Polyurethane (PU), full name of polyurethane, is a polymer material with excellent mechanical properties formed by polycondensation of polyols and polyisocyanates[1], extremely plastic.Its synthesis can be traced back to 1937. Professor Byaer first synthesized linear polyurethane resin with 1,6-hexamethylene diisocyanate and 1,4-butanediol as raw materials[2]。Several main categories of polyurethane include polyether type, polyester type, polyimide type, polyurea type, etc. They can be made into polyurethane plastic (mainly foam plastic), polyurethane fiber (called spandex in China), polyurethane rubber and elastomer[3]And other materials.
After nearly 80 years of technological development, this kind of material has been widely used, involving products such as coatings, adhesives, fabric finishing agents, leather modifiers, polyurethane soft/hard foams, elastomers, etc., and applied in textile, construction, aviation, shipping, transportation, medicine, electronics and other fields.Soft polyurethane is mainly a thermoplastic linear structure. It has better stability, chemical resistance, resilience and mechanical properties than PVC foam, and has less compression deformation.Good thermal insulation, sound insulation, seismic resistance and anti-virus performance.Therefore, it is used as packaging, sound insulation and filtering materials.Rigid polyurethane plastic is light in weight, excellent in sound insulation and heat insulation, resistant to chemicals, good in electrical performance, easy to process, and low in water absorption.It is mainly used in construction, automobile, aviation industry, thermal insulation structural materials.The performance of polyurethane elastomer is between plastic and rubber, oil resistant, wear-resistant, low temperature resistant, aging resistant, high hardness and elastic.It is mainly used in shoemaking industry and medical industry.Polyurethane can also be used to make adhesives, coatings, synthetic leather, etc[4]。
Polyurethane (PU) was first developed by German scientists in the 1930s. German scientists condensed liquid isocyanate and liquid polyether or glycol polyester to form a new material. The physical property parameters of this material are different from those of polyolefin materials at that time. Scientists named it polyurethane.With the end of World War II, the chemical manufacturing industry in the United States flourished. In the 1950s, flexible polyurethane foam was synthesized, which was an important landmark research in the chemical industry at that time and provided a solid technical foundation for the development of polyurethane industry in the future[5]。
Overview of main raw material markets
The main raw materials of polyurethane include diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI) and polypropylene glycol (PPG), which have become international commodities.The production technology and equipment of these raw materials are very complex, and the product competition is quite fierce. As a result of long-term development, the production is relatively concentrated[6]。
Isocyanate is the general name of various esters of isocyanate.If classified by the number of - NCO group, it includes monoisocyanate R-N=C=O, diisocyanate O=C=N-R-N=C=O and polyisocyanate;It can also be divided into aliphatic isocyanates and aromatic isocyanates. At present, the use of aromatic isocyanates is the largest, such as toluene diisocyanate TDI and diphenylmethane diisocyanate MDI[6]。
TDI is a light yellow or colorless transparent liquid with pungent odor, which has the advantages of good economy, storage and convenient transportation, but is not resistant to yellowing. Its reaction activity is high, mainly due to the electron absorption of the benzene ring in the structure, and the reaction temperature needs to be controlled during synthesis (Figure 1).
Figure 1 TDI Structure Diagram
It is mainly divided into 2 isomers, 2,4-TDI and 2,6-TDI. 1.Industry is mainly divided into three types:
(1) TDI-100 contains 100% 2,4-TDI;
(2) TDI-80 contains 80% 2,4-TDI and 20% 2,6-TDI;
(3) TDI-65 contains 65% 2,4-TDI and 35% 2,6-TDI, among which TDI-80 is the most common.
MDI mainly includes pure MDI, polymerized MDI, liquefied MDI, crude MDI, etc. (Figure 2).Pure MDI is white or light yellow solid, toxic, unstable at room temperature, easy to self polymerize, and needs to be stored below 5 ℃.Polymerized MDI is a brown transparent liquid, which can be stored at room temperature.Crude MDI is a mixture of polymerized MDI and pure MDI, also known as PAPI.MDI has good molecular regularity, hardness, solvent resistance and water resistance, but is not resistant to yellowing. It is mainly used for plastic runways, soft and hard foams and other raw materials.
