Selection and control of raw materials for friction materials
This paper comprehensively expounds the selection principle of raw materials for friction materials, the types and varieties of commonly used raw materials, the main technical indicators and their significance of various raw materials, and the management of raw materials.
The friction brake pad used in automobile is a kind of main safety component which seems to be ordinary on the vehicle, but actually it is the main safety part to determine the safety of people and vehicles. It is the responsibility of our colleagues in friction material industry to continuously improve the technical indicators of products and ensure the stability of product quality.
There are four key links in the production of friction materials, that is, formulation design, selection and control of raw materials, production technology and quality control. Every step of the formula must be meticulous.
Friction material is a compound composed of many kinds of materials. When the vehicle is braked, the friction dual interface composed of the friction brake block and the brake drum or brake disc absorbs the kinetic energy of the vehicle motion through friction, and the molecular motion accelerates and the temperature rises. In this way, a series of physical changes and a series of chemical changes that change the structure and composition of the substance will take place at the interface. It is the key to improve the technical level of friction materials to prevent and eliminate the influence of these changes on the friction properties of friction materials and friction dual contact interface. Therefore, the raw materials of friction materials are the premise of the production of friction materials, and the physical and chemical properties of relevant raw materials must be studied when designing the formula.
Determine several links of raw materials.
一、 Determine the standards and specifications of raw materials to achieve scientific and reasonable application.
(1) Raw material standards and specifications should be as close as possible to national standards or industrial standards. This is conducive to the quality control in the process of procurement and future use.
(2) If the existing standards can not meet the use requirements, special processing can be negotiated with the manufacturer.
(3) Learn from the advanced raw material standards of international advanced friction material industry.
(4) According to the characteristics of the design formula, special raw materials and composite materials are designed or processed by special processes, so that they have some special functions needed for composite.
(5) Materials are widely available, easy to obtain and reasonable in price.
二、 The type and variety of raw materials of friction materials can be selected preferentially
(1) . binder
At present, the common binders for friction materials are phenolic resin and rubber. Binder is the key material of friction material.
Phenolic resin: commonly used and relatively mature technology, relatively stable quality of pure phenolic resin, nitrile rubber modified phenolic resin, cashew nut shell oil modified phenolic resin.
Rubber: nitrile rubber, styrene butadiene rubber.
(2) Reinforcement materials: aramid pulp, glass fiber, steel fiber, cotton fiber, ceramic fiber, mineral fiber, acrylic pulp, wood fiber, copper wire, etc.
(3) . friction materials: corundum, calcined alumina, silicon carbide, silicon dioxide.
(4) . antifriction materials: flake graphite, earth graphite, artificial graphite, molybdenum disulfide
(5) Friction material performance regulator: antimony sulfide, molybdenum disulfide, potassium titanate whisker, zircon powder, zinc sulfide, stannous sulfide, chrome green, copper powder, tin powder, zinc powder, friction powder, calcined petroleum coke, heavy magnesium oxide, reduced iron powder, nitrile rubber powder, tire powder, etc.
(6) . filling materials: barium sulfate, heavy calcium, light calcium, calcium hydroxide, vermiculite, mica, talcum powder, kaolin.
(7) . Colorants (pigments): iron black, iron red, carbon black, iron yellow, iron green
Classification is not absolute. Many materials have many functions. Many materials in the filler play an important role in friction materials. For example, barite powder (barium sulfate), calcium hydroxide, vermiculite, etc. It also has a good adjustment effect on the properties of friction materials. The same importance must be attached to it.
三、 Main items required by material technical index:
(1) Purity / chemical composition: refers to the content of the required substance.
The higher the purity of the selected material, the more it can reflect the inherent properties of the material, and the more able to achieve the original intention of the formula design. It is assumed that 90% flake graphite is selected, but the actual purchased material is less than 90%, which can not achieve the expected effect of reducing friction and wear. Barite powder contains SiO2 / silicate, which causes great wear, noise and friction damage.
Friction materials are ultimately mixtures, so the use of materials does not require high purity. The higher the purity, the higher the cost. It is necessary to determine an indicator value based on the data, the experience of the identified company, and the application practice of the company. Reliable methods should be combined with national standards or industry standards. High purity and high price.
As long as the standard is determined, the standard requirements can not be reduced at will with the price. I have encountered more than a few companies to apply, flake graphite contains 28% sand, vermiculite contains 31.4% sand, barite powder contains 28% silica and 45% calcium. Such materials must not be used. It seriously affects the product quality.
