Plastic modification is a relatively broad concept. Whether it is to improve the original properties of the resin through physical, chemical, mechanical or other means, it is called plastic modification. The application range of plastic modification is relatively wide.
Definition
It can be said that all plastics can be modified to improve their quality, such as density, hardness, precision, appearance, processability, transparency, mechanical properties, electromagnetic properties, chemical properties, corrosion resistance, aging resistance, wear resistance, thermal properties, flame retardancy, barrier properties and cost. Plastic modification is the most effective way to reduce costs and improve performance.
Technical methods
The commonly used methods for plastic modification are as follows:
1. Add modification
(1) Adding small molecule inorganic or organic matter
A modification method that adds small molecule inorganic or organic substances to polymers (resins) to obtain certain expected properties through physical or chemical effects. This method is one of the earliest modification methods. It has obvious modification effects, simple processes, and low costs, so it is widely used. I believe that those who have done graduation projects in colleges and universities have come into contact with and understand this method.
This modification method is divided into cost reduction (adding various cheap inorganic and organic fillers), strength improvement (adding various reinforcing fibers), toughness improvement (adding elastomers and ultrafine fillers, etc.), flame retardancy improvement (adding metal oxides, metal hydroxides, inorganic phosphorus, organic halides, organic phosphides, silicones and nitrides, etc.), life improvement (adding various antioxidants, light stabilizers, etc. ), processability improvement (adding plasticizers, heat stabilizers, lubricants and processing aids, etc.), wear resistance improvement (adding graphite, MoS2, SiO2, etc.), crystal structure improvement (adding nucleating agents, specifically organic carboxylic acids, sorbitol, etc.), antistatic and conductivity improvement (adding antistatic agents and conductive agents), degradability improvement (starch filling, degradation additives, etc.), radiation resistance improvement, etc.
The additives commonly used in this method are: inorganic additives (fillers, reinforcing agents, flame retardants, colorants and nucleating agents, etc.), organic additives (plasticizers, organic tin stabilizers, antioxidants and organic flame retardants, degradation additives, etc.).
(2) Adding polymer substances
This method is also called blending modification. Its main method is to add one or more other resins (including plastics and rubbers) to a resin to change the original resin properties. Since the composite system of blending modification is composed of high molecular substances, its compatibility is better than that of the system with small molecules added. The modification does not have much effect on other properties of the original resin. Our common polymer alloys are the modified products of this method. Blending modification is the most effective way to develop new polymer materials, and it is also the main way to achieve high performance and refinement of existing plastic varieties.
2. Morphology and structure modification
This method mainly aims at modifying the resin morphology and structure of the plastic itself. The usual methods are to change the crystal state of the plastic, cross-link, copolymerize, graft, etc.
(1) Morphology control modification
The morphology control modification of plastics is to control the different aggregation forms of plastic products so that they can achieve the expected performance. This method is to control the morphology by adjusting the processing conditions without external forces. It is generally called self-modification, among which self-reinforcement is the most commonly used. Through plastic morphology control, many properties of plastics can be improved, such as mechanics, thermal, optics and other aspects. The modification effects in some aspects are very obvious. For example, the crystallization quality is controlled by nucleation technology, and a high degree of orientation is obtained by biaxial stretching technology.
(2) Cross-linking modification
Crosslinking should be familiar to everyone. Generally, the linear structure is crosslinked into a network structure or a three-dimensional structure. External conditions are required to initiate crosslinking, usually different forms of energy (such as light, heat, radiation, etc.). Due to external effects, the macromolecular chains produce reactive free radicals or functional groups, thereby forming new chemical bonds between the macromolecular chains, so that the linear structure polymer forms a network structure polymer to varying degrees. For example, the crosslinking modification of polypropylene can improve its mechanical properties.
(3) Copolymerization and grafting modification
This method mainly adds other molecular segments or functional groups to the original molecular chain. Copolymerization refers to the polymerization reaction of two or more monomers, which can expand the performance of polymers and is an important way to improve the performance and use of polymers. For example, copolymerization of polystyrene and acrylonitrile improves the weakness of polystyrene’s brittleness; copolymerization of polyvinyl chloride and vinyl acetate improves the plasticity of polyvinyl chloride. Grafting includes chain transfer grafting, chemical grafting, and radiation grafting. Its modification is widely used in rigid bodies and elastomers. For example, styrene-butadiene grafted copolymer improves the impact performance of PS.
3. Composite modification
The composite modification of plastics is a method of forming a multilayer film, sheet or other material by combining two or more layers of film, sheet or other material together through adhesives or hot melt methods. The composite modification of plastics is actually an extreme form of laminar blending in the plastic blending modification method, and can also be regarded as a special plastic blending modification.
4. Surface modification
Plastic surface modification refers to a type of modification method that changes the surface properties of plastic products through physical or chemical methods. There are two differences between plastic surface modification and other modifications: first, its modification is limited to the surface of the product, and its internal properties do not change; second, its modification is implemented after the plastic product is once molded, which belongs to secondary processing modification.