Figure 2 MDI Structure Diagram
In actual industrial production, TDI and MDI are substitutes. The foam density produced by TDI system is relatively high, and the polyurethane produced by MDI has good plasticity.
The production technology and equipment requirements of MDI are complex, and the production technology is controlled by several giant enterprises around the world.More than 95% of MDI manufacturing enterprises in the world are concentrated in Asia and Europe.Compared with the two, the growth rate of the European market is not as fast as that of Asia, but its MDI industry started earlier and the market is more mature[6]。
After the signing of the China South Korea Free Trade Agreement on June 1, 2015, the tax rate of polyurethane raw materials has changed. The export tax rate of China's polymerization MDI has decreased faster than the import tax rate.With the increase of domestic aggregate MDI market capacity and output, the dependence on imports decreases and the dependence on exports increases[6]。
3. Polypropylene oxide glycol PPG
The ether bond of polyether polyol is easy to rotate, which can prepare water-borne polyurethane with good flexibility. The ether bond is resistant to hydrolysis, and the polyurethane has good hydrolysis resistance, but its carbon atom is easy to be oxidized, so the thermal stability of the material is poor (Figure 3).PPG is one of the main raw materials for the production of polyurethane products. The usage of PPG in polyurethane foam can reach more than 90%, accounting for the largest proportion.Due to the large scale of polyether polyol production plant, the production capacity is mainly concentrated in BASF, Bayer, Dow Chemical, Shell and other large multinational companies.The polyether polyol manufacturers in China have formed a certain scale only through technology introduction and independent research and development.In recent years, with the expansion of large manufacturers, the varieties and specifications of polyether polyols are basically complete, and the product quality and stability are also greatly improved[6]。
Figure 3 Structural Diagram of Polyether Polyol
Polyurethane classification
Announce
edit
Classification according to appearance
Table 1 Appearance Classification
name
Polyurethane aqueous solution
Polyurethane emulsion
Polyurethane dispersion
state
colloidal dispersion
dispersed
dispersed
appearance
transparent
White turbidity
Milky white translucent
Particle size/nm
Less than 10
10~100
Greater than 100
Relative molecular weight
100~1000
Greater than 5000
10 million~200000
According to the appearance (dispersion form and particle size) of polyurethane, it is classified as follows: Aqueous solution of polyurethane, Dispersions of polyurethane, and Emulsion of polyurethane[7](Table 1).
Classification according to hydrophilic groups
According to different types of ionic groups and charges, they are classified as anionic, cationic and non-ionic[8](Table 2).
Table 2 Classification of hydrophilic groups
name
characteristic
Anionic
Anionic groups are attached to the main chain or side chain, mainly sulfonic acid type and hydroxy acid type.Ions are introduced through chain extender and emulsified to form anionic polyurethane
Cationic
The tertiary amine group is introduced into the main chain or side chain, and it contains ammonium ion or matte ion (mostly quaternary ammonium ion) after quaternization
Non ionic
There is no ionic group in the molecule. The emulsifier and polyurethane are emulsified with a shearing machine or non-ionic hydrophilic groups are used as raw materials
Classification according to use form
According to the use form, it can be divided into single component and two component[9](Table 3).
Table 3 Classification of Use Forms
name
characteristic
One component
The required waterborne polyurethane can be obtained without adding crosslinking agent and can be used directly
Bicomponent
Crosslinking agent must be added, which can not be used alone, or cross-linking agent is added to one component waterborne polyurethane to increase adhesion, so that polyurethane raw materials and cross-linking agent form two components
Research status of polyurethane modification
Waterborne polyurethane (WPU) has a wide variety of advantages, such as strong functionality, weak volatility, small odor, good wear resistance, and is widely used in all walks of life.However, poor water resistance and heat resistance limit the development of waterborne polyurethane to some extent, which can be treated by modification[10]。
Generally, acrylic acid modification, organic fluorine modification, organic silicon modification, nano material modification, epoxy resin modification, bio based material modification and cross-linking modification are adopted.In addition, the modification of other materials by polyurethane is also a research hotspot.
