I. Introduction
Compared with traditional materials, composite materials have a series of irreplaceable characteristics and have developed rapidly since the second occupation. Although the output is small (according to the statistics of Vetrotex company in France, the global composite materials reached 7 million tons in 2003), the level of composite materials is one of the signs to measure the scientific, technological and economic level of a country or region. The United States, Japan and Western Europe have a high level. North America and Europe account for 33% and 32% of the global output respectively, and Asia, mainly China (including Taiwan Province Province) and Japanese, accounts for 30%. In 2003, the output of glass fiber reinforced plastics (glass fiber and resin composite materials, commonly known as "glass fiber reinforced plastics") in Chinese mainland exceeded 900,000 tons, ranking second in the world (in 2003, it was 6,543,800 tons in the United States and less than 700,000 tons in Japan).
Composite materials are mainly composed of reinforcing materials and matrix materials:
Reinforcing material: The main mechanical properties given to the composite material by not forming a continuous phase in the composite material. For example, glass fiber in FRP and carbon fiber (carbon fiber reinforced plastic) in CFRP are reinforcing materials.
Matrix: A single material that constitutes the continuous phase of composite materials, such as the resin in glass fiber reinforced plastic (GRP) (epoxy resin mentioned in this paper) is the matrix. y
According to the different matrix materials, composite materials can be divided into three categories:
Resin composite material
Metal matrix composites
Inorganic non-metal matrix composites, such as ceramic matrix composites.
In this paper, epoxy resin matrix composites are discussed.
1, why use epoxy resin as matrix?
Curing shrinkage is low, only 1%-3%, while unsaturated polyester resin is as high as 7%-8%;
Strong adhesion;
B stage, which is beneficial to the production process;
Can be cured at low pressure and has very low volatility;
After curing, it has good mechanical properties, chemical resistance and electrical insulation.
It is worth pointing out that epoxy resin has better resistance to organic solvents and alkali than phenolic resin and unsaturated polyester resin, but its acid resistance is poor. After curing, it is generally brittle and has poor toughness.
2. Performance of epoxy FRP (according to ASTM)
Taking the glass fiber reinforced epoxy resin products manufactured by FW (filament winding) method as an example, it is compared with steel.
Comparison of table 1 GF/EPR and steel properties
Glass content GF/EPR (glass fiber content 80wt%) AISI 1008 cold-rolled steel.
The relative density is 2.08-7.86V.
The tensile strength is 55 1.6Mpa 33 1.0MPa.
The tensile modulus is 27.58GPa 206.7GPa.
Elongation 1.6% 37.0%
The bending strength is 689.5MPa.
The flexural modulus is 34.48GPa.
Compressive strength 3 1.3 MPa 33 1.0 MPa.
Cantilever impact strength 2385J/m
Flammability (UL-94) V-O
Specific heat capacity 535J/kg? k 233J/kg？ k
The expansion coefficient is 4.0×10-6k-16.7×10-6k-1.
Thermal deformation temperature 204? C (1.82mpa)
Thermal conductivity 1.85W/m? k 33.7W/m？ k
The dielectric strength is11.8×106v/m.
Water absorption 0.5%(24 hours)
Table 2 Specific Strength and Modulus of Several Common Materials and Composites
Material name density g/cm3 tensile strength×104mpa elastic modulus×106mpa specific strength×106cm specific modulus×109cm.
Steel 7.810.10 20.59 0.130.27
Aluminium 2.8 4.617.35 0.17 0.26
Titanium 4.59.41.1.1.1.80.21.25
FRP 2.010.40 3.92 0.53 0.5438+0
Carbon fiber/epoxy resin1.4514.7113.73
Carbon fiber/epoxy resin 1.6 1049 23.54
Aramid fiber/epoxy resin 1.4 13.73 7.85
Boron fiber/epoxy resin 2. 1. 13.53 20.59
Boron fiber/aluminum 2.65 9.81.1.9 4438+0.75c2
Second, the fiber reinforced epoxy resin composite molding process.
1, layup.
(1) summary is applied to the mold surface in turn.
demould
Gel coating
A layer of moderately active liquid thermosetting resin with a viscosity of 0.3-0.4PaS (after the gel coat is cured).
A layer of fiber reinforced material (glass fiber, aramid fiber, carbon fiber, etc.), including felt, untwisted coarse gauze (plaid cloth), etc. Impregnate the fiber reinforced material with resin with a hand-held roller or brush, and drive out bubbles to compact the base. The layering operation is repeated many times until the design thickness of the product is reached.
Due to polymerization, the resin is cured at room temperature. It can be heated to accelerate curing.
