Introduction To Polyurethane Materials
Polyurethane is a kind of polymer with many repeated carbamate groups in its molecular structure, which is fully called polyurethane, or PU for short. Polyurethane can be made into thermoplastic polyurethane with linear molecules or thermosetting polyurethane with body molecules according to its composition. The former is mainly used for elastomers, coatings, adhesives, synthetic leather, etc., while the latter is mainly used for manufacturing various soft, semi-rigid and hard foamed plastics.
Polyurethane was first developed by German scientists in 1937 and began industrial production in 1939. The manufacturing method is that isocyanate reacts with compounds containing active hydrogen (such as alcohol, amine, carboxylic acid, water, etc.) to form compounds with carbamate groups. The reaction between isocyanate and polyol is the basic reaction for manufacturing Pu, and the reaction formula is:
The reaction belongs to stepwise addition polymerization, and no small molecular by-products are generated in the reaction process. If one of isocyanates or polyols has more than three functional groups, a three-dimensional network structure will be formed.
1、 Basic raw materials for Polyurethane Synthesis
The basic raw materials for the synthesis of polyurethane are isocyanate, polyol, catalyst and chain extender.
(1) Isocyanate isocyanate generally contains two or more isocyanate groups. Isocyanate groups are very active and can react with alcohols, amines, carboxylic acids, water, etc. At present, the main isocyanates used in polyurethane products are toluene diisocyanate (TDI), dibasic methane diisocyanate (MDI) and polymethylene p-benzene polyisocyanate (Papi). TDI is mainly used for soft foamed plastics; MDI can be used for semi-rigid and hard foam plastic machine adhesives; Papi can be used in thermosetting rigid foams, mixing and casting products because of its three functions.
(2) Polyol polyol constitutes the elastic part of polyurethane structure. Polyether polyol and polyester polyol are commonly used. The content of polyol in polyurethane determines the hardness, flexibility and rigidity of polyurethane resin. Polyether polyols are formed by ring opening polymerization of polyols, polyamines or other organic compounds containing active hydrogen with oxidized olefins. They have the advantages of high elasticity and low viscosity. This kind of polyol is widely used, especially in soft foamed plastics and reaction injection molding products. Polyester polyols are obtained by esterification of various organic polybasic acids and polyols. The linear polyester polyol synthesized by dibasic acid and diol is mainly used for soft polyurethane, and the branched polyester polyol synthesized by dibasic acid and triol is mainly used for hard polyurethane.
(3) Catalysts also need to be added in the polyurethane polymerization process to accelerate the polymerization process. Generally, there are two kinds of amines and tin. The commonly used amines are triethylenediamine, n-aminomorphorphine, etc., and tin includes dibutyltin dilaurate, stannous octanoate, etc
(4) Chain extenders commonly used chain extenders are diols and diamines with low relative molecular weight, which react with isocyanates to form hard segments in polymers. Common chain extenders include ethylene glycol, propylene glycol, butanediol, hexanediol, etc. Aromatic binary amines are generally used, such as diphenylmethane diamine, dichlorodiphenylmethane diamine, etc.
2、 Effect of structure on Performance
The properties of any polymer material are determined by its structure. Polyurethane structure includes chemical structure and aggregation structure. Chemical structure, that is, molecular chain structure, is an important factor to be considered in the formulation design at the beginning of synthesis; Aggregation structure refers to the stacking state of macromolecular segments, which is affected by molecular chain structure, synthetic process, service conditions and so on. Specifically, it has the following impacts:
(1) Effect of soft segment on Performance
Polyether, polyester and other oligomer polyols form a soft segment. The soft segment accounts for the majority of polyurethane, and the properties of polyurethane prepared by different oligomer polyols and diisocyanates are different.
The polyurethane elastomer and foam with strong polarity polyester as soft segment have good mechanical properties. Because the polyurethane made of polyester contains ester groups with high polarity, the polyurethane can not only form hydrogen bonds between hard segments, but also the polar groups on the soft segments can partially form hydrogen bonds with the polar groups on the hard segments, so that the hard phase can be more evenly distributed in the soft phase and play the role of elastic intersection point. At room temperature, some polyesters can form soft segment crystallization, which affects the properties of polyurethane. The strength, oil resistance and thermal oxidation stability of polyester polyurethane are higher than those of PPG polyether, but the hydrolysis resistance is worse than that of polyether. Polytetrahydrofuran (PTMEG) polyurethane is easy to form crystallization due to the regular structure of PTMEG, and its strength is equal to that of polyester. Generally speaking, polyether polyurethane has good flexibility and excellent low-temperature performance because the ether group in the soft segment is easy to rotate, and there is no ester group that is relatively easy to hydrolyze in the polyether, so its hydrolysis resistance is better than that of polyether. Of ether bond in polyether soft segment α Carbon is easily oxidized to form peroxide free radicals, resulting in a series of oxidative degradation reactions. Polyurethane with polybutadiene as soft segment has weak polarity, poor compatibility between soft and hard segments and poor elastomer strength. The soft segment with side chain has weak hydrogen bond and poor crystallinity due to steric hindrance, and its strength is worse than that of the side free polyurethane with the same soft segment main chain.