The company does not necessarily determine the chemical composition of raw materials, because the chemical analysis is complex, but the supplier must guarantee.
(3) Specific gravity: the inherent property of an object. Each raw material has a standard value, data. Reference books / Handbooks of science and technology all contain information on the standard specific gravity / density of substances. The more composite standard value of the purchased material is, the more this parameter of the test material reflects its purity to a certain extent. When selecting raw materials, the specific gravity should be close to the standard value. For example, barium sulfate, barite powder, the specific gravity should be 4.5 g / [cm] 3. Baidu found 4.2-4.5g / [cm] 3. According to the purity on the market. The standard adopted by our company requires 4.35g / [cm] 3, the content of BaSO4 is 92%, and the minimum is 88%. Frodo requires 4.15-4.4g / [cm] 3, and BaSO4 content is 90-98%. Afania requires 4.35g / [cm] 3, and BaSO4 content is 94%.
测试比重最经典的方法是比重瓶。不困难，需要的是细致。可以查到测定方现在市售的有很先进的测试设备。如美国产的Accupyc 1330 真密度分析仪。
The most classic way to test specific gravity is the pycnometer. It's not difficult. It needs to be meticulous. It can be found that the testing side is now on the market with very advanced testing equipment. Such as accupyc 1330 real density analyzer made in the United States.
（3） Granularity: a very important physical parameter. Generally, the particle size of domestic materials is expressed by mesh number. The particle size has a great influence on the properties of friction materials. The smaller the particle size of the material is, the larger the surface area is, that is, the larger the contact surface is. The greater the binding force between the material and the organic binder is, the better the product performance is, and the better the mechanical properties such as rigidity, toughness and impact strength of the product are. If the hardness and particle size of the friction increasing material is high, it is easy to produce noise and damage friction duality. It is necessary to find the appropriate particle size according to the function of materials in friction materials. For example, the particle size of molybdenum disulfide is small, easy to disperse and uniform, and the effect is good; heavy calcium carbonate is a rubber reinforcing agent, which needs it to disperse and combine into the rubber branch chain, so it needs small particles
The size of material is generally measured by mesh number in China. According to my experience, many materials below 320 mesh can not be simply measured by their mesh size, and there must be provisions for particle size. For example, barite powder, molybdenum disulfide, antimony sulfide, white corundum, silicon carbide, calcium hydroxide, heavy calcium carbonate, talc powder.
Standards adopted by our company (examples)
The particle size of materials was determined by the following methods：
Materials with particle size greater than 325 mesh are generally measured by vibrating screen.
If the particle size is less than 325 mesh, it can be sieved by sample screen, but auxiliary measures must be taken. Rubber ball is added on the vibrating screen or washed with water to screen.
The traditional test methods for materials with particle size less than 325 mesh include sedimentation method, scraper method and electron microscope method.
Now there are many models of "laser particle size analyzer" to measure the actual size of particles. Very convenient and practical.
(4) Particle size distribution: the particle size of a material, that is, all the particle sizes are absolutely not consistent. There is a particle size distribution, so we can not say how many items in general. Different material particle size distribution, the product has different mechanical structure. The change of particle size distribution of each batch of materials will cause the change of friction properties. Everyone has encountered this situation, nothing has changed, the friction coefficient has increased or decreased inexplicably, and the wear rate has also increased or decreased. Nothing moved. It recovered. It is likely to be caused by the change of particle size distribution of the material. Therefore, the physical and chemical indexes of each batch of materials must be consistent and stable.
For example, for the particle size distribution of commonly used calcined petroleum coke, we use the following indicators.
(5) Loose density and tap density:
松散密度是粉末自然堆积的密度,取决于颗粒间的黏附力、滑动的阻力以及粉体孔隙被小颗粒填充的程度、 粉末体的密度、颗粒形状、 颗粒密度和表面状态、 粉末的粒度及粒度组成等因素。粉末颗粒形状愈规则, 其松散密度就愈大; 颗粒表面愈光滑, 松散密度也愈大; 粉末颗粒愈粗大, 其松散密度就愈大; 细粉末形成拱桥和互相黏附妨碍了颗粒相互移动, 故粉末的松散密度减小。粉末颗粒越致密, 松散密度就越大。粉末粒度范围小的粗细粉末, 松散密度都较低。当粗细粉末按一定比例混合均匀后, 可获得最大松散密度, 因为粗颗粒之间的大孔隙可被一部分细颗粒填充。
Loose density is the density of natural accumulation of powder, which depends on the adhesion between particles, the resistance of sliding, the degree of filling pores by small particles, the density, shape, density and surface state of powder, particle size and particle size composition. The more regular the shape of powder particles, the greater the loose density; the smoother the surface of the particles, the greater the loose density; the coarser the powder particles, the greater the loose density; the arch bridge formed by fine powder and adhesion to each other hinder the particles from moving each other, so the loose density of the powder decreases. The denser the powder particles are, the greater the loose density is. The loose density of coarse and fine powders with small particle size range is low. When the coarse and fine powders are mixed evenly in a certain proportion, the maximum loose density can be obtained because the large pores between the coarse particles can be filled by some fine particles.
