2.3 Modification formula design
The modification of any plastic material consists of a matrix resin, filler, modifier, additives, etc., and polypropylene is no exception.
There are many kinds of PP materials at home and abroad, and polymerization process is also different, so properties of produced PP materials are different.
In general, polypropylenes existing on market can be divided into homopolymerization and copolymerization according to type of polymerization. From point of view of domestic use, T30S is typical of homopolymer polypropylene and EPS30R is typical of copolymer type, which are also two most widely used polypropylene materials with different types of polymerization on market.
According to state of existence, it is divided into polypropylene pellets obtained by loop reactor and polypropylene powder obtained by small boiler method. The molecular weight of polypropylene granules is stable, comprehensive physical properties and mechanical properties are good, and performance of different batches of products is relatively stable. Because necessary antioxidant has been added in granulation step, weather resistance is better, but price is relatively high. And mixing with powdered inorganic materials, pre-mixing process is difficult process. However, polypropylene powder has a wide molecular weight distribution, high ash content and high halogen content, and performance between batches is not sufficiently stable. The physical properties, mechanical properties and weather resistance are poor, so it cannot be used directly. can be modified by adding appropriate additives. , but market price is cheap, and it is easy to pre-mix evenly with inorganic filler of same powder.
Which type and grade of polypropylene should be chosen for formulation development should be chosen according to specific technical requirements.
The fillers used in modification of PP are all-inclusive, including common filler talc, calcium carbonate, industrial waste residues, etc., as well as various fibers, wollastonite, mica, etc. for reinforcement; barium sulfate is also present which can improve gloss; there are magnesium hydroxide and aluminum hydroxide that can be used as flame retardants; there are also kaolin and montmorillonite, which can be used as special additives; there are both round and fibrous The presence of shapes and scales, etc.
The specific choice should be made according to function that formula should implement.
There doesn't seem to be much to choose from. Although SBS, EPR, EPDM, EVA, POE, etc. maybe used as reinforcing additives for polypropylene, as technology of POE application in field of PP has been developed, other strengthening additives have been removed. Toughening of PP market.
Because polypropylene is a non-polar polymer and commonly used fillers are polar fillers, it is especially necessary to use a compatibilizer (compatibilizer) in field of modifying polypropylene. It is especially used in field of polypropylene alloys.
Commonly used substances fall into two categories: compatibilizers and coupling agents.
Commonly used coupling agents are aluminate coupling agents, titanate coupling agents and silane coupling agents. Among coupling agents, aluminate coupling agents are most commonly used in field of polypropylene modification, followed by silane coupling agents. Among many coupling agents, there is a coupling agent that is not a coupling agent - bismaleylimine, which is especially important in modification of glass fiber reinforced polypropylene, is often ignored by many formulators.
The compatibility compatibilizer will not be described in this chapter, and a special chapter will be described later. It is only indicated here that among many compatibilizers, PP-g-MAH is most commonly used. to common compatibilizers more Used in polypropylene alloys such as PP/PP-g-MAX/PA etc.
2.3.5 Other additives
Other additives include dispersants for dispersing fillers, lubricants for lubrication systems, absorbing residual halogen scavengers in polymerization of polypropylene, antioxidants for thermal and oxidative aging, UV absorbers for UV protection, flame retardants. Flame retardants, etc. will not be described. one.
2.4 Sample recipe
There are many types of PCB modifications, and modification requirements vary widely. The author cannot describe all modifications, and only a few examples can be given by way of example.
2.4.1 Special materials for rice cookers
PP-modified materials used for plastic parts of rice cookers are mainly divided into high gloss and heat resistant materials. The rice cooker body requires high gloss, and middle ring and base require high temperature resistant materials. , development of this series of materials is mainly based on physical modification. The main raw materials are polypropylene homopolymer, various inorganic fillers and additives. The development of polypropylene modified materials for rice cookers revolves around blending with appropriate processes and formulations. Key technologies are designed with following aspects in mind.