(1) Acrylic modification
Acrylic acid has good light stability, weather resistance, acid and alkali resistance, water resistance, and is not prone to yellowing, and its price is low.At present, the modification of acrylic acid mainly adopts block, graft, core shell emulsion polymerization and other methods.The adhesion, weather resistance and solid content of polyurethane can be improved by acrylic modification. The polyurethane prepared by this method with low price has broad application prospects and is widely used in many fields at present[11-12]。For example, super hydrophobic layer is prepared by using acrylic polyurethane and hydrophobic ZnO bactericidal suspension[13]Reactive emulsifier is used instead of traditional emulsifier to prepare waterborne polyurethane with small particle size and narrow distribution and high solid content[14-15], or improve the thermal stability and tensile strength of the adhesive film through hexafluorobutyl acrylate (FA)[16]。
(2) Organic fluorine modification
The fluorine atom in organic fluorine has high C-F bond energy, short bond length, good water resistance, chemical solvent resistance, heat resistance, biological resistance, wear resistance, and insulation[17-18]。Introducing organic fluorine into polyurethane is one of the common modification methods. The modified polyurethane film has improved heat resistance and water resistance[19-20]。
(3) Silicone modification
The main chain of organosilicon contains Si-O-Si bond, and the side group is formed by connecting organic groups with silicon atoms. It has the advantages of thermal stability, water resistance, weather resistance, corrosion resistance, non-toxic, tasteless and biocompatible.Polyurethane modified by organosilicon can improve the mechanical properties of the adhesive film, endow polyurethane with water resistance, weather resistance, low surface energy and other characteristics.At present, five methods are mainly used: (1) silicone alcohol modification method;(2) Aminoalkyl polysiloxane modification method;(3) Hydroxyalkyl polysiloxane modification method;(4) Alkoxysilane crosslinking modification method;(5) Cage type silsesquioxane (POSS) modification method[21-25]。After modification, the material properties are effectively improved[26-29]。
(4) Nano material modification
The modification of nano materials can improve the mechanical properties of the adhesive film, endow it with thermal insulation, hydrophobic, conductivity and other functions, and can also form an open hole structure.At present, metal nanomaterials and carbon nanomaterials are mainly used, including TiOtwoGraphene and carbon nanotubes have been extensively studied.Modified polyurethane is mainly used in medicine, aerospace, electronics, etc[30-33]。
(5) Epoxy resin modification
Epoxy resin has epoxy group, ether bond, hydroxyl group and other groups. It has good stability, mechanical strength, adhesion, low curing shrinkage and low price.The modification method is mainly to introduce the ring opening of epoxy group into polyurethane, and improve the water resistance, solvent resistance and other properties of the modified polyurethane emulsion.However, after modification, the brittleness is large and the toughness is insufficient, and the application in high-end fields is limited, mainly used in electronics, aerospace, transportation and other fields[34][35-39]。
(6) Crosslinking modification
Cross linking modification refers to the cross linking between the polyurethane molecule and the introduced molecule. Cross linking modification can be divided into internal cross linking, external linking and self crosslinking.Generally, polyurethane modified by internal crosslinking belongs to one component polyurethane, while polyurethane modified by external crosslinking belongs to two component polyurethane. Self crosslinking modification refers to the introduction or co mixing of crosslinking groups.The water resistance, heat resistance, solvent resistance and mechanical properties of the modified polyurethane film are significantly improved[40-43]。
(7) Modification of bio based materials
Bio based materials are mainly used to modify polyurethane with environmentally friendly materials.For example, chitosan, lignin, cellulose, etc., have the advantages of renewable, green, environmental protection, etc[44-48]。
Classification of polyurethane products
Announce
edit
Polyurethane products mainly include the following: foam plastics, elastomers, fiber plastics, fibers, leather shoe resins, coatings, adhesives and sealants, among which foam plastics account for the largest proportion[6]。
Polyurethane foam
Polyurethane foam plastics are divided into hard foam and soft foam, which have excellent elasticity, elongation, compression strength, softness, and good chemical stability.In addition, polyurethane foam has excellent processability, adhesion, thermal insulation and other properties, which are excellent cushioning materials[6]。
Polyurethane elastomer
Polyurethane elastomer has two segments, soft and hard, so it can endow materials with high strength, good toughness, abrasion resistance, oil resistance and other excellent properties through the design of molecular chain. The polyurethane, known as "abrasion resistant rubber", has both the high elasticity of rubber and the rigidity of plastic[6]。
Polyurethane fiber plastics
Polyurethane fiber plastic has high tensile strength and toughness, making it the preferred material in many applications.It has good abrasion resistance and excellent oil resistance, making it suitable for applications requiring abrasion resistance, such as ropes, sports shoe materials, etc., and can maintain stable performance in the presence of some grease and chemicals.Polyurethane fiber shows high elasticity and resilience, which makes it widely used in applications with high elasticity requirements, such as elastic fabrics, springs, etc.
