2.1.4 Modification methods
There are many ways to modify polypropylene, which can be divided into chemical modification and physical modification, depending on change in structure.
A. Chemical modification
Chemical modification refers to introduction of other components into molecular chain of polypropylene through copolymerization, grafting, etc., or modification of crosslinking by adding a crosslinking agent to give polypropylene higher impact resistance, low temperature resistance. , anti-aging properties, etc. Chemical modification mainly includes: copolymerization, grafting, cross-linking, chlorination, etc.
(a) Copolymerization modification
In PP synthesis stage, ethylene or other monomers are added for random, alternating or block copolymerization, which can improve performance of polypropylene in certain aspects, for example, adding a small amount of ethylene for random copolymerization of polypropylene, which greatly reduces melting point of polypropylene, improves low temperature performance and impact resistance of polypropylene, improves transparency. In conventional random copolymers, for every 1% increase in ethylene content, melting point of copolymer decreases by 5°C; Montell Combines Clarity and High Clarity of Random Copolymers The impact strength of flowable copolymers has been developed to meet impact and transparency requirements at -40°C. Another example is polypropylene chain, block copolymerization of 2-3% ethylene monomer can produce ethylene propylene rubber, which is a thermoplastic elastomer that can withstand low temperatures at -30℃. However, after copolymerization of polyethylene, rigidity and high temperature performance of polypropylene will deteriorate. In addition, polypropylene can also be modified by copolymerization with styrene, cycloolefin, etc.
(b) Graft modification
Grafting changes structure of molecular chain and radically changes physical and chemical properties of polypropylene. There are many methods for grafting polypropylene, including melt method, solution method, and solid phase grafting method. For example, melting method is to add graftable monomers to polypropylene resin under action of an initiator, heat and melt, and carry out grafting reaction in mixing process, usually grafting polar low molecular weight substances such as maleic anhydride, acrylic acid, methyl alcohol, etc. Introduction of polar groups into chain of non-polar PP can significantly improve compatibility of PP with polar polymers and inorganic substances. This type of polypropylene grafting is mainly used as a compatibilizer and coupling agent necessary for blending polypropylene with other polymers.and and inorganic substances; other chlorinated polypropylene is for grafting -Cl groups onto molecular chain of polypropylene Modified polypropylene has good wear resistance, aging resistance and acid resistance, and has become one of important products of chemical modification of polypropylene.
(c) Cross reference modification
The purpose of crosslinking is to increase stability of form of aggregation of polymer chains, improve mechanical properties, heat resistance, creep resistance and melt strength. Polypropylene crosslinking can use organic peroxide crosslinking, nitrogen chemical crosslinking, radiation crosslinking, thermal crosslinking and other methods. After crosslinking, polypropylene can obtain higher hardness, better solvent resistance and excellent low temperature resistance. However, due to fact that side group in structure of PP itself contains -CH3, PP is difficult to crosslink.
(d) Chlorinated polypropylene
Chlorinated polypropylene is essentially a type of graft, a polar thermoplastic resin in which chlorine atoms are embedded in polypropylene molecular chain. The chlorine content of 20%-40% is low chlorination, and chlorine content of 63%-67% is high chlorination. They are all white powder or granules, but colorless after film formation. Melting point <150°C (those containing 30% chlorine have lowest melting point), decompose at 180-190°C. Insoluble in ethanol and paraffin, soluble in aromatic hydrocarbons, esters and ketones. Oil resistance, heat resistance and light resistance, it can resist corrosion of strong oxidizing acid and strong alkali. CPP membrane can be immersed in 10% sodium hydroxide solution or 10% nitric acid solution for 144 hours without swelling. Good compatibility with most resins, such as coumarone resin, petroleum resin, phenolic resin, alkyd resin, coal tar, turpentine, etc. It can be used as plastic and metal bonding adhesive, paint coating, paper coating, waterproofing agent , fire retardant, printing ink additives, etc. It can be obtained by chlorination of polypropylene by solution, suspension or solid phase method.