The molecular weight of the soft segment has an effect on the mechanical properties of polyurethane. Generally speaking, assuming that the molecular weight of polyurethane is the same, if the soft segment is polyester, the strength of polyurethane increases with the increase of the molecular weight of polyester glycol; If the soft segment polyether is used, the strength of polyurethane decreases with the increase of polyether glycol molecular weight, but the elongation increases. This is because the polarity of polyester soft segment itself is strong, and large molecular weight leads to high structural regularity, which is beneficial to improve the strength, while the polarity of polyether soft segment is weak. If the molecular weight increases, the relative content of hard segment in polyurethane decreases and the strength decreases.
The crystallinity of the soft segment contributes greatly to the crystallinity of the linear polyurethane segment. Generally speaking, crystallinity is beneficial to improve the properties of polyurethane products, but sometimes crystallization will reduce the low-temperature flexibility of materials, and crystalline polymers are often opaque. In order to avoid crystallization, the regularity of molecules can be disturbed, such as copolyester or copolyether polyol, mixed polyol, mixed chain extender, etc.
(2) Effect of hard segment on Performance
The hard segment of polyurethane is composed of isocyanate or polyisocyanate after reaction and chain extender. It contains strong polar groups such as aryl, carbamate and substituted urea. Generally, the conformation of the rigid segment formed by aromatic isocyanate is not easy to change and extends into a rod shape at room temperature. Hard segments usually affect the softening melting temperature and high temperature properties of polymers.
The structure of isocyanate affects the rigidity of the hard segment, so the type of isocyanate has a great influence on the properties of polyurethane materials. The existence of rigid aromatic rings in aromatic isocyanate molecules and the generated carbamate bonds give polyurethane strong cohesion. Symmetrical diisocyanate makes the molecular structure of polyurethane regular and orderly and promotes the crystallization of polymer. Therefore, 4,4 '- diphenylmethane diisocyanate (MDI) has higher cohesion, modulus, tear strength and other physical and mechanical properties than polyurethane made of asymmetric diisocyanate (such as TDI). The polyurethane prepared by aromatic isocyanate has a rigid aromatic ring in its hard segment, which increases the cohesion strength of its hard segment. The material strength is generally higher than that of aliphatic isocyanate polyurethane, but it has poor UV degradation resistance and is easy to yellowing. Aliphatic polyurethane will not turn yellow. Different isocyanate structures also have different effects on the durability of polyurethane. Aromatic polyurethane has better thermal oxidation resistance than aliphatic isocyanate polyurethane, because the hydrogen on the aromatic ring is difficult to be oxidized.
Chain extenders also affect the properties of polyurethane. Compared with aliphatic diol chain extended polyurethane, diol containing aromatic ring has better strength. Binary amine chain extender can form urea bond, and the polarity of urea bond is stronger than that of urethane bond. Therefore, polyurethane with binary amine chain extender has higher mechanical strength, modulus, adhesion, heat resistance and better low-temperature performance than polyurethane with diol chain extender. Cast polyurethane elastomers mostly use aromatic diamine MOCA as chain extender. In addition to curing process factors, it is because the elastomer has good comprehensive properties.
The soft segment of polyurethane will not be oxidized and degraded quickly in a short time under high temperature, but the heat resistance of the hard segment affects the temperature resistance of polyurethane. Several bond groups formed by isocyanate reaction may appear in the hard segment, and the order of thermal stability is as follows:
Isocyanurate > urea > carbamate > biuret > urea formate
The most stable isocyanate began to decompose at about 270 ℃. The thermal stability of urethane bond decreases with the increase of substituents on adjacent oxygen and carbon atoms, the increase of isocyanate reactivity or the increase of steric hindrance. In addition, the aromatic or aliphatic groups on both sides of the urethane bond also affect the thermal decomposition of the urethane bond. The stability order is as follows:
R-NHCOOR＞Ar-NHCOOR＞R-NHCOOAr＞Ar-NHCOOAr
Increasing the content of hard segment in polyurethane usually increases the hardness and reduces the elasticity.