A. Matrix Resin Selection
There are many varieties of polypropylene produced by domestic large petrochemical enterprises. According toAnd with requirements of material processing and flowability, injection molding polypropylene can be selected with a melt index range of 6-25. However, due to different synthesis processes, performance of PP from different manufacturers is quite different. Generally speaking, polypropylene can be selected as a matrix material from three aspects: polypropylene pellets obtained in a loop reactor, polypropylene powder obtained by a small-scale boiler method, and recycled polypropylene materials. The molecular weight of polypropylene granules is stable, comprehensive physical properties and mechanical properties are good, and performance of different batches of products is relatively stable. Because necessary antioxidant has been added in granulation step, weather resistance is better, but price is relatively high. And mixing with powdered inorganic materials, pre-mixing process is difficult process. However, polypropylene powder has a wide molecular weight distribution, high ash content, and high halogen content. The performance between batches is not stable enough, and physical properties, mechanical properties, and weather resistance are poor. It cannot be used directly. It needs to be modified by adding appropriate additives. But market price is cheap, and it is easy to pre-mix evenly with inorganic filler of same powder. Although recycled materials are very cheap, they are generally not used when they are not needed or guaranteed because it is difficult to ensure hygiene and stable physical properties.
B. The choice of filling material
A large number of inorganic fillers are used in modified polypropylene materials for rice cookers. There are many inorganic materials that can be used to fill or reinforce polypropylene, including talc, calcium carbonate, kaolin, barium sulfate, mica, wollastonite, silica, fiberglass, various industrial waste residues, etc. The materials are different. Regardless of which type of filler is chosen, characteristics of inorganic powder that affect final performance of modified polypropylene material are as follows.
Geometric shape of powder - thickness to diameter ratio (flake), aspect ratio (fibrous), flake and fibrous inorganic fillers have a reinforcing effect, such as talc, wollastonite and round granular inorganic powder Only body has a filling effect, such as barium sulfate.
Particle size - inorganic powder has a strong polarity and is easy to agglomerate. Therefore, primary particle size of inorganic powder and size of its dispersed particles in PP are not same concept. The first choice of inorganic powder should be with a sufficiently coarse mesh, powder can contain as small as possiblefine aggregates in PP using additives and a mixing process. The smaller size of dispersed particles, better overall characteristics of modified material. It is best to achieve nanoscale dispersion. Generally, a dispersed particle size of 1-10 µm can provide good material performance.
Surface properties. Inorganic materials are highly polar, while polypropylene is non-polar and they are incompatible. Appropriate additives are needed to reduce free surface energy of powder particles during activation treatment, prevent powder agglomeration, and increase affinity with polypropylene matrix. The powder sold by some manufacturers is activated, but it does not necessarily meet requirements of users. Thus, conventional inorganic powders must be handled separately before use.
Physical properties including powder density, hardness, optical properties, thermal properties, electrical properties and magnetic properties will affect performance of PP-modified materials. For example, density affects amount of filler, hardness affects wear resistance, optical properties affect color, and thermal, electrical, and magnetic properties affect thermal conductivity, electrical conductivity, and anti-magnetic radiation of polypropylene-modified materials. . In addition, physical properties of inorganic powder will have an important effect on heat resistance and flowability of materials modified with polypropylene. Both talc and calcium carbonate can significantly increase heat distortion temperature of polypropylene, but calcium carbonate impairs processing of polypropylene. Studies have shown that when using a PP homopolymer with a melt index of 11, filled with 15% talc and calcium carbonate, respectively, melt index of talc + polypropylene system is slightly higher than 11, while melt index of calcium carbonate + PP system is reduced to about 8.
Thermal and chemical properties——Inorganic powders in process of using PP-modified materials Chemical stability also causes some special functions, such as Mg(OH)2, Al(OH)3, etc., burn with loss of water and play role of a flame retardant.
In development of high gloss polypropylene materials, product gloss is a very important control. Studies have shown that various fillers affect gloss of polypropylene in following ways.
Types of fillers—with exception of glass beads, almost all fillers will reduce gloss of products made from PP-modified materials, but extent varies. With same amount of fillers, fillers will affect gloss of products. The general order of sizes is as follows:
Shape of filler particles——Microscopic shapefiller particles also affect glossiness of filled products differently. General order:
Particle size and distribution of fillers - smaller particle size of filler, less its effect on gloss of filled product, and smaller particle size, less effect on gloss. This is due to fact that large particles on surface of product easily scatter reflection of light.
Level of filler - higher level, less shine.
