The so-called polyurethane is the abbreviation of polyurethane, which is formed by the reaction of polyisocyanates and polyols, and contains many repeated amino ester groups (- NH-CO-O -) on the molecular chain. In actual synthesized polyurethane resins, in addition to the amino ester group, there are also groups such as urea and biuret. Polyols belong to long-chain molecules with hydroxyl groups at the end, which are called “soft chain segments”, while polyisocyanates are called “hard chain segments”.
Among the polyurethane resins generated by soft and hard chain segments, only a small percentage are amino acid esters, so it may not be appropriate to call them polyurethane. In a broad sense, polyurethane is an additive of isocyanate.
Different types of isocyanates react with polyhydroxy compounds to generate various structures of polyurethane, thereby obtaining polymer materials with different properties, such as plastics, rubber, coatings, fibers, adhesives, etc. Polyurethane rubber
Polyurethane rubber belongs to a special type of rubber, which is made by reacting polyether or polyester with isocyanate. There are many varieties due to different types of raw materials, reaction conditions, and crosslinking methods. From a chemical structure perspective, there are polyester and polyether types, and from a processing method perspective, there are three types: mixing type, casting type, and thermoplastic type.
Synthetic polyurethane rubber is generally synthesized by reacting linear polyester or polyether with diisocyanate to form a low molecular weight prepolymer, which is then subjected to chain extension reaction to generate a high molecular weight polymer. Then, appropriate crosslinking agents are added and heated to cure it, becoming vulcanized rubber. This method is called prepolymerization or two-step method.
It is also possible to use a one-step method – directly mixing linear polyester or polyether with diisocyanates, chain extenders, and crosslinking agents to initiate a reaction and generate polyurethane rubber.
The A-segment in TPU molecules makes the macromolecular chains easy to rotate, endowing polyurethane rubber with good elasticity, reducing the softening point and secondary transition point of the polymer, and reducing its hardness and mechanical strength. The B-segment will bind the rotation of macromolecular chains, causing the softening point and secondary transition point of the polymer to increase, resulting in an increase in hardness and mechanical strength, and a decrease in elasticity. By adjusting the molar ratio between A and B, TPUs with different mechanical properties can be produced. The cross-linking structure of TPU must not only consider primary cross-linking, but also secondary cross-linking formed by hydrogen bonds between molecules. The primary cross-linking bond of polyurethane is different from the vulcanization structure of hydroxyl rubber. Its amino ester group, biuret group, urea formate group and other functional groups are arranged in a regular and spaced rigid chain segment, resulting in a regular network structure of rubber, which has excellent wear resistance and other excellent properties. Secondly, due to the presence of many highly cohesive functional groups such as urea or carbamate groups in polyurethane rubber, hydrogen bonds formed between molecular chains have high strength, and the secondary crosslinking bonds formed by hydrogen bonds also have a significant impact on the properties of polyurethane rubber. Secondary cross-linking enables polyurethane rubber to possess the characteristics of thermosetting elastomers on one hand, and on the other hand, this cross-linking is not truly cross-linked, making it a virtual cross-linking. The cross-linking condition depends on temperature. As the temperature increases, this cross-linking gradually weakens and disappears. The polymer has a certain fluidity and can be subjected to thermoplastic processing. When the temperature decreases, this cross-linking gradually recovers and forms again. The addition of a small amount of filler increases the distance between molecules, weakens the ability to form hydrogen bonds between molecules, and leads to a sharp decrease in strength. Research has shown that the order of stability of various functional groups in polyurethane rubber from high to low is: ester, ether, urea, carbamate, and biuret. During the aging process of polyurethane rubber, the first step is the breaking of the cross-linking bonds between biuret and urea, followed by the breaking of the carbamate and urea bonds, that is, the main chain breaking.
01 Softening
Polyurethane elastomers, like many polymer materials, soften at high temperatures and transition from an elastic state to a viscous flow state, resulting in a rapid decrease in mechanical strength. From a chemical perspective, the softening temperature of elasticity mainly depends on factors such as its chemical composition, relative molecular weight, and crosslinking density.
Generally speaking, increasing the relative molecular weight, increasing the rigidity of the hard segment (such as introducing a benzene ring into the molecule) and the content of the hard segment, and increasing the crosslinking density are all beneficial for increasing the softening temperature. For thermoplastic elastomers, the molecular structure is mainly linear, and the softening temperature of the elastomer also increases when the relative molecular weight is increased.
For cross-linked polyurethane elastomers, crosslinking density has a greater impact than relative molecular weight. Therefore, when manufacturing elastomers, increasing the functionality of isocyanates or polyols can form a thermally stable network chemical cross-linking structure in some of the elastic molecules, or using excessive isocyanate ratios to form a stable isocyanate cross-linking structure in the elastic body is a powerful means to improve the heat resistance, solvent resistance, and mechanical strength of the elastomer.