Polyurethane coating
1. Application status
With the continuous improvement of the material living standard of the public, polyurethane coatings, with its outstanding performance advantages, began to enter a new stage of rapid development.According to the statistical data, from 1980 to 2004, the usage of polyurethane coatings in various industries has shown a very rapid development trend. The total usage of polyurethane coatings has achieved a breakthrough development from 17000 to 200000 tons. The output of polyurethane coatings is second only to alkyd resin coatings, acrylic resin coatings and phenolic resin coatings,It has become the fourth largest variety in the paint field, and this development trend continues to this day, its output and scope of use still maintain a very rapid development trend[49]。
From the perspective of research, development and application of polyurethane coatings, the most mainstream is still two-component polyurethane coatings, which has been continuously expanded and improved in the field of wood furniture painting.In addition, single component polyurethane coatings still have very strong application advantages in automobile processing, basement waterproofing and other fields. With polyurethane as the main material, such coatings have very good application value in the production of various paints and varnishes.The newly developed acrylic polyurethane paint uses biuret as the curing agent, which is very reliable in the application of automotive repair paint.Especially for light vehicles, large buses and vans, the coating function in vehicle processing is very worthy of affirmation, and the market prospect is considerable.A variety of new polyurethane coatings developed on the basis of acrylic polyurethane coatings can also play an application value in other manufacturing and processing fields to meet the requirements of processing products such as household appliances and trains on processing quality[49]。
In addition, special polyurethane coatings applicable to floor coating, machine tool coating, aerospace equipment surface coating and other fields are in the process of accelerated development and research.The classification, characteristics and application fields of polyurethane coatings are reviewed and analyzed in relevant reports[49]。
2. Research and development progress
(1) Waterborne polyurethane coating
In 1942, PSchlack synthesized waterborne polyurethane emulsion for the first time, and polyurethane development entered a new era[50-51]。In 1967, the United States took the lead in realizing industrialization[52]Then in 1972, Dietrich of Bayer Company in Germany successfully prepared waterborne polyurethane coating with diisocyanate and diol as raw materials[53], and PCompared with Schlack synthesis, the particle size is smaller (0.03-100.00 μ m)[54]。
At present, the annual output of waterborne polyurethane resin in the world is about 50000~60000 tons.Water is selected as the dispersion medium of waterborne polyurethane coatings. Organic solvents are rarely found in the entire polyurethane coating structure system, which meets the requirements of energy conservation and emission reduction in the coating field proposed by the current environmental protection. Therefore, the application and development of waterborne polyurethane coatings in related fields are increasingly concerned and valued by industry insiders.Up to now, although the application proportion of polyurethane coatings in the whole coating field in China is only about 4%, the engineering application of waterborne polyurethane coatings is maintaining a growth rate of nearly 10% in recent years.Generally, waterborne polyurethane coatings do not need additional dispersants or emulsifiers, and the molecular size and molecular structure can be properly adjusted as appropriate.In view of this feature, compared with the latex coatings that are widely used in the traditional sense, waterborne polyurethane coatings can have better low-temperature film forming characteristics, without adding plasticizers and film forming additives with the same proportion of income.Compared with other coatings, waterborne polyurethane coatings not only have a good appearance, but also have a short drying time, showing unique advantages in the field of wood coatings.The solvent based acrylic leather coating in the traditional sense has gradually been replaced by water-borne polyurethane leather coating. With its advantages in chemical resistance and low temperature resistance, it has attracted the attention of people in the leather coating field.In addition, this kind of material also plays a very specific application value in plastic, vehicles, industry, anti-corrosion and other related fields, and has quite broad development space[49]。