Surface treatment of fillers - surface treatment improves compatibility of fillers and polypropylene, and also facilitates dispersion of fillers. Typically, gloss of treated fillers is 10-40% higher than that of untreated products filled with polypropylene.
Effects of Additives - General lubricants are useful in improving product gloss, while inorganic flame retardants such as antimony trioxide reduce gloss.
By minimizing influence of various components of modified polypropylene material on gloss of surface of product, some additives to improve gloss of polypropylene product, such as nucleating agents, external lubricants, surfactants, fluorescents, etc. additives can greatly improve shine of surface of filled polypropylene.
The choice of inorganic materials is very important. In order to select suitable inorganic materials for various requirements, many factors must be considered. For example, whether it is filling or reinforcing, improving heat resistance or fire resistance, considering optical properties or electrical conductivity, etc., it is necessary to study various inorganic materials for surface gloss, mechanical properties and heat resistance of PP modified materials products Performance characteristics, rheological properties.
C. Auxiliary selection
In order to provide a comprehensive performance of PP-modified materials and emphasize their performance in a certain aspect, various additives must be added during modification process.
Functional additives are auxiliary substances that specifically improve weather resistance, heat resistance, frost resistance and gloss of surface of materials, such as antioxidants and gloss.
Technological additives - additives that improve machinability of materials, such as lubricants; additives that improve mixing effect, promote powder dispersion and bonding at interface between inorganic powder and polypropylene matrix, such as a binder, co-binding agent, dispersant, etc.
Strengthening additives are additives that can significantly improve characteracting materials, such as nucleating agents.
Everyone should pay attention: The role of different additives is not same, but many additives must be compounded to obtain a certain material characteristic. Only by studying in detail effect of each additive on characteristics of product, we will be able to adjust characteristics of product according to needs of user.
D. Choice of nucleating agent
Although first chapter mentioned careful use of nucleating agents, it is very important that rice cooker materials based on nucleating agents have gloss requirements, so we should talk about them here.
One of most important modification methods for polypropylene is nucleation modification. Adding a nucleating agent to special modified rice cooker materials is an important measure to improve comprehensive performance of modified polypropylene materials. The crystallization of polypropylene basically has two modes of nucleation: homogeneous phase and heterogeneous phase.
Homogeneous nucleation means that, in absence of foreign matter, polypropylene melt mainly depends on thermal fluctuations of polymer during cooling process, and ordered arrangement of polymer chain segments provides nucleation points. At low temperature, this is easily destroyed by molecular thermal motion and can only form at more than low temperature.
Heterogeneous nucleation consists of adding an external nucleator to provide nucleation points. The nucleating agent reduces free energy of nucleation of polypropylene. A small amount of additive can make polypropylene work at a higher temperature at a higher speed. Nucleation and crystallization. The role of nucleating agent manifests itself in following aspects:
(a) Acts as a seed to crystallize polypropylene.
(b) Shorten crystallization induction time, decrease degree of supercooling, increase crystallization temperature, crystallization speed and degree of crystallization orientation, and shorten processing cycle.
(c) Provides more nucleation points, cleans spherulites, reduces size of spherulites, and plays a self-reinforcing role. Transparent polymers are obtained when size of spherulite is smaller than wavelength of visible light.
(d) Change in crystal form of polypropylene depending on various nucleating agents.
(e) Reduce melt viscosity and improve workability.
(f) Improve crystallinity of polypropylene and improve material performance in various aspects, such as improving tensile strength and modulus, tensile strength and flexural modulus, impact strength, heat distortion temperature, hardness, surface gloss, etc.p>
The efficiency of nucleating agent is very significantna. In recent years, many types of polypropylene nucleating agents have been developed. When designing PP-modified materials for a rice cooker, you can choose from following aspects.
One of them is an inorganic nucleating agent. In fact, inorganic powders used in development of polypropylene modified materials for rice cookers, such as talc and kaolin, have a certain germ effect. The price of inorganic substances is low, but nucleation effect is relatively low. Low and can only nucleate at small particle sizes (<5 µm).