(2) Raw material f gb ng
Resin unsaturated polyester resin, hexene ester resin, epoxy resin, phenolic resin, etc.
Glass fiber, carbon fiber, aramid fiber, etc. Although it is difficult to impregnate thick aramid fabric with resin by hand, it can also be used.
The core material is arbitrary.
(3) Advantages
1) is suitable for small-scale production;
2) it can be molded at room temperature, with less equipment investment and low depreciation cost of the mold;
3) Large products and complex products can be manufactured;
4) the resin and the reinforcing material can be freely combined, and the material design is easy;
5) Local reinforcement can be adopted, and metal parts can be embedded;
6) The surface with free color can be obtained by using the gel coat layer (one side is not smooth if the mold is opened);
7) The glass fiber content is higher than that of spray molding.
Twistless coarse gauze accounts for about 50%
35%-45% fabric
30-40% chopped strand mat
(4) Disadvantages
1) belongs to labor-intensive production, and the product quality is determined by the training level of workers; ;
2) The glass fiber content should not be too high; Resin needs low viscosity for manual handling, and the amount of solvent/styrene is high, which limits its mechanical and thermal properties.
3) The molecular weight of resin for hand paste is low; Generally, resins with large molecular weight may be harmful to human health and safety.
(5) Typical products
Ship, wind turbine blade, amusement equipment, cooling tower shell, building model.
2. Resin transfer molding (RTM)
(1) summary
RTM is a closed-die low-pressure forming method.
Placing the fiber reinforced material between the upper die and the lower die; Closing the mold and clamping the mold; Injecting resin under pressure; After the resin is cured, open the mold and take out the product.
Before the gelation process of the resin begins, the resin must be filled into the mold cavity, and the pressure urges the resin to quickly transfer to the mold to impregnate the fiber material.
RTM is a low-pressure system, and the injection pressure of resin is between 0.4-0.5MPa and 0.5MPa. When manufacturing products with high fiber content (over 50% by volume), such as aerospace parts, the pressure even reaches 0.7MPa.
Fiber reinforced materials can sometimes be preformed in one mold (with adhesive) in advance and then injected into a second mold. Variable vacuum resin injection can be selected to improve the ability of resin impregnating fibers.
Note that once the fiber material is saturated with resin, the resin injection port should be closed so that the resin can be cured. Injection and curing can be carried out at room temperature or under heating conditions. The mold can be made of composite materials and steel materials. If a heating process is used. Steel molds should be used.
(2) Raw materials
Resin: epoxy, unsaturated polyester, vinyl ester and phenolic resin are generally used; When heated, high temperature resin can also be used as bismaleimide resin.
The French company Vetrotex developed the thermoplastic resin RTM.
Fiber: Any. Commonly used glass fiber continuous felt, sewing material (the gap between fibers is obtained by resin transfer) and untwisted coarse gauze; Composite yarn of glass fiber and thermoplastic and its fabric and sheet (Vetrotex trade name TWINTEX, France).
Core material: there is no need for honeycomb, because the honeycomb space is filled with resin, and the pressure will lead to its destruction. Solvent-resistant foaming materials such as PU, PP, CL and VC can be used.
(3) Advantages
1) the fiber content of the product can be very high, and the part that is not saturated by the resin is very small;
2) closed mold forming, good production environment;
3) The labor intensity is low, and the requirements for workers' technical proficiency are lower than those of hand paste and spray molding;
4) the product is polished on both sides, which can be used as a product with a gel coat on the surface, and the accuracy is relatively high;
5) short molding cycle;
6) The product can be enlarged;
7) Strength can be oriented according to design requirements;
8) It can be formed integrally with the core and the insert;
9) Compared with injection molding equipment and molds, the cost is lower.
(4) Disadvantages
1) It is not easy to make smaller products;
2) Because of the pressure, the mold is heavier, more complicated and more expensive than the mold used in hand paste and spraying process;
3) There may be unimpregnated materials, resulting in waste.
(5) Typical products
Small aircraft and automobile parts, bus seats, instrument housings.
3. Fiber winding
(1) summary
Direct roving is usually used as reinforcing material. Roving is arranged on the creel. Roving is unwound from creel, passed through tension system, resin tank and winding nozzle, driven by trolley to reciprocate, and wound on rotating mandrel (die). Fiber winding angle and fiber arrangement density are designed according to strength, and accurately controlled by the ratio of spindle (die) speed to trolley reciprocating speed. And demoulding the wound composite product after curing.
For some products with closed ends, demoulding is not needed, and the core mould is wrapped in composite products as lining.
