays and other active substances, resulting in loss of tensile strength and cross-linking and hardening of rubber chains. In order to prevent the degradation of natural rubber, adding antioxidants is an extremely effective method. A good antioxidant can increase the service life of rubber products by several tens of times. The antioxidants commonly used in natural rubber are: amine antioxidants (antioxidants A, D , 4010 , 4010NA , 4020, etc.), phenolic antioxidants (antioxidants 264 , SP, etc.), bonded antioxidants (antioxidants NDPA, etc.) ), and paraffin and so on.
6 ) Reinforcing filling system
According to the requirements of the formula, adding porous powder quartz filler can improve the physical properties of the rubber compound and show a better reinforcing effect. Compared with other silicon powders, its natural amorphous silica has the characteristics of high fineness, high activity and light specific gravity, so it is easier to match and fuse with other materials such as high molecules in rubber. According to the test results, the filler can replace part of carbon black and white carbon black to reduce production costs.
When abrasion resistance is required in the blending of natural rubber, super wear-resistant carbon black (N110), medium-super wear-resistant carbon black ( N220 ) and high wear-resistant carbon black ( N330 ) can be added. When it is required to improve the tear resistance, channel carbon black has the best effect. When the strength is not high, the elasticity is good, and the processing performance is good, the fast pressing furnace black ( N550 ), general furnace black ( N660 ), semi-reinforced furnace black ( N770 ), etc. can be used. When selecting carbon black, pay attention to the particle size, structure, surface activity and PH value of carbon black . PH value affects the vulcanization speed. Therefore, the variety and dosage of the vulcanization system should be adjusted according to the acidity and alkalinity of the carbon black. The aftereffect accelerator should be selected for furnace carbon black, and the amount should not be too large.
For light-colored and brilliant-colored products, natural rubber can be added with precipitated white carbon black, activated white carbon black, ultra-fine porous powder quartz, etc. as a reinforcing agent. When adding precipitated white carbon black, it should be equipped with an organic active agent diethylene glycol (one shrinkage). Diethylene glycol), glycerin (glycerol), ethanolamine, polyvinyl alcohol, etc. adjust the vulcanization speed, and the dosage is generally 5% to 10% of silica .
The amount of reinforcing agent can be determined according to product performance requirements and gel content index. Generally speaking, to obtain the highest strength, the appropriate amount is 40 to 45 parts. If the amount is too large, the strength will decrease. If low strength and low cost are required, fillers such as pottery clay and CaCO3 can be added to reduce the gel content.
Carbon black is the most commonly used reinforcing agent. The properties of rubber products are closely related to the properties of the selected carbon black. Therefore, we must choose carbon black according to the performance requirements of rubber products. Generally speaking, when the wear resistance of rubber products is required to be improved, super wear resistance, super wear resistance and high wear resistance furnace black can be selected; when the tear strength is required to be improved, channel carbon black has the best effect; For tensile stress, high-density furnace black can be selected; when high elasticity is required, fine-particle furnace black and medium-particle thermal cracking furnace black can be selected; when good processing performance is required, fast-pressed furnace black, general furnace black, and full Use furnace black. In addition, when selecting furnace black, the particle size, structure, surface activity and PH value of carbon black should be considered.
In addition to carbon black, silica (chemical composition is SiO 2) is another important reinforcing filler, which can improve the tear strength of natural rubber, reduce crack expansion, and improve the adhesion of rubber and cord.合 performance.
The common heavy/light calcium carbonate, titanium dioxide, talcum powder, clay, etc. are non-reinforcing fillers, which can increase the volume and specific gravity of the rubber and reduce the cost of the product. If these fillers are activated or added with a coupling agent, they can sometimes obtain a certain reinforcing effect (such as activated calcium carbonate, clay treated with coupling agent, etc.).
7 ) Softening and plasticizing system
1. Physical plasticizer
The use of softening and plasticization is to obtain good process performance, such as viscosity and flexibility, to facilitate mixing, calendering, extrusion molding and other process operations, and to adjust the hardness of the rubber compound. The choice of softening plasticizer should consider the impact on physical and mechanical properties. Commonly used varieties of softening plasticizers are pine tar and solid coumarone. The general dosage is 4 to 5 parts, and various types of petroleum softeners are 5 to 15 parts.
2. Chemical plasticizer (plasticizer)
Smoked sheet rubber and crepe sheet rubber in natural rubber have high Mooney viscosity, which brings certain difficulties to the mastication process. In the past, mastication was generally added with additives M and DM for plastification, but the effect was not satisfactory. Adding chemical peptizers can improve the plasticizing effect of raw rubber, shorten the time and improve efficiency. Commonly used peptizers for natural rubber include SJ-103 and 12-I, etc., with an amount of 0.1 to 0.3 parts. The former is suitable for open mixer and internal mixer, while the latter is only suitable for internal mixer plastic refining process.
Reprinted from: Rubber Technology Li Xiuquan Studio