Another type is organic nucleating agents, including anti-reflective ones such as sorbitol, improved ones such as organophosphates, and special types such as crystal nucleating agents. The nucleation effect of organic nucleating agents is obvious, but cost is high. There are many types of organic nucleating agents, and quality of such products varies greatly. The nucleating agents greatly change submicroscopic structure of aggregate of matrix material, which has a great influence on performance of product. The use of processing technology must go through many experiments.
With a comprehensive analysis of above-mentioned different types of materials, combined with specific technical requirements of rice cooker, it will not be difficult to develop a reasonable formula and conduct a large number of experimental tests.
2.4.2 Special materials for car bumpers
To develop a car bumper formula, first thing to understand is function of a car bumper.
First of all, car bumper is installed in front of car as an independent unit, which mainly plays a protective role in event of a collision and meets requirements of lightness of car.
Secondly, as an element of external protection of car, it must have a relatively high impact strength and elasticity, and at same time have a simple molding method.
These two functional requirements dictate that bumper must have very high impact strength and elongation at break, while requiring lowest possible density and moderate flow. Among these requirements, elongation at break is particularly important, which determines ability of bumper to deform and absorb energy at moment of impact.
A. Matrix Resin Selection
The types of polypropylene materials have been described above and will not be repeated here.
The requirements of high toughness and moderate fluidity require us to choose strongest resin possible when choosing polypropylene resin, and resin has a certain high fluidity. According to these two selection principles, most suitable resin is Qilu. Designed by SP179. from Sinopec, which received second prize of National Achievement Award inpart of science and technology, is most suitable, but biggest disadvantage of this resin is that elongation at break is too low, so it does not meet requirements of high elongation at break. break.
In process of researching application of materials for automobile bumpers, author found that it is difficult for PP resin alone to meet above needs. From point of view of economy and practicality, it is most appropriate to use polypropylene homopolymerization and copolymerization as matrix resin.
B. Hardener selection
Composite polypropylene homopolymerization and copolymerization systems alone are difficult to meet impact strength needs, so stiffening of blending system is inevitable. Resilient has no choice. Since POE is a vinyl resin, PE is chosen as system compatibility agent. The study showed that use of PE as a compatibility component has an obvious synergistic effect with POE.
C. The choice of filling material
The choice of inorganic materials is considered from three points of view:
(a) Cost reduction.
(b) Reduce bumper softness.
(d) affect elongation at break as little as possible.
According to selective description of inorganic materials in B in 2.4.1, inorganic materials with a reinforcing effect can be excluded, since effect of reinforcing inorganic materials on elongation at break is quite obvious. Therefore, choose talc or calcium carbonate in terms of overall cost and effect on elongation at break.
To ensure high elongation at break, amount of inorganic materials should not be too high.
D. Auxiliary selection
According to needs of bumper casting, we need to make a reasonable choice of related additives.
Technological additives - to ensure uniform mixing of inorganic materials and polypropylene matrix resin, increase adhesion strength at interface between inorganic materials and matrix resin and ensure molding fluidity, add lubricants, binding agents. , and dispersants are inevitable. We need to reasonably test appropriate processing aids to ensure mixing system is impacted to a minimum.
Functional additives - scope of bumper determines that bumper can withstand wind, sun and ultraviolet radiation. For uniform mixing, PP should be split as best as possible. As described in 2.3.5, PP is internal and therefore contains residual halogens for protection. Optionally, antioxidants, UV absorbers and halogen absorbers must be added. However, since bumpers are usually used for painting, amount of additives should not be too high.
Compatibilizer—To further improve interfacial combination of componentsin a mixed system, an appropriate amount of compatibilizer must be added.
According to above principles, author has developed a series of material formulations for bumpers, in which design range of tensile strength is 16-35 MPa, and design range of impact strength is 250-560 J/m. The performance characteristics of one of materials are given in Table 2.2.
Table 2.2. Bumper material test results
The bumper made of this material has passed vibration test, continuous vibration test, high temperature test, cold-heat alternation test, aging test and other elements to meet technical requirements of automobile bumpers.
2.4.3 Sample recipe
The following formula is a formula developed by author for a product, and structure of formula will not be analyzed here, it is just an example.
One of biggest technical requirements that this formula meets is that a hollow cylinder with a wall thickness of 3 mm can withstand a load of 500 kg and withstand it for 60 minutes without deformation. And through equation to predict life over 30 years.