(2) Raw materials
Resin: Any. Epoxy resin, unsaturated polyester resin, vinyl ester resin and phenolic resin.
Fiber: Any. Untwisted roving, sewing thread and non-woven fabric. When producing pipes and cans, surface felt and chopped strands are often used as lining materials.
Core material: available. Although composite products are usually single shell, they are generally not used.
(3) Advantages
1) Because the fibers are laid in a reasonable linear shape and bear the load, the structural characteristics of composite products can be very high;
2) due to the combination with the lining, it can be made into corrosion-resistant, pressure-resistant and heat-resistant products;
3) products with closed ends can be manufactured;
4) Fast and economical paving, untwisted roving and low material cost;
5) Resin can be used for metering, and then the resin content of impregnated fiber can be controlled by extrusion or die;
6) It can be produced and automated in Dali;
7) mechanical molding, the composite material has uniform material and directionality and stable quality.
(4) Disadvantages
1) The shape of the product is limited to cylindrical or other revolving bodies;
2) The fibers are not easy to be arranged accurately along the length direction of the product;
3) For large products, the mandrel cost is high;
4) The appearance of the finished product is not "molded", which is not satisfactory;
5) For products under pressure, if the resin is not suitable or there is no lining, it is easy to leak.
(5) Typical products
Pipes, storage tanks, gas cylinders (fire breathing gas cylinders, compressed natural gas cylinders, etc.). ), solid rocket engine shell.
4. Reaction injection molding
(1) summary
Two or more components are mixed at a low pressure (0.5MPa) in the mixing zone, and then injected into a closed mold at a low pressure (0.5- 1.5MPa) for reaction molding, that is, the technological process. If the first component is polyol and the second component is isocyanate, the reaction will produce polyurethane. In order to increase strength, ground glass fiber precursors and/or fillers can be directly added to a component. The machine can be reinforced by long fibers (such as continuous fiber felt, fabric, composite felt, preform of chopped strand, etc.). ) and the long fiber reinforced material is placed in the mold in advance before injection. Products with high mechanical properties can be obtained by this method. This process is called SRIM (structural reaction injection molding).
(2) Raw materials
Resin: commonly used polyurethane system or polyurethane/urea mixed system; Can also use epoxy, nylon, polyester and other basic;
Fiber: commonly used wool glass fiber with a length of 0.2-0.4 mm;
Core material: no.
(3) Advantages
1) The manufacturing cost is lower than that of thermoplastic injection molding process;
2) Large products with complex beginnings can be manufactured;
3) fast curing, suitable for rapid production.
(4) Disadvantages
It is expensive to use ground glass fiber to reinforce raw materials, so it is suggested to use mineral composite materials instead.
(5) Main products
Automobile dashboard, bumper, building door, window, table, sofa, electrical insulation.
5. Pultrusion molding
(1) summary
Mainly use glass fiber roving (put on the creel in advance before use) to provide longitudinal reinforcement (along the direction of production line).
Other types of reinforcement materials include continuous precursor mats, fabrics, and the like. , that is, to supplement the transverse reinforcement, and the surface felt is used to improve the surface quality of the finished product. Fillers can be added to the resin to improve the material properties (such as flame retardancy) and reduce the cost.
The process of pultrusion is as follows
1) impregnating glass fiber reinforced material with resin;
2) after the glass fiber is preformed, it enters a heating mold for further impregnation (glue extrusion), curing of basic resin and setting of composite materials;
3) Cut the profile according to the required length. At present, the forming technology of pultruded products with variable cross-section and arc length has been developed. In pultrusion, there are two ways to impregnate reinforcing materials with resin:
Glue tank dipping method: This method is usually adopted, that is, the reinforcing material is dipped in the resin tank and then enters the mold. The method has the advantages of cheap equipment and good operability, and is suitable for unsaturated polyester resin and vinyl ester resin.
Injection impregnation method (Figure 6): The glass fiber reinforced material is impregnated by the resin injected into the mold after entering the mold. This method is suitable for producing resin matrix with short gel time and high viscosity, such as phenolic resin, epoxy resin and bismaleimide resin.
(2) Raw materials
Resin: commonly used unsaturated polyester resin, epoxy resin, vinyl ester resin, phenolic resin;
Fibers: glass fiber roving, continuous felt, stitched felt, stitched composite felt, fabric, glass fiber surface felt, polyester fiber surface felt, etc.
Core material: generally not used. At present, polyurethane foam is used as core material, and continuous pultruded frame profile is used as thermal insulation wallboard.