Tamping density refers to the mass per unit volume of the powder in the container measured under specified conditions. When the dry powder particle group is applied with vibration and other external forces, it reaches the limit packing density, which is called vibrating density.
Apparent density (bulk density) and tap density are one of the measurable macro characteristics related to the size, morphology, size distribution and degree of drying (moisture content) of ultrafine particles. They are also the most commonly used quality control parameters for the production and application of ultra-fine powder products.
After the purity and particle size of the material are determined, the loose density and tap density should be a fixed range, which can roughly measure the purity and quality stability of the material, and can be used as a reference for calculating the feeding amount and mold cavity.
For example: loose density and tap density of commonly used calcined petroleum coke
(6) . specific surface area of powder material
The total surface area of a powder particle by volume. It is usually expressed in m2 / g or m2 / m3.
The specific surface area of powder material is a kind of powder property used to indicate the activity of powder. The former is called mass specific surface, and the latter is called volume specific surface.
Surface area: in general, the specific surface area of powder refers to the sum of the external surface area of unit mass powder particles and the surface area of internal pore structure. Specific surface area is divided into internal specific surface area, external specific surface area and total specific surface area. Generally, materials with large specific surface area have high activity and strong adsorption capacity.
The finer the powder material, the higher the surface roughness and the larger the specific surface area.
Materials with large surface area have high adsorption capacity, such as activated carbon, with large specific surface area and strong adsorption capacity. Used in friction materials, one of the functions is to absorb the gas generated in the reaction to avoid blistering and cracking during the pressing process.
The material with large surface area has strong adsorption capacity, which affects the amount of phenolic resin.
The finer the dust particles are, the larger the specific surface area is. Fine particles often exhibit significant physical and chemical activities, such as oxidation, dissolution, evaporation, adsorption, catalysis and physiological effects, which can be accelerated due to their larger specific surface area. The explosion risk and toxicity of some dust increase with the decrease of particle size. The wettability and adhesiveness of dust are also related to its specific surface area. Dust is a headache in the production of friction materials. The smaller the particle size of powder material, the more serious the dust.
The technical index of materials should be considered comprehensively. The combination of different materials makes the comprehensive performance and processability of the product reach the best state.
(7) . other technical specifications of materials
Apparent volume in compression (fibrous materials)
DBP oil absorption rate: the absorption amount of dibutyl phthalate (DBP) absorbed by a certain amount of materials --- grams. Reference to determine the amount of phenolic resin.
Volatile matter at 105 ℃, ignition loss / residue at 800 ℃, acetone extraction, pH value, hardness, melting point and decomposition temperature.
Specific heat: if the specific heat of the selected material is large, the temperature rise of the brake pad will be low and the damage will be small.
Thermal conductivity: low thermal conductivity, poor heat dissipation and fast temperature rise.
High thermal conductivity, good heat dissipation performance, heat transfer to the back plate and brake pump, high oil temperature rise.
The characteristics of various materials need to be taken into account.
四、 Technical index of phenolic resin:
After the formulation of friction material is determined, the selection of binder phenolic resin is the key to ensure the technical index of friction material.
At present, the friction material products at home and abroad are basically traditional organic matrix composites, and most of the binders are phenolic resin. Some companies add a small amount of nitrile rubber and styrene butadiene rubber in the formula, its purpose and function is to increase the toughness and reduce the hardness of products. The technical index and quality of phenolic resin as binder play a key role in friction performance, hardness and impact strength of products.
Good selection of resin products have stable friction performance, good heat degradation, stable friction coefficient, long wear-resistant life, moderate hardness, and low noise generation probability.