When PPDI (p-phenyldiisocyanate) is used as the raw material, due to the direct connection of two isocyanate groups to the benzene ring, the formed hard segment has a higher benzene ring content, which improves the rigidity of the hard segment and thus enhances the heat resistance of the elastomer.
From a physical perspective, the softening temperature of elastomers depends on the degree of microphase separation. According to reports, the softening temperature of elastomers that do not undergo microphase separation is very low, with a processing temperature of only about 70 ℃, while elastomers that undergo microphase separation can reach 130-150 ℃. Therefore, increasing the degree of microphase separation in elastomers is one of the effective methods to improve their heat resistance.
The degree of microphase separation of elastomers can be improved by changing the relative molecular weight distribution of chain segments and the content of rigid chain segments, thereby enhancing their heat resistance. Most researchers believe that the reason for microphase separation in polyurethane is the thermodynamic incompatibility between the soft and hard segments. The type of chain extender, hard segment and its content, soft segment type, and hydrogen bonding all have a significant impact on it.
Compared with diol chain extenders, diamine chain extenders such as MOCA (3,3-dichloro-4,4-diaminodiphenylmethane) and DCB (3,3-dichloro-biphenylenediamine) form more polar amino ester groups in elastomers, and more hydrogen bonds can be formed between hard segments, increasing the interaction between hard segments and improving the degree of microphase separation in elastomers; Symmetric aromatic chain extenders such as p, p-dihydroquinone, and hydroquinone are beneficial for the normalization and tight packing of hard segments, thereby improving the microphase separation of products.
The amino ester segments formed by aliphatic isocyanates have good compatibility with the soft segments, resulting in more hard segments dissolving in the soft segments, reducing the degree of microphase separation. The amino ester segments formed by aromatic isocyanates have poor compatibility with the soft segments, while the degree of microphase separation is higher. Polyolefin polyurethane has an almost complete microphase separation structure due to the fact that the soft segment does not form hydrogen bonds and hydrogen bonds can only occur in the hard segment.
The effect of hydrogen bonding on the softening point of elastomers is also significant. Although polyethers and carbonyls in the soft segment can form a large number of hydrogen bonds with NH in the hard segment, it also increases the softening temperature of elastomers. It has been confirmed that hydrogen bonds still retain 40% at 200 ℃.
02 Thermal decomposition
Amino ester groups undergo the following decomposition at high temperatures:
- RNHCOOR – RNC0 HO-R
- RNHCOOR – RNH2 CO2 ene
- RNHCOOR – RNHR CO2 ene
There are three main forms of thermal decomposition of polyurethane based materials:
① Forming original isocyanates and polyols;
② α— The oxygen bond on the CH2 base breaks and combines with one hydrogen bond on the second CH2 to form amino acids and alkenes. Amino acids decompose into one primary amine and carbon dioxide:
③ Form 1 secondary amine and carbon dioxide.
Thermal decomposition of carbamate structure:
Aryl NHCO Aryl,~120 ℃;
N-alkyl-NHCO-aryl,~180 ℃;
Aryl NHCO n-alkyl,~200 ℃;
N-alkyl-NHCO-n-alkyl,~250 ℃.
The thermal stability of amino acid esters is related to the types of starting materials such as isocyanates and polyols. Aliphatic isocyanates are higher than aromatic isocyanates, while fatty alcohols are higher than aromatic alcohols. However, the literature reports that the thermal decomposition temperature of aliphatic amino acid esters is between 160-180 ℃, and that of aromatic amino acid esters is between 180-200 ℃, which is inconsistent with the above data. The reason may be related to the testing method.
In fact, aliphatic CHDI (1,4-cyclohexane diisocyanate) and HDI (hexamethylene diisocyanate) do have better heat resistance than commonly used aromatic MDI and TDI. Especially the trans CHDI with symmetrical structure has been recognized as the most heat-resistant isocyanate. Polyurethane elastomers prepared from it have good processability, excellent hydrolysis resistance, high softening temperature, low glass transition temperature, low thermal hysteresis, and high UV resistance.
In addition to the amino ester group, polyurethane elastomers also have other functional groups such as urea formate, biuret, urea, etc. These groups can undergo thermal decomposition at high temperatures:
NHCONCOO – (aliphatic urea formate), 85-105 ℃;
- NHCONCOO – (aromatic urea formate), at a temperature range of 1-120 ℃;
- NHCONCONH – (aliphatic biuret), at a temperature ranging from 10 ° C to 110 ° C;
NHCONCONH – (aromatic biuret), 115-125 ℃;
NHCONH – (aliphatic urea), 140-180 ℃;
- NHCONH – (aromatic urea), 160-200 ℃;
Isocyanurate ring>270 ℃.