B. Physical modification
Physical modification refers to use of polypropylene as a base material along with other polymers, inorganic materials and special functional additives by mixing and kneading to obtain polypropylene composite materials with certain properties. Physical modification can be divided into filling, reinforcement, mixing and nucleation modification.
(a) Infill Modification
In order to reduce cost of materials and improve performance of polypropylene without affecting overall performance of polypropylene, other additives are added to polypropylene.Low cost materials such as calcium carbonate, talc, wollastonite, mica and sulfuric acid. Barium, wood flour. , etc. There is a lot of research on filling modification, and field of industrial application is also very wide. Filling polypropylene with filler can improve stiffness, heat distortion temperature, and dimensional stability of polypropylene, but it will affect other mechanical properties, especially toughness. However, with application of surface modification technology and nanotechnology, filling modification technology is developing in direction of hardening and toughening. Adding large amounts of magnesium hydroxide or aluminum hydroxide to PP can also result in flame retardant polypropylene materials.
(b) extended modification
The materials used to reinforce polypropylene include glass fiber, asbestos fiber and various fibrous or flake minerals such as acicular or flake talc, acicular wollastonite, flaky mica, etc., reinforced modified polypropylene has excellent mechanical properties and heat resistance, so that it can replace some engineering plastics.
(c) Blend Modification
Polypropylene blending modification refers to polypropylene and other plastics, rubber, or thermoplastic elastomers, as well as addition of certain additives, mixing under a certain temperature and shear force to form macroscopically uniform mechanical and optical properties, or process of obtaining new materials. with improved thermal properties. For example, blending with rubber can improve impact resistance of PP at low temperatures, blending with low density polyethylene can improve transparency of PP, blending with polyvinyl alcohol can improve antistatic performance of PP, etc. These blends are also called polymer blends. alloys, more than 80% of commercially available PP in developed countries are mixed materials, so development prospects are very wide.
(d) Nucleation modification
Adding a nucleating agent to PCB is a simple and effective method for modifying PCB. PP has a polymorphic structure, under different crystallization conditions, it can form five crystal forms α, β, γ, δ and pseudo-hexagonal, most common of which are α-type monoclinic system and β-type hexagonal system. For example, addition of sorbitol-based nucleating agents can improve transparency and rigidity of polypropylene, addition of organophosphorus nucleating agents can greatly increase heat distortion temperature of polypropylene, addition of β nucleating agents promotes formation of β crystal form, and enhancement effect is obvious. . The addition of a nucleating agent greatly increases crystallization temperature of polypropylene, refines crystallized particles, and comprehensively improves performance of polypropylene.The nucleation modification method is featured by less addition of nucleating agent, low cost, simple process, and obvious effect. In recent years, it has become a hot spot for polypropylene modification.
Modification technology makes polypropylene a high-performance engineering material. In actual process of developing new materials from polypropylene, various modification technologies are usually used together to achieve best value for money. There have been many studies on modification of PP at home and abroad, and scope of PP is constantly expanding, and it has become a general-purpose plastic second only to polyethylene in consumption.
For most enterprises, physical modification is mainly used to modify PP, so this chapter mainly explains physical modification of PP.
2.2 Additional Effects
Polypropylene modification is divided into chemical modification and physical modification. Among these two different modification methods for modified plastics enterprises, most of them use physical modification. Therefore, only physical modification will be described in this chapter, and for contents of graft modification in chemical modification, see Chapter 7.
In physical modification, no matter what method of modification is, it is nothing more than application of several types of materials: thickeners, fillers, reinforcing materials, flame retardants, and other types of additives.
Let's characterize influence of various types of additives on properties of polypropylene materials.
2.2.1 Hardener effect
Although there are many types of reinforcers, there are quite a few reinforcers that can be applied to polypropylene, but commonly used reinforcing agents for polypropylene have been around for decades and are none other than polyethylene, ethylene vinyl acetate, and styrene-styrene. , EPR, EPDM and POE. Among these types of hardeners, PE, EVA, and then SBS were first materials used for hardening. EPR was then applied, but since EPR needs to break down during use, therefore it was not widely used in modifications to increase stiffness of polypropylene. With advent of granulated EPDM, EPR has left market for polypropylene modifications. With introduction of POE and constant rise in price of EPDM, EPDM is also gradually leaving PP modification market. At present, except for elastomer modification field, SBS and EPDM have practically disappeared in field of ordinary PP modification. Being one of first domestic researchers and users of hardened modification of PP POE, author began to conduct experimental studies in 1998, and in 1999 applied it on a car bumper.