(3) Morphology and structure of polyurethane
In the final analysis, the performance of polyurethane is affected by the morphology and structure of macromolecular chain. Especially for polyurethane elastomer materials, the phase separation of soft segment and hard segment is very important to the performance of polyurethane. The unique flexibility and wide range of physical properties of polyurethane can be explained by two-phase morphology. The properties of polyurethane materials largely depend on the phase structure of soft and hard segments and the degree of microphase separation. Moderate phase separation is beneficial to improve the properties of polymers.
From the microscopic structure, in polyurethane, due to the large cohesion energy of strong polar and rigid carbamate groups, hydrogen bonds can be formed between molecules, which gather together to form hard segment micro phase regions. At room temperature, these micro regions are glassy secondary crystals or microcrystals; Polyether segments or polyester segments with weak polarity gather together to form soft segment phase region. Although the soft segment and hard segment have certain miscibility, the hard segment phase region and soft segment phase region have thermodynamic incompatibility, resulting in micro phase separation, and the soft segment micro region and hard segment micro region show their respective glass transition temperatures. The soft segment phase region mainly affects the elasticity and low temperature properties of the material. The chain segment attraction between hard segments is much greater than that between soft segments. The hard phase is not dissolved in the soft phase, but distributed in it to form a discontinuous microphase structure. It plays the role of physical cross-linking point and enhancement in the soft segment at room temperature. Therefore, the hard segment has an important influence on the mechanical properties of the material, especially the tensile strength, hardness and tear strength. This is why polyurethane elastomers can show high strength and high elasticity at room temperature even without chemical crosslinking. Whether micro phase separation can occur in polyurethane elastomer, the degree of micro phase separation and the uniformity of hard phase distribution in soft phase all directly affect the mechanical properties of elastomer.
(4) Hydrogen bond
Hydrogen bonds exist between groups containing nitrogen atoms, oxygen atoms and H atoms with strong electronegativity, which is related to the cohesion energy of the group. Carbamate or urea groups in hard segments have strong polarity, and hydrogen bonds mostly exist between hard segments. It is reported that most of the imino groups (NH) of various groups in polyurethane can form hydrogen bonds, most of which are formed between NH and carbonyl groups in hard segments, and a small part is formed between NH and ether oxy or ester carbonyl groups in soft segments. Compared with the bonding force of intramolecular chemical bond, hydrogen bond is a kind of physical attraction, and the close arrangement of polar segments promotes the formation of hydrogen bond; At higher temperature, the chain segment receives energy and moves, and the hydrogen bond disappears. Hydrogen bond plays the role of physical crosslinking, which can make polyurethane elastomer have high strength and wear resistance. The more hydrogen bonds, the stronger the intermolecular force, and the higher the strength of the material.
(5) Crosslinking degree
Moderate intramolecular crosslinking can increase the hardness, softening temperature and elastic modulus of polyurethane, and reduce the elongation at break, permanent deformation and swelling in solvent. For polyurethane elastomer, proper crosslinking can produce materials with excellent mechanical strength, high hardness, elasticity, excellent wear resistance, oil resistance, ozone resistance and heat resistance. However, if the crosslinking is excessive, the tensile strength, elongation and other properties can be reduced.
Polyurethane chemical crosslinking is generally caused by Polyols (occasionally polyamines or other multifunctional raw materials) or crosslinking bonds (urea formate and biuret, etc.) formed by high temperature and excess isocyanate. The crosslinking density depends on the amount of raw materials. Compared with the physical crosslinking caused by hydrogen bond, chemical crosslinking has better thermal stability.
Polyurethane foam is a cross-linked polymer. The flexible foam is made of long-chain polyether (or polyester) glycol, triethanol, diisocyanate and chain extending cross-linking agent, which has good elasticity and softness; Rigid foams are made of polyether polyols and polyisocyanates (Papi) with high functionality and low molecular weight. Due to the high degree of crosslinking and the presence of more rigid benzene rings, the materials are brittle. Studies have shown that the fatigue resistance of flexible polyurethane foam decreases with the increase of urea based formate, biuret and other groups.
3、 Several practical applications of polyurethane
(1) Shark skin swimsuit
Shark skin swimsuit is a nickname given by people according to its shape characteristics. Its core technology is to imitate shark skin. Biologists have found that the rough V-shaped folds on the surface of shark skin can greatly reduce the friction of water flow, make the water flow around the body flow more efficiently, and sharks can swim quickly. The super stretched fiber surface of the fast skin is completely made from the surface of shark skin. In addition, this swimsuit also fully integrates the principles of bionics: imitating human tendons at the seams to provide power for athletes to stroke backward; It imitates human skin on the cloth and is elastic. Experiments show that the fiber of shark skin can reduce the resistance of water by 3%, which is of great significance in swimming competitions where the outcome can be determined in one second. Root cause: "shark skin" uses polyurethane fiber materials that can increase buoyancy.