The performance of waterborne polyurethane coatings still has some limitations, and the lack of water resistance is one of the most important factors affecting the application of waterborne polyurethane coatings in practice.In addition, there are also some problems in the application of waterborne polyurethane coatings during the construction period. For example, for two-component waterborne polyurethane coatings, the drying speed is relatively slow after being mixed with construction materials, which requires a long time for maintenance.Carbon dioxide bubbles generated during the reaction of waterborne polyurethane coating with water may remain in the coating film in large quantities, affecting its performance.Moreover, the high cost has also become one of the main factors affecting the industrial application of waterborne polyurethane coatings.More importantly, a large amount of waterborne coatings added to waterborne polyurethane coatings may have an impact on iron substrates in engineering buildings, resulting in flash corrosion, and even affecting the wettability and appearance performance of surface coatings.In view of the above problems, in order to promote the further development of waterborne polyurethane coatings, composite modified waterborne polyurethane coatings should be taken as the development direction and research focus in the future, and some molecular structures with special functions, such as silicone polymer chains and fluoropolymer chains, should be introduced into the polyurethane chain to improve the comprehensive performance of the coating film and give full play to its high temperature resistanceAdvantages in water resistance and weather resistance.Low VOC and high performance two-component waterborne polyurethane coatings can also be taken as the focus of research and development to reduce the cost of polyurethane coatings and improve the use efficiency[49]。
(2) Modified polyurethane coating
As for the relatively single polyurethane coating, its appearance, gloss, water resistance and hardness still have some limitations in the process of industrial production and application. Therefore, we can try to improve its performance level by developing modified polyurethane coating.At the current technical level, polyurethane materials can be modified in two ways: the first is through chemical intervention to enable polyurethane coatings to have two or more properties;The second is to mix two or more resin materials with complementary characteristics through physical intervention, so that polyurethane coatings can have diversified properties.Among them, for the silicone material, it has a series of characteristics and advantages, such as non corrosion, non-toxic, flame resistance, ozone resistance, weather aging resistance, electrical insulation, etc., and has a very good application value in the modification and processing of polyurethane coatings.Bayer took the lead in the basic research and development of polyurethane powder coatings, and successfully developed a closed isocyanate cross-linking system. The commonly used is caprolactam closed IPDI curing system, whose curing temperature is above 170 ℃. This high temperature curing is conducive to the high leveling of the coating film, and is a variety without volatile by-products.It has also been pointed out in previous reports that the combination of organosilicon materials and polyurethane coatings and the application of appropriate methods for modification processing can obviously overcome the performance defects of polyurethane materials, which is of great value for expanding the application field of polyurethane materials.At the same time, the chemical structure of polysiloxane is special, showing excellent stability, biocompatibility, electrical insulation and high and low temperature resistance.Since the 1940s, it has been widely used in industrial production practice. In the process of research and development of modified polyurethane materials, we can try to use polysiloxane as the soft segment to synthesize polysiloxane polyurethane block copolymers to highlight the advantages of polyurethane and polysiloxane, and highlight the surface enrichment of the modified polyurethane coatingOutstanding advantages in dielectric and biocompatibility, with very large application space and development potential[49]。
(3) Environmental friendly polyurethane coating
Affected by environmental protection requirements and the requirements for sustainable development of the entire coating industry, relevant personnel in the industry must actively explore and develop polyurethane coating products with environmental benefits, such as UV curable polyurethane coatings, high solid content low viscosity polyurethane materials, and powder polyurethane materials.The polyurethane coating presented in the form of powder agent has good physical and mechanical properties and chemical resistance, and also shows good appearance.Compared with polymer materials of other structures, waterborne polyurethane dispersion resin materials have a lot of advantages, and conform to environmental protection requirements. They can be dispersed in water, free of free isocyanates, non-toxic, and have good adhesion to the substrate.Waterborne two-component polyurethane coating is composed of water-borne polyol containing - OH group and low viscosity polyisocyanate curing agent containing - NCO group. Its film performance is mainly determined by the composition and structure of hydroxyl resin[49]。