(3) Advantages
1) The typical pultrusion speed is 0.5-2m/min, which has high efficiency and is suitable for mass production of long-sized products;
2) the resin content can be accurately controlled;
3) Because the fiber is longitudinal, the ratio of body to species can be very high (40%-80%), so the axial structural characteristics of the profile can be very good;
4) Roving is the main reinforcement, the cost of raw materials is low, and the mechanical properties of products can be adjusted through the combination of various reinforcement materials;
5) The product quality is stable and the appearance is smooth.
(4) Disadvantages
1) The mold cost is high;
2) Generally, it is limited to producing products with equal cross section.
(5) Typical products
Building roof beams, rafters, door and window frame profiles, wallboard, oil pumping rod, tent rod, ladder, bridge, tool handle, mobile phone microwave station cover, automobile leaf spring, transmission shaft, cable tube, optical fiber cable core, fishing rod, grille, automobile air conditioning cover, rail telescopic cover. 0} 1x p* V
6. Vacuum bag technology
(1) Summary:
This method is an extension of hand paste method and spray method. Put the hand-pasted or sprayed laminate together with the mold into the A-grade resin, cover the laminate with a plastic bag, seal the periphery, and then vacuumize with a vacuum pump to make the laminate compact and shape under the pressure not exceeding 1 air pressure.
(2) Raw materials
Resin: epoxy resin and phenolic resin are mainly used. Unsaturated polyester resin and vinyl ester resin are generally not used, because styrene (crosslinking agent) in the resin is excessively pumped out by vacuum pump, which may cause problems;
Fiber: the same hand paste method;
Core material: any.
(3) Advantages
1) products with high fiber content can usually be obtained by ordinary wet lamination technology;
2) large-size products can be manufactured;
3) The product is double-sided;
4) Compared with wet lamination, the porosity is lower;
5) Due to the pressure, the resin flows through the structural fiber, and the fiber can be well impregnated with the resin;
6) Conducive to the health and safety of operators; Vacuum bag reduces volatile substances escaping during curing.
(4) Disadvantages
1) The extra procedures increase the labor and packaging costs;
2) Operators are required to have high technical proficiency;
3) resin mixing and content control basically still depend on the skills of operators;
4) The production efficiency is not high.
(5) Typical products
Ship, racing car, core material bonding, airplane nose cone radome, wing, rudder.
7. Resin film perfusion (RFI- resin film perfusion)
(1) summary
Dry strength objects and resin sheets (provided on a layer of release paper) are alternately placed in the mold. The lay is covered by a vacuum bag, and that air in the dry fabric is evacuated by a vacuum pump. Then heating, so that the resin melts and flows into the fabric that has been evacuated, and then cures after a period of time.
(2) Raw materials
Resin: generally only epoxy resin is used; ?
Fiber: arbitrary;
Core material: A variety of core materials can be used. Due to the high temperature during processing, PVC foam needs special treatment to avoid foam damage.
(3) Advantages
1) has low porosity and can accurately obtain high fiber content;
2) The layer is clean, which is beneficial to health and safety (like prepreg);
3) The cost can be lower than that of the prepreg method, which is the main advantage;
4) Because the resin can only pass through the thickness direction of the fabric, the resin will not penetrate into the white spot area, which can be smaller than the Siman composite resin infiltration molding process.
(4) Disadvantages
1) is currently only used in the aerospace industry and has not yet been popularized;
2) Although the autoclave system used in aerospace industry is always essential, the curing of composite materials, heating chamber and vacuum bag system are always essential;
3) The mold should be able to withstand the process temperature of resin diaphragm (low temperature curing 60- 100? c)；
4) It is required that the core material used can withstand the process temperature and pressure;
(5) Typical products
Aircraft radome, warship sonar fairing.
8, prepreg (autoclave) molding
(1) summary
Pre-impregnating fabrics and/or fibers with pre-catalytic resin under the conditions of heating, pressure or using solvents. Most curing agents can be stored at ambient temperature for weeks or months, and can still be used in good quality. When the shelf life is extended, the materials must be stored in frozen conditions. Resin is usually in a critical solid state at ambient temperature. Therefore, there will be a slight adhesion when touching the prepreg, like adhesive tape. The fiber used to make unidirectional prepreg is directly taken off the creel and combined with resin. Spread the prepreg on the surface of the mold manually or mechanically, and vacuum it through a vacuum bag, usually heating it to 120- 180? C. Allow the resin to flow back and finally cure. The extra pressure for frost spraying is usually provided by an autoclave (actually a pressure heating tank), which can exert a pressure of up to 5 atmospheres on the layer.