Figure 2.7 Effect of POE, EPDM, SBS on polypropylene impact strength and flexural modulus
Study of reinforced polypropylene SBS, EPDM and POE (see Fig. 2.7) showed that effect of SBS, EPDM and POE on impact strength and flexural modulus of polypropylene is consistent, and impact strength of these three components is increased. The bending modulus and bending modulus reduction inflection points are about 15 phr.
In addition, more in-depth studies have shown that POE has widest adaptability to PP in hardening and modifying PP among three. POE can get same application effect in any liquid polypropylene resin, while SBS, due to its own viscosity, modifying effect of EPDM on low flow polypropylene is obviously higher than on high flow polypropylene.
2.2.2 Influence of filler
There is no doubt that main purpose of using fillers in plastics is to reduce costs. When author was studying, teacher clearly pointed out when talking about excipient modification: whether you are a qualified formulation engineer depends on whether formula you are developing can add excipients and how many excipients are added. Of course, with development of filler technology, use of fillers can now change many of properties of polymers, but cost reduction is still top priority.
In general, effect of inorganic fillers on product characteristics is as follows:
A. The specific gravity increases with amount of added filler.
B. The surface hardness increases with amount of added filler.
C. Rigidity increases with amount of added filler.
D. Flexural strength decreases with increasing filler content.
E. Elongation at break decreases with increasing filler content.
F. Surface gloss decreases with increasing filler content.
G. Impact strength generally decreases as amount of filler added increases; if shape of filler is acicular or fibrous, it usually rises.
H, heat resistance increases with addition of fillers.
Influence of organic fillers on product characteristics.
Usually, addition of organic fillers has a great influence on physical and mechanical properties of plastics, and no general patterns have yet been found (sometimes it also affects chemical properties). general:
A. The specific gravity of product decreases as amount of filler increases.
B. The surface gloss of product decreases as amount of filler added increases.
C. The impact strength of a product usually increases with addition of fillers, especially when form of powder is acicular or fibrous, it usually increases.
The above laws of influence are general laws, not absolute.Just as there are special cases of Hund's rule in chemistry, there are also exceptions to fill in, so do not take above laws for golden words.
The fillers used in modification of polypropylene are mainly calcium carbonate, talc, etc., and amount of talc used in modification of polypropylene is largest. becomes higher and higher. This also highlights cost advantage of calcium carbonate. Therefore, filler for modified PP is gradually shifting towards calcium carbonate or calcium carbonate and talc. Since author is currently experimenting with calcium carbonate-filled cost reduction experiments, behavior of calcium carbonate-filled PCB is shown in Fig. 2.8. The effect of other polypropylene fillers on material properties is similar.
It can be seen that addition of fillers will reduce tensile strength and toughness of mixed system to a different extent, but thermal deformation temperature and bending modulus will increase to a different degree, of which bending modulus increases to a certain degree The application scope of fillers is very important to compose formulations according to specific performance requirements.
In order to better understand effect of different size fillers on performance of modified mixture system, nanosized calcium carbonate was chosen (although problem of nanosizing was described in Chapter 3, experimental studies cannot overcome this obstacle). , and in future chapters there will be more nanofillers (use of introduction) and micron-sized calcium carbonate have been studied and found that finer filler, more favorable mechanical properties of material (see Fig. 2.9). But everything must be considered in two parts. The finer filler, more complex technological requirements. If you are not careful, there will be opposite effect. Therefore, you need to be careful with fineness of the filler.
Figure 2.8 Effect of calcium carbonate-filled polypropylene on properties
The author found an exception during an experiment on performance of PP with various fillers in production, that is, effect of barium sulfate on elastic modulus of system of PP mixtures, different from calcium carbonate filling. , talc, etc. See fig. 2.10.