One component waterborne polyurethane coating is a kind of coating with waterborne polyurethane resin as the base material and water as the dispersion medium.The waterborne polyurethane coating modified by crosslinking has good storage stability, film mechanical properties, water resistance, solvent resistance and aging resistance, and is similar to the performance of traditional solvent based polyurethane coating. It should be one of the very important development directions of environment-friendly polyurethane coating.From the perspective of future development trend, polyurethane powder coatings have become one of the most mainstream development directions in the current entire coating field, and their proportion in various powder coatings and even polyurethane coatings has been increasing.For example, for the automotive coating processing market, in the process of coating product development, polyurethane powder coatings should be the best choice and development direction. At the same time, through technological innovation and research and development, a new generation of polyurethane powder coatings with good adaptability to low temperature environment and no volatile by-products should be developed,The new powder coating has good similarity with the two-component solvent based polyurethane coating in performance, which can be applied to the coating manufacturing and processing field of the new generation automobile production line. While ensuring the coating performance, it reflects good environmental protection benefits and has a very considerable comprehensive benefit[49]。
Polyurethane adhesive
The synthesis of polyurethane adhesive is based on the unique chemical properties of isocyanate.Isocyanates are compounds containing isocyanate group (- NCO) in the molecule, which has a highly unsaturated bond structure with overlapping double bonds, and can react with various compounds containing active hydrogen.In the field of polyurethane adhesive, isocyanates containing two or more - NCO characteristic groups are mainly used.The polyurethane adhesive is divided into general isocyanate polyurethane adhesive and yellowing resistant isocyanate polyurethane adhesive according to whether the product yellows under light[55]。
1. Universal PU adhesive
Universal isocyanate, namely aromatic isocyanate, is the most widely used isocyanate in the polyurethane industry at present. Because the methylene linked to the benzene ring in the structure is easy to be oxidized to form quinone chromophore, the material will turn yellow.Commonly used general-purpose isocyanates include TDI, MDI and polymethyl polyphenyl isocyanate (PAPI).TDI is liquid at room temperature and easy to use. It is the first isocyanate used in polyurethane industry[55]。
2. Yellow resistant PU adhesive
In order to improve the yellowing of polyurethane materials caused by general isocyanates, in addition to the use of relevant additives, the generation of quinone chromophores with benzene ring conjugation should be avoided.For this reason, researchers have developed many yellowing resistant isocyanates, such as XDI, HDI, IPDI, etc[55]。
3. Modification of polyurethane adhesive
Although polyurethane adhesive has excellent performance, it is vulnerable to the influence of external environment such as light, heat, oxygen, water, etc., reducing its use value.With the development of society, the single performance of polyurethane adhesive can no longer meet the application needs. The research on the modification of polyurethane adhesive has become a hot field, in which physical modification and chemical modification are the main modification methods[55]。
(1) Physical modification
Physical modification is a method to improve the performance of polyurethane adhesive by doping some fillers and additives under certain conditions during the preparation of polyurethane adhesive.Quartz powder has good compatibility with polyurethane adhesive system, which can significantly improve the tensile strength, elongation at break and tear strength of polyurethane adhesive products.Nano boron nitride (BN) was ultrasonic dispersed in polyols, and then reacted with MDI to prepare polyurethane adhesive films for food packaging.Compared with the adhesive without nano BN, the water vapor permeability of the film decreased by 50%, the adhesive strength increased by 37%, and the peel strength increased by 7.14%.Adding SiO2 nanofibers to polyurethane matrix, it is found that the hydroxyl groups on the surface of SiO2 nanofibers form a close cross-linking structure with polyurethane, which improves the adhesive, the hardness of the adhesive film, and the tensile strength, but also increases the viscosity of the colloid[55]。
(2) Chemical modification
Chemical modification is a modification method that changes the type of atoms or atomic groups on the molecular chain and their combination mode through the chemical reaction of polymer. Block and graft are several common chemical modification methods of polyurethane adhesive.Among them, high-performance epoxy resin modified polyurethane adhesive, acrylate modified polyurethane adhesive and silicone resin modified polyurethane adhesive are the targets of the industry.Epoxy resin has many advantages such as good adhesion, corrosion resistance, high strength, etc., but its toughness is poor. Introducing epoxy group into polyurethane system can obtain products with better performance.Silane modified polyurethane adhesive can not only improve flexibility, but also avoid the shortcomings of traditional polyurethane adhesive, such as easy foaming and poor adhesion to smooth substrates[55]。