The purpose of plastic surface modification can be divided into two categories: one is direct application modification, and the other is indirect application modification.
(1) Directly applied plastic surface modification Directly applied modification refers to some modifications that can be directly applied, including surface gloss, surface hardness, surface wear resistance and friction, surface anti-aging, surface flame retardancy, surface conductivity and surface barrier, etc. This aspect of plastic surface modification has been developed and applied rapidly in recent years. For example, in the aspect of plastic barrier modification, surface barrier modification occupies a very important position.
(2) Indirect application of plastic surface modification Indirect application modification refers to some modifications that lay the foundation for direct application, such as the modification of plastic surface tension to improve the adhesion, printability and stratification of plastics. For example, taking plastic electroplating as an example, only the ABS coating fastness of plastic varieties without surface treatment can meet the requirements; especially polyolefin plastic varieties, the coating fastness is very low, and surface modification must be carried out to improve the bonding fastness with the coating before electroplating treatment can be carried out.
Related knowledge
Plastic modification methods
1. Filling modification: By adding a certain amount of filler to the plastic, the production cost of the plastic can be effectively reduced. In addition, adding nanopowders with special functions can make corresponding functional masterbatches.
2. Blending modification: Two or more polymer compounds with similar properties are mixed in a certain proportion to form a polymer blend.
3. Copolymerization modification: Two or more monomers undergo polymerization reaction to obtain a copolymer, such as ethylene + propylene = ethylene-propylene rubber ; acrylonitrile + butadiene + styrene = ABS resin.
Reduce density
Reducing the density of plastics means reducing the original relative density of plastics through appropriate means to meet the needs of different applications. There are three ways to reduce the density of plastics: foaming modification, adding lightweight fillers, and blending lightweight resins.
1. Foaming reduces the density of plastics. Foaming molding of plastic products is the most effective way to reduce their density. However, the two modification methods of adding lightweight additives and blending lightweight resins can only slightly reduce the density, and the reduction is generally only about 50%, and the minimum relative density can only reach about 0.5. The density of plastic foam products varies widely, and the minimum relative density can reach 10-3.
2. Add lightweight fillers to reduce the density of plastics. This method reduces the density to a relatively small extent, generally down to a relative density of about 0.4-0.5. The relative density of fillers is generally greater than that of plastics. There are only a few types of fillers with a lower relative density than plastics:
(1) Microspheres a. Glass hollow microspheres ( floating beads ) have a relative density of 0.4-0.7 and are mainly used for thermosetting resins ;
b. The relative density of phenolic microbeads is 0.1.
(2) Organic fillers
a. Cork powder: relative density 0.5, apparent density 0.05-0.06;
b. Relative density of fiber dust and cotton dust: 0.2-0.3;
c. Fruit shell crops such as straw powder, peanut powder and coconut shell powder. The amount of light filler added is generally less than 50%, in principle, not seriously affecting its original performance.
3. Blending lightweight resins to reduce the density of plastics. This method has a smaller reduction range and is generally only suitable for plastics with a relatively high density, such as fluoroplastics, POM, PPS, HPVC, PA66, PI and thermosetting plastics. The optional lightweight plastics refer to several resins with a relative density of less than 1, such as poly-4-methylpentene – 1, EPR ( ethylene-propylene copolymer ), PE, PP, EVA, etc. The amount of addition is mainly based on not affecting other properties of the plastic, generally about 20%-40%.
Increase density
Increasing the density of plastics is a method of increasing the relative density of the original resin, mainly by adding heavy fillers and blending heavy resins.
1. Add heavy fillers to increase the density of plastics
(1) Metal powder
(2) Heavy mineral fillers
2. Blending heavy resins to increase the density of plastics. This method has a relatively small increase, generally only up to about 50%. It is mainly suitable for some light resins such as PE, PP, PS, EVA, PA1010 and PPO. Commonly added heavy resins include: PTFE, FEP, PPS and POM.
Improved transparency
The transparency of plastic is a measure of the transparency of a material, and there are many performance indicators that need to be considered. Commonly used indicators include: transmittance, haze, refractive index, birefringence and dispersion. Among the above indicators, transmittance and haze are mainly used to characterize the transmittance of the material, while refractive index, birefringence and dispersion are mainly used to characterize the transmittance quality of the material. A good transparent material requires the above performance indicators to be excellent and balanced. Classification of transparency: According to the transmittance of the material, it can be divided into the following three categories:
Transparent material: the transmittance of visible light with a wavelength of 400nm-800nm is above 80%;
Translucent material – the transmittance of visible light with a wavelength of 400nm-800nm is between 50% and 80%;
Opaque material – the transmittance of visible light with a wavelength of 400nm-800nm is less than 50%.