It can be seen from Figure 2.10 that when barium sulfate is added, flexural modulus of mix system drops significantly and reaches bottom by about 20 parts. On this basis, increase amount of barium sulfate and mixture begins to recover and increase flexural modulus of system, which formulators should pay attention to Special attention.
Figure 2.9 Effect of calcium carbonate with different particle sizes on properties of polypropylene
Figure 2.10 Influence of BaSO4 content on elastic modulus of a polypropylene blend system
2.2.3 Influence of reinforcing material
Generally speaking, reinforcing materials used to reinforce PP materials are glass fiber, carbon fiber, wollastonite, mica, etc. Among these reinforcing fillers, glass fiber reinforcement material is usually used, which is not required to improve strength. In high cases, use of wollastonite and mica to reinforce polypropylene can be considered. Generally speaking, wollastonite can be used in combination with glass fiber to reduce costs. In general, when it is comprehensively balanced, glass fiber can be used in all reinforcing materials. Medium is best in balance. Figure 2.11 is a graph showing relationship between tensile strength and toughness of glass fiber reinforced polypropylene.
It can be seen from figure that when glass fiber content is about 30-35 parts, both strength and toughness reach highest point. Increasing glass fiber content does not improve performance of glass fiber. mixed material, and it's even more detrimental to performance. Therefore, unless there is a special need, amount of fiberglass used should not exceed 35 parts.
Figure 2.11 Influence of glass fiber content on polypropylene properties
At same time, we tested wollastonite-reinforced polypropylene with a high L/D value, as shown in fig. 2.12.
Figure 2.12. Mechanical properties of polypropylene reinforced with wollastonite
Figure 2.13. Mechanical properties of polypropylene reinforced with glass fiber / wollastonite
It can be seen that when content of wollastonite reaches 25 parts, it has a good effect on polypropylene mixture system, but reinforcement effect is not very ideal. In order to expand use of wollastonite in field of reinforcement, an experiment was carried out to add wollastonite to reinforced PP under condition of a fixed dosage of glass fiber, results are shown in Fig. 2.13.
It can be seen from figure that addition of an appropriate amount of wollastonite can synergistically strengthen polypropylene materials with glass fibers. At same time, due to cost factor, addition of wollastonite can effectively reduce cost of formulation. from reinforced material. However, attention must be paid to retention of L/D wollastonite during co-treatment, otherwise hardening effect will not give desired effect.
Based on results of experiments carried out by author in last two years with polypropylene and polyamide reinforced with wollastonite, in order to obtain an ideal material reinforced with wollastonite, granulator used should be selected. it is easy to destroy L/D of wollastonite during production, so that wollastonite loses its reinforcing effect; at same time, twin screw granulator used in production of wollastonite reinforced materials should not be cut too much, which is also easy to damage limestone L/D silicon, affect performance of reinforced materials.
2.2.4 Lubrication effect
The author did not conduct experimental studies of influence of lubricants on properties of polypropylene materials. From inquiry of data, it appeared that Li Ming and others from Shanghai Jiaotong University studied mechanical properties of polypropylene/polyethylene materials with various lubricants (see Table 2.1). Right or wrong, author dares not make false statements.
Table 2.1 Effect of different lubricants on properties of a mixed system
It can be seen from table above that stearate lubricants have little effect on tensile strength and toughness of system, while EBS, polypropylene wax and stearic acid have a large effect on toughness of system. And it can be seen that lubricant has a certain effect on flexural modulus and thermal deformation temperature of polypropylene/polyethylene.
Therefore, in a modified PP system, lubrication system must be carefully designed during formulation development, and it can be seen from Table 4.1 that, in general, lubricants that have least effect on mixture performance of system are calcium stearate and barium stearate. However, in terms of individual impact, different lubricants have different effects. When developing a formula, we can choose a comprehensive balance of all additives, including lubricants, according to specific needs of customers.
Source: "Practical Guide to Rubber and Plastic Technology"
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