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Plastic modification is a method to change the properties and improve the quality of plastics by adding additives and fillers. At the same time, plastic modification is also the most effective way to reduce the cost of plastics. The elements of formula design in plastic modification mainly include: material selection, compounding, dosage and mixing mode. The interaction of various elements can be complementary or offset each other. Therefore, it is not easy to design a formula with high performance, easy processing and low cost.
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Selection of resin
1. Type and model of resin
The selected target resin has the closest performance to the target performance to be modified: first, select the variety of resin, such as improving the toughness of resin, and selecting ABS, SBS and other resins; For transparent modification, three transparent resins PS, PMMA and PC should be selected first.
After determining the target resin, select the brand of the resin. The specific brand of resin has more specific performance indicators, which will make the target selection more deterministic. Selection of resin: large brand and general resin are preferred to ensure that it is more convenient to purchase resin in the later stage.
2. Compounding of resin
The combination of the selected target resin and the main resin, that is, the selected target resin and the main resin are compatible and can form a whole without phase separation. The target resin can play a modified performance target.
3. Viscosity of resin and fluidity of composite resin
In the formula, the viscosity of the selected target resin is close to that of the main resin. If the viscosity of the selected target resin is too large, the viscosity of the composite resin will be large. Modifiers need to be added to reduce the viscosity gradient. For example, PA66 is toughened and modified, and PA6 is added as modifier in flame retardant formula; PA6 was toughened and modified, and HDPE was added as modifier in the flame retardant formula.
The change of viscosity will affect the change of fluidity, and finally affect the change of processing mode, such as injection molding stage, extrusion stage, blow molding stage, calendering stage, etc. The main reasons for the influence of the same resin on its fluidity are the molecular weight and the arrangement of molecular chains, such as linear molecules, body molecules and so on. Among them, high fluidity resins include PS, hips, ABS, PE, PP, PA, etc; Low fluidity resin: PC, MPPO, etc; Non flowing resin: polytetrafluoroethylene, UHMWPE, PPO, etc.
Selection of additives
1. Purposeful selection of additives
Different types of additives have different target properties, and the additives added into the system can fully their expected functions and achieve the specified targets. Among them, the specified indicators are generally national standards, international standards, or performance requirements put forward by customers. The properties of common target resins are as follows.
1) Flame retardancy: inorganic phosphorus, organic halide, organic phosphide, organic silicon and nitride, etc.
2) Reinforcement: fiber and whisker. Glass fiber, asbestos fiber, carbon fiber, whisker, quartz fiber, graphite fiber and ceramic fiber; PAN fiber, polyethylene fiber, PA fiber, PC fiber, PVC fiber and polyester fiber; Boron fiber and aluminum, titanium, calcium and other metal whiskers.
3) Toughening: high impact resin, such as CPE, MBS, ACR, SBS, ABS, EVA, modified petroleum resin (MPR), etc; High impact rubber, such as ethylene propylene rubber (EPR), ethylene propylene diene monomer (EPDM), nitrile rubber (NBR), styrene butadiene rubber, natural rubber, CIS polybutadiene rubber, chloroprene rubber, polyisobutylene and butadiene rubber.
4) Wear resistance: graphite, MoS2, silica, etc.
5) Degradability: starch filling, degradable additives, etc.
2. Effect of additives on composite properties
The morphological distribution of the same additives is also different, and the effect relative to the modification is also different. For powder additives, the key factor is the particle size.
1) The smaller the particle size, the more beneficial to the tensile strength and impact strength of the filling material. For example, in terms of impact strength, the particle size of antimony trioxide decreases by 1 μ m. The impact strength will be doubled;
2) The smaller the particle size of flame retardant, the better the flame retardant effect.
3) The smaller the particle size of the colorant, the higher the coloring power, the stronger the covering power and the more uniform the color. However, the smaller the particle size of the colorant, the better. There is a limit value, and the limit values for different properties are different;
4) Taking carbon black as an example, the smaller its particle size is, the easier it is to form a network conductive path to achieve the same conductive effect, and the amount of carbon black added is reduced. However, like colorants, the particle size also has a limit value. If the particle size is too small, it is easy to aggregate and difficult to disperse, and the effect is not good.
For fiber additives, the higher the fiber degree of additives, the better the reinforcement effect. Among them, the degree of fibrosis of additives can be expressed by length diameter ratio (L / D). The molten state is better than the powder state to maintain the aspect ratio and reduce the probability of fiber breaking, so it is also a better addition method.