According to the above classification method, resins can be divided into the following categories:
(1) Transparent resins mainly include: PMMA, PC, PS, PET, PES, JD series, CR-39, SAN (also known as AS), TPX, HEMA and BS (also known as K resin ), etc. Among them, PES is polyethersulfone, JD series optical resin is a copolymer derivative of PES, SAN is a styrene / acrylonitrile copolymer, TPX is polymethylpentene -1, BS is a 25% butadiene/75% styrene copolymer, CR-39 is a bis( allyl diglycol carbonate) polymer, and HEMA is poly (hydroxyethyl methacrylate).
(2) Translucent resins mainly include PP and PA.
(3) Opaque resins mainly include ABS, POM, PTFE and PF.
Crystalline resin molded products contain a mixture of crystalline and amorphous parts. During crystallization, the random molecular chains are regularly arranged, so the volume of the resin decreases. This phenomenon is called “shrinkage”.
Once the resin is cured, its molecular chains are fixed, and the ratio of the crystalline part to the amorphous part in the solid molded product seems to no longer change. However, the actual situation is that when the molded product encounters a certain degree of high temperature, the molecular chains of the amorphous part sometimes rearrange, resulting in crystallization. As a result, the volume of the molded product decreases. This phenomenon is called “post-shrinkage”. Post-shrinkage can easily lead to dimensional changes, dents, warping and other failures.
When the temperature of the molded product is high, it is easy to cause post-shrinkage. Rapid cooling and solidification during molding may sometimes lead to insufficient crystallization, which is easy to cause post-shrinkage.
To prevent post-shrinkage, the molded product should be fully crystallized before it is actually used. Specifically, it should be left to stand for 2 to 3 hours in an environment with a temperature about 20°C higher than the ambient temperature when the molded product is used. This is called ” annealing treatment “. If the dimensional tolerance is within the annealing treatment, there is usually little problem.
Crystalline resin
There are many kinds of resin materials, and “crystalline resin” is one of them. Here is an introduction to its basic situation:
Resins are roughly divided into thermoplastic resins and thermosetting resins. Thermoplastic resins are hot-melting and cold-setting resins. Thermosetting resins are resins whose raw materials undergo a chemical reaction when heated and no longer melt after solidification. Thermoplastic resins can be further divided into crystalline resins and non-crystalline resins.
When melted, the molecular chains of the resin are randomly mixed and move. When the resin cools, the molecular chains begin to align, and eventually the crystalline and non-crystalline parts are mixed together and solidify. Even crystalline resins are sometimes not 100% crystalline, and there must be non-crystalline parts mixed in. On the other hand, non-crystalline resins solidify in a random state as shown in Figure (A).
ISO and ASTM physical property determination methods
With the advancement of international standardization, most chinese measurement methods specified by ISO are used as standards. How different is this from the previous ASTM standard method?
Background: Advanced countries in the world have their own unified industrial standards. For example, Japan has JIS (Japanese Industrial Standards), the United States has ANSI (American Standards Association), Germany has DIN (German Standards), etc. In addition, the standards of independent groups such as ASTM (American Society for Testing and Materials Standards) in the United States are also widely recognized. These various standards have many differences even for the same test items. With the acceleration of the process of international marketization, its disadvantages are becoming more and more prominent. In order to eliminate these disadvantages, everyone urgently needs an international standardization. In January 1995, the WTO (World Trade Organization) issued TBT (Agreement on Technical Barriers to Trade), and all must be carried out in accordance with ISO international regulations. Japan’s JIS (Japanese Industrial Standards) has also been gradually replaced by ISO specifications, and engineering plastics are no exception.
The difference between ASTM and ISO: Taking the tensile test as an example to illustrate the difference between ASTM and ISO.
Other
(1) Synthesis modification of copolymerized polyimide: Polyimide is usually obtained by polycondensation of diamine and dianhydride monomers in a certain solvent to generate polyamic acid, which is then dehydrated by thermal cyclization or chemical cyclization. Studies have found that when a third monomer is added to the system to form two dianhydrides and one diamine or two diamines and one dianhydride, the properties of the resulting polymer will change. If all aromatic dianhydrides or diamines are used, the heat resistance and strength of the synthetic material will be improved; if aliphatic dianhydrides or diamines are used, the solubility will be appropriately increased.
(2) Modification of the main chain of polyimide: The main chain modification mainly involves the introduction of flexible groups, silicon elements or liquid crystal units into the polyimide monomer. The modified material has improved processing fluidity, flexibility and other properties.
(3) Polyimide side chain modification: There are two methods to introduce functional side groups: one is to first synthesize monomers containing functional groups, and then further polymerize them into side chain PIs; the other is to first synthesize PIs that have been imidized and have active groups on the main chain, and then connect some functional groups to the PI main chain. Due to the differences in the main chain and side chain structures and the different aggregate structures, the processability and solubility of PI can be improved after the introduction of side groups, and the mechanical properties of long side chains are better than those of short side chain polymers. The functional side groups introduced are generally organosiloxane side groups, chromogenic side groups, acetylene- containing side groups, etc.