Solid buoyancy material is a kind of porous structural material with low density and high strength. Polyurethane elastomer sprayed on the surface of the material as a water barrier can effectively reduce the water absorption and volume deformation rate of the material, which is of great significance to improve the safety and reliability of the underwater use of solid buoyant materials
(2) Athlete shoe sole
Characteristics of polyurethane base:
Polyurethane sole is very light, with better viscose ratio than rubber sole and ox tendon sole, and better comfort than rubber sole and ox tendon sole.
Polyurethane base, good dimensional stability and long storage life; Excellent wear resistance and deflection resistance; Excellent shock absorption and anti-skid performance; Good temperature resistance; Good chemical resistance, etc. However, polyurethane base can be divided into two types: densified polyurethane base and foamed polyurethane base.
The density of foamed polyurethane is lower than that of encrypted polyurethane, the foamed polyurethane is softer than that of encrypted polyurethane, the weight of foamed polyurethane is lighter than that of encrypted polyurethane, and the cost of foamed polyurethane is half cheaper than that of encrypted polyurethane. The gloss of foamed polyurethane is not as bright as that of densified polyurethane; The wear resistance of foamed polyurethane is not as good as that of densified polyurethane
The wear resistance of encrypted polyurethane sole is 5 times that of ordinary rubber sole, and the wear resistance of foamed polyurethane sole is 1 / 2 of that of ordinary rubber sole.
Performance of polyurethane base:
Polyurethane soles usually form various bubbles in production and are elastic Abrasion resistance, light, chemical resistance Anti corrosion and other characteristics, Pu shoes mainly made of microporous polyurethane elastomer feel soft, comfortable, warm, elastic and shockproof non-slip.
Polyurethane soles are divided into encryption type and foaming type. Encryption type polyurethane soles are very light, moderate soft and hard, hand-made, wear-resistant and durable, convenient maintenance, and are not easy to break. Foamed polyurethane base is naturally very soft due to its few components, but it is not wear-resistant and difficult to open glue. Once it is opened, it cannot be repaired.
Polyurethane soles are widely used in the production of casual shoes, sports shoes and work shoes Sandals. Tourist shoes, men's and women's leather shoes, protective shoes, etc The polyurethane sole and upper are bonded together with adhesive. Because they are light, the degumming rate is lower than that of rubber sole.
(3) Polyurethane coating
Performance:
Excellent wear resistance
Excellent chemical and oil resistance
Strong adhesion
Low temperature curing performance
High decorative performance
Performance diversity and adjustability. Through the improvement of formula, the polyurethane coating can be made into high hardness coating or elastic coating with excellent flexibility, which greatly strengthens the application range of polyurethane coating.
High and low temperature resistance.
The film is non-toxic after curing.
Environmental friendly waterborne polyurethane coatings contain no or very little organic solvents
Purpose:
Aircraft exterior wall coating.
Wood coatings.
Means of transportation.
Anti corrosion coating.
Coating of machine tools and instruments.
Plastic coatings.
Polyurethane coatings are widely used. In addition to the above purposes, acrylic polyurethane can be used as magnetic recording coating, polyester polyurethane as electrical insulation coating, transparent elastic polyurethane as antifogging coating, etc. In short, polyurethane coatings can be used in automotive industry, aviation, marine, construction, plastics, electromechanical, petrochemical and other fields.
(4) Polyurethane adhesive
Principle of adhesive:
Polyurethane adhesives contain -nco- (isocyanate) and -nhcoo- (carbamate group) with strong polarity and chemical activity, and have excellent chemical adhesion with substrates containing active hydrogen, such as porous materials such as foam, plastic, wood, leather, fabric, paper, ceramics, and materials with smooth surfaces such as metal, glass, rubber, plastic, etc.
characteristic:
It has excellent shear strength and impact resistance, is suitable for various structural bonding fields, and has excellent flexibility;
Polyurethane adhesive can adapt to the adhesion of substrates with different thermal expansion coefficients. It forms a soft hard transition layer between substrates, which not only has strong adhesion, but also has excellent cushioning and damping functions;
The low-temperature and ultra-low-temperature properties of polyurethane adhesives exceed all other types of adhesives;
Waterborne polyurethane adhesive - waterborne polyurethane adhesive has the characteristics of low or no environmental pollution and non combustion, which is the key development direction of polyurethane adhesive.