3. Compatibility between additives and resin
The good compatibility between resin and additives is an important basis to ensure the good performance of additives, and ensure their good durability, extraction resistance, migration resistance and precipitation resistance during use. Good compatibility is the basic requirement. Even the barrier formula requires the layered distribution of additives in the resin. Main methods to improve resin compatibility:
1) The addition of compatible additives, such as surfactants, is better to give full play to its best effect.
2) Surface treatment. Using compatibilizer or coupling agent as surface treatment agent to improve the compatibility between resin and additives. Common coupling agents include silane, titanate and aluminate; The compatibilizer includes maleic anhydride grafted polymer corresponding to the resin. Additives to be surface treated include inorganic additives and fiber additives.
4. Amount of additives added
The appropriate amount of additives is not only to improve the appropriate performance of the target resin, but also based on the economic basis of low cost. The requirements for the addition amount of different additives are different: 1) for flame retardant, toughening agent, magnetic powder, barrier agent, etc., although the more the addition amount is, the better, the cost should also be verified; 2) Conductive additives generally form a circuit path; 3) Antistatic agent, and a charge discharge layer can be formed on the surface; 4) Coupling agent to form surface coating.
5. Relationship between resin and additives
The addition of additives can not lead to the deterioration of the properties of the resin. For example, PPS can not add additives containing lead and copper, and PC can not use antimony trioxide, which can lead to depolymerization; At the same time, the acidity and alkalinity of additives should be consistent with that of resin, otherwise they will react.
6. Relationship between additives and other components
In order to take into account multiple purposes at the same time, a variety of different additives need to be added to a formula. The interaction of additives is very complex. The main overview is as follows: 1) they are independent and have no influence on each other; 2) Synergy: multiple additives with heavy formula promote each other, so that the overall effect is higher than the average value of a single additive; 3) Mutual elimination: when two or more additives are added together, the effect is lower than the average value when they are added alone. For example, in the formula of anti-aging plastics, sulfide auxiliary antioxidants and HALS light stabilizers are not used at the same time, because the acid components bred by sulfide inhibit the light stability of HALS.
Uniform mixing is not only the basic requirement of plastic formula modification, but also the requirement of uniform distribution of additives in the formation of resin. The uneven distribution of additives can not improve the performance of the original resin, but also because of the uneven distribution of fillers, the performance is worse than that of pure resin. For example, the mechanical properties and processing properties of the material are not evenly dispersed by the use of fillers, which will not only greatly reduce the mechanical properties, but also affect the processing properties. Common ways to improve the uniform mixing of additives: 1) reasonably sort the added fillers, such as: in the process of coupling treatment, the fillers are advanced, and then add coupling agent after heating and dehydration; 2) The components shall be added into the system in primary and secondary order. A large number of fillers can be added in multiple batches to facilitate uniform mixing. In the treatment of coupling agent, it is generally sprayed three times to disperse evenly, and the coupling effect is good.
Selection of formula
The processability of the formula, that is, the modified plastic formula has processability, which is to meet the care of both product molding after the modification of molding plastics. The main embodiment of the processability of the formula: the additive has good heat resistance, no evaporation loss, decomposition and deactivation; The formed formula has little wear and corrosion to the equipment and releases non-toxic gas, which can be used as the evaluation standard.
1. Fluidity of modified plastics
The fluidity of plastics is the main aspect affecting the processing performance of plastics, and it is also an important basis for choosing which plastic processing method to adopt. Generally speaking, the addition of inorganic filler will reduce the flow performance, which needs to be optimized by adding flow modifier.
2. Heat resistance
Heat resistance shall be considered in two aspects: 1) reduce the processing temperature of the resin, and the processing temperature shall not exceed the decomposition temperature of the additives, so as to ensure the stable existence of additives and reflect the modification performance; 2) Add a certain amount of antioxidant additives to prevent thermal decomposition and affect the performance of the resin. Generally speaking, most organic dyes have low decomposition temperature and are not suitable for engineering plastics processed at high temperature; The decomposition temperature of spices is low, generally below 150 ℃, and only EVA and other resins with low processing temperature can be used as the carrier.
3. Environmental protection of formula
The environmental protection of the formula includes: harmless to human body, equipment, environment and users. Human health - the resin and selected additives should be absolutely non-toxic, or their content should be controlled within the specified range. Environmental pollution - the selected components cannot pollute the environment.
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