Several indexes of phenolic resin were selected：
1. Softening point: the softening point indicates the condensation degree of phenolic resin. High softening point, high condensation degree, high molecular weight and good heat resistance. The hardness of the product is low. Phenolic resin with low softening point has low molecular weight, high hardness and low decomposition temperature. When mixing, it is easy to agglomerate and has poor looseness.
2. Flow length: the flow length is inversely proportional to the molecular weight. The flow length is large, the molecular weight is small, the process performance is good, the product structure is uniform and the molding is good.
3. Polymerization speed: an important technical index, which is used to determine the pressing process and reasonably arrange the times of outgassing and holding time. Long polymerization speed, good processability and low pressing efficiency.
4. Free phenol content: should not be too high. The free phenol in phenolic resin is easy to cause foaming, delamination and thermal degradation. High content of free phenol indicates low degree of polymerization. The free phenol content of foreign high-grade phenolic resin is less than 1%.
5.moisture / volatile: should not be too high. High moisture / volatile content indicates that the dehydration of phenolic resin is not good in the production process. Moisture / volatile matter is easy to cause defects such as bubbles and delamination.
6.50% acetone solution viscosity: this test shows the degree of polymerization and molecular weight of phenolic resin. The larger the data, the higher the degree of polymerization, the larger the molecular weight, the better the relative adhesion.
7.Content of hexamethylene tetramine: hexamethylenetetramine is the curing agent of thermoplastic resin, the general dosage is 6-10%. The supplier adjusts the dosage according to the specific data of the resin to adjust the polymerization rate of phenolic resin. Phenolic resin is active in summer and should be added less. The activity of phenolic resin is low in winter and its dosage is less than that in summer. The redundant hexamethylene tetra press is one of the main causes of product delamination and foaming. The content of hexamethylene tetrakis in high quality phenolic resin is only 6.5%.
1. Phenolic resin for drum brake lining: in general, the molecular weight is greater than 4000, and the distribution of molecular weight is controlled.
Polymerization speed 60-80 seconds
Flow length 23-33mm
The viscosity of 50% acetone solution is 60mpas
The contents of water and free phenol were less than 1%
The softening point is about 102 ℃
2. Application examples of disc brake pads
The polymerization speed is 18-28 seconds
Flow length 35-50mm
50% acetone solution viscosity 37 MPAs
The contents of water and free phenol were less than 1.5%
Softening point, 90 ℃
Because of the different kinds of phenols and aldehydes, the types of catalysts and the molar ratio of phenol to aldehyde, a variety of phenolic resins with different indexes can be produced, which should be comprehensively analyzed.
The raw material of friction material is the premise of the production of friction material products. The physical and chemical properties of relevant raw materials must be studied when designing the formula, and high-quality raw materials must be purchased.
五、 Raw material management
After the raw materials are purchased and put into storage, management is an important factor to determine whether the raw materials can play their inherent role.
According to the technical indicators of raw materials, formulate corresponding and reliable inspection methods, and purchase necessary inspection equipment. Only in this way can the purchased raw materials meet the use requirements.
2. 准确标识，合理存放，避免混杂 。
2. Accurate identification, reasonable storage, avoid mixing.
3. Take different storage measures according to different characteristics of raw materials
(1) Phenolic resin, rubber, binder and other organic materials are easy to decompose and deteriorate, or gradually reduce their effect. Temperature is the key factor to increase the rate of chemical reaction. When exposed to the environment above 18 ℃, the technical properties will change within a few days.
For example, the best storage condition of phenolic resin is 0-25 ℃. The longest storage period is 180 days. Store in the workshop for no more than 9 days. If possible, they should be kept in the cold room.
(2) All raw materials should be purchased in the order of purchase, and the earliest batch should be used first. If the storage period or storage temperature is exceeded, new sampling must be done. The raw material testing laboratory will specify a new application time limit according to the measured data.
(3) Some materials are easy to absorb water and must be stored in the drying room. After opening the package, the bag must be tied tightly and kept airtight.
(4) Some materials are easy to agglomerate and need to be stored in a dry room. They must be crushed before use. But the phenolic resin was basically scrapped after caking.
(5) Some materials are very active and easy to oxidize. Such as metal powder: iron powder, zinc powder, aluminum powder, steel fiber, copper powder, copper wire dimension, etc. Some materials such as calcium hydroxide are easy to absorb carbon dioxide in the air and produce calcium carbonate through chemical reaction.
Therefore, reasonable storage measures should be taken according to the nature of the materials.
The above is our company's practical experience, please correct me.
Zhang Hao, Shijiazhuang Shuo Ruo Technology Co., Ltd