(2) Raw materials
Resin: usually epoxy resin, unsaturated polyester resin, phenolic resin and high temperature resistant resin, such as polyimide, cyanate ester, bismaleimide resin, etc. ;
Fiber: Any. Although high temperature has a certain influence on the core material in the process, some special foam core materials should be used.
(3) Advantages
1) prepreg manufacturers can accurately adjust the level of resin/curing agent and the content of resin in fiber; A high fiber content can be reliably obtained.
2) The material is very safe for the operator, without any harm to health, and the operation is clean;
3) The cost of unidirectional belt fiber is the lowest, because there is no need to convert the fiber into fabric for secondary processing in advance;
4) Because the manufacturing process adopts permeable high-viscosity resin, the chemical properties, mechanical properties and thermal properties of the resin can be the most suitable;
5) The effective time of the material is long (it can be guaranteed for several months at room temperature), which means that the structure can be optimized and the composite material is easy to lay;
6) Automation can be realized and labor can be saved.
(4) Disadvantages
1) For prepreg, the material cost is high;
2) It is usually necessary to cure composite products in an autoclave, which is costly, slow to operate and limited in product size;
3) The mold should be able to withstand the working temperature;
4) Core materials need to withstand working temperature and pressure.
(5) Typical products
Aircraft structural composite materials (such as wings and tails), satellite and launch vehicle structural parts (solar cell substrates, sandwich structural plates, satellite interface brackets, rocket fairings, etc. ), racing cars, sports equipment (such as tennis rackets, snowboards, etc. ).
9, low-temperature curing prepreg molding
(1) summary
The low-temperature curing prepreg was prepared completely according to the usual prepreg method, but the chemical properties of the resin enabled it to be cured at 60- 100? Cure at c temperature. 60? C. The operable storage period of materials can be as small as 1 week, but it can also be extended to several months. The flow section of resin system is suitable for vacuum bag pressure and avoids autoclave.
(2) Materials |
Resin: generally only epoxy resin is used;
Fiber: arbitrary, the same as the usual prepreg;
Core material: any, although general PVC foam needs special attention.
(3) Advantages
1) has the advantages of (1)-(6) the traditional prepreg method;
2) Mold materials are cheaper, such as wood, because of its low curing temperature;
3) Large structures can be easily manufactured. Because only the vacuum bag pressure is needed; The curing temperature is low, and a simple hot air circulation heating chamber can be used (a heating chamber larger than the product is often built on site).
4) ordinary PVC foam core material can be used, and it can be treated slightly;
5) Low energy consumption.
(4) Disadvantages
1) The material cost is still higher than that of prepreg;
2) a heating chamber and a vacuum bag system are needed to cure the product;
3) The mold should be able to withstand the temperature higher than the ambient temperature (usually 60- 100? c)；
4) It still consumes energy, because it needs to be cured at a temperature higher than the ambient temperature.
(5) Typical products
High performance wind turbine blades, rowing boats, lifeboats and train parts.
10, Scripps, Rift Valley, VARTM
Figure 1 1 sketch, Rift and Vartm.
(1) summary
Scrimp (Seeman Composite Infiltration Molding Process-Resin Infiltration Molding Method of Seaman Composite Company), Rift (umder flexibe tooling) and Vartm (VSCUUM Assisted Transfer Molding-Vacuum Assisted Resin Transfer Molding) have similar principles.
Put the fabric into the mold as a dry paving material, just like RTM. And then cover with peeled protective lay and stitched non-structural fabric. The whole floor is covered with vacuum bags. After the bag has no leakage, let the resin flow to the laminated layer. The resin can easily flow through the unstructured fabric and be distributed throughout the layer. SCRIMP method can place pressure module between vacuum bag and layer, which is beneficial to improve the appearance and structural density of products.
(2) Materials
Resin: epoxy resin, unsaturated polyester, vinyl ester resin;
Fiber: Any ordinary fabric. These processes are very easy to use, because the gap makes the resin flow quickly;
Core material: all kinds of core materials can be used except honeycomb.
(3) Advantages
1) is the same as RTM, but the product has only one light, unlike RTM, which has two lights;
2) Because half of the mold is a vacuum bag, the main mold only needs low strength, so the mold cost is very low;
3) large-size products can be manufactured;
4) For these molding methods, ordinary wet paving tools can be improved;
5) The core material structure can be produced in one operation.
(4) Disadvantages
1) to complete the relatively complicated operation process;
2) The viscosity of the resin must be very low, which limits the mechanical properties of the product;
3) The waste products caused by the resin not saturated in the layer are very wasteful;
Some process elements of SCRIMP are restricted by patents.
(5) Typical products
Body panels for semi-finished ships, trains and trucks.