PP (homo) 100
PP (copolymerization) 28
Precipitated barium sulfate 25
Sealing agent 0.3
Sodium Benzoate 1
Polyethylene wax 0.35
Cooling Super Concentrate 0.2
Titanium dioxide 1
Broadcast 2 0.15
The performance parameters of this formulation are listed in Table 2.3.
Table 2.3 Efficiency indicators of tests of above formulations
2.5 Dynamic Cure Technology
As a dynamic vulcanization technology that has been widely used in recent years, it has breathed new life into development of polypropylene modification.
Bucknell's research has shown that when elastomers stiffen a plastic, elastomer particle morphology and properties, matrix properties, and interface conditions between elastomer particles and matrix are three main factors affecting compound strength. Elastomer particles can absorb some of impact energy and serve as stress concentration points, inducing and inhibiting crack growth, increasing shear yield strength of matrix, and thus making plastic more rigid. Dynamic crosslinking can crosslink an elastomer in a mixture system to increase modulus of elastomer, reduce difference in properties between elastomer and polypropylene, and facilitate interfacial bonding of two phases.
During study of mixed thermoplastic elastomers, it was proved that dynamic crosslinking provides interpenetration of amorphous part of plastic and amorphous elastomer at interface between two phases. properties of mixtures is a reflection of increase in strength of interfacial adhesion between two phases of mixtures. Dynamic crosslinking causes elastomer particles to crosslink to form a three-dimensional network structure, and these crosslinked elastomer particles inhibit movement of polypropylene molecular chains to a certain extent. In addition, dynamic crosslinking can also cause weak crosslinking of polypropylene phase, which can also reduce fluidity of system. Therefore, compared with a simple mixing system, MFR of dynamic crosslinking system is small and equilibrium torque is large.
This is only a correct description of PP/EVA dynamic vulcanization system studied by author.
In order to obtain high-strength modified polypropylene materials, author conducted a study on use of a polypropylene / ethylene vinyl acetate mixture system. After simple mixing and shaping, it was found that performance of system was not very ideal. After adding 0.6 parts of DCP initiator and 0.2 parts of co-crosslinking agent S to PP/EVA=80/20 system after dynamic vulcanization, it was found that impact strength of notched dynamic vulcanization system reached 55.7 kJ/m2. , which is 4 times more than with simple mixing (14.1 kJ/m2). In addition, tensile strength, elongation at break, flexural strength and modulus of elasticity in dynamic vulcanization are higher than those of plain compound system, see Table 2.4. This result also gives same effect in same amount of dynamically vulcanized EPDM polypropylene.
Table 2.4. Comparison of performance of a simple mixing system and a dynamic vulcanization system
As a result of analysis of SEM photograph of system (see Fig. 2.14), it was found that in a simple mixing system, EVA phase is dispersed phase, polypropylene phase is continuous phase, and EVA particles are dispersed phase. dispersed in a polypropylene matrix with larger particles; in dynamic crosslinking system, EVA phase is still dispersed phase, but EVA particle size is much smaller than in simple mixing system, and EVA particle distribution is relatively uniform. When elastomer content is constant, finer particle size and more uniform distribution within a certain range, better silver entrainment initiation and termination effect, and more obvious hardening effect will be. For PP/EVA dynamic crosslinking system, dynamic vulcanization makes EVA particles smaller and more uniform in size, so strength enhancing effect is obvious.
However, it cannot be denied that although dynamic vulcanization and hardening technology improves overall performance of mixing system, it also worsens fluidity of mixing system (Figure 2.15). Therefore, dynamic vulcanization technology should be used with caution if you are unsure or material is being used in large injection molded parts.
Figure 2.14. SEM photo of mixing system
Figure 2.15. Logηa/T-1 curves for simple mixtures and dynamic sulfides at different values of γw
Source: "Practical Guide to Rubber and Plastic Technology"
Disclaimer: All text/images and other manuscripts marked "Source: XXX" are intended to provide additional information and facilitate industry discussion under this heading and do not constitute agreement with them. The content of article is for reference only. If there is any infringement, please contact us to have it removed. All texts/figures and other manuscripts in which author is indicated at beginning of the article are originals with this heading number, if necessary, it is necessary to obtain authorization with this heading number.