The thermal decomposition temperature of biuret and urea based formate is much lower than that of aminoformate and urea, while isocyanurate has the best thermal stability. In the production of elastomers, excessive isocyanates can further react with the formed aminoformate and urea to form urea based formate and biuret cross-linked structures. Although they can improve the mechanical properties of elastomers, they are extremely unstable to heat.
To reduce the thermal unstable groups such as biuret and urea formate in elastomers, it is necessary to consider their raw material ratio and production process. Excessive isocyanate ratios should be used, and other methods should be used as much as possible to first form partial isocyanate rings in the raw materials (mainly isocyanates, polyols, and chain extenders), and then introduce them into the elastomer according to normal processes. This has become the most commonly used method for producing heat-resistant and flame resistant polyurethane elastomers.
03 Hydrolysis and thermal oxidation
Polyurethane elastomers are prone to thermal decomposition in their hard segments and corresponding chemical changes in their soft segments at high temperatures. Polyester elastomers have poor water resistance and a more severe tendency to hydrolyze at high temperatures. The service life of polyester/TDI/diamine can reach 4-5 months at 50 ℃, only two weeks at 70 ℃, and only a few days above 100 ℃. Ester bonds can decompose into corresponding acids and alcohols when exposed to hot water and steam, and urea and amino ester groups in elastomers can also undergo hydrolysis reactions:
RCOOR H20- → RCOOH HOR
Ester alcohol
One RNHCONHR one H20- → RXHCOOH H2NR -
Ureamide
One RNHCOOR-H20- → RNCOOH HOR -
Amino formate ester Amino formate alcohol
Polyether based elastomers have poor thermal oxidation stability, and ether based elastomers α- The hydrogen on the carbon atom is easily oxidized, forming a hydrogen peroxide. After further decomposition and cleavage, it generates oxide radicals and hydroxyl radicals, which eventually decompose into formates or aldehydes.
Different polyesters have little effect on the heat resistance of elastomers, while different polyethers have a certain influence. Compared with TDI-MOCA-PTMEG, TDI-MOCA-PTMEG has a tensile strength retention rate of 44% and 60% respectively when aged at 121 ℃ for 7 days, with the latter being significantly better than the former. The reason may be that PPG molecules have branched chains, which are not conducive to the regular arrangement of elastic molecules and reduce the heat resistance of the elastic body. The thermal stability order of polyethers is: PTMEG>PEG>PPG.
Other functional groups in polyurethane elastomers, such as urea and carbamate, also undergo oxidation and hydrolysis reactions. However, the ether group is the most easily oxidized, while the ester group is the most easily hydrolyzed. The order of their antioxidant and hydrolysis resistance is:
Antioxidant activity: esters>urea>carbamate>ether;
Hydrolysis resistance: ester<biuret urea formate ester<amino formate ester urea group<ether.
To improve the oxidation resistance of polyether polyurethane and the hydrolysis resistance of polyester polyurethane, additives are also added, such as adding 1% phenolic antioxidant Irganox1010 to PTMEG polyether elastomer. The tensile strength of this elastomer can be increased by 3-5 times compared to without antioxidants (test results after aging at 1500C for 168 hours). But not every antioxidant has an effect on polyurethane elastomers, only phenolic 1rganox 1010 and TopanOl051 (phenolic antioxidant, hindered amine light stabilizer, benzotriazole complex) have significant effects, and the former is the best, possibly because phenolic antioxidants have good compatibility with elastomers. However, due to the important role of phenolic hydroxyl groups in the stabilization mechanism of phenolic antioxidants, in order to avoid the reaction and “failure” of this phenolic hydroxyl group with isocyanate groups in the system, the ratio of isocyanates to polyols should not be too large, and antioxidants must be added to prepolymers and chain extenders. If added during the production of prepolymers, it will greatly affect the stabilization effect.
The additives used to prevent hydrolysis of polyester polyurethane elastomers are mainly carbodiimide compounds, which react with carboxylic acids generated by ester hydrolysis in polyurethane elastomer molecules to generate acyl urea derivatives, preventing further hydrolysis. The addition of carbodiimide at a mass fraction of 2% to 5% can increase the water stability of polyurethane by 2-4 times. In addition, tert butyl catechol, hexamethylenetetramine, azodicarbonamide, etc. also have certain anti hydrolysis effects.
04 Main performance characteristics
Polyurethane elastomers are typical multi block copolymers, with molecular chains composed of flexible segments with a glass transition temperature lower than room temperature and rigid segments with a glass transition temperature higher than room temperature. Among them, oligomeric polyols form flexible segments, while diisocyanates and small molecule chain extenders form rigid segments. The embedded structure of flexible and rigid chain segments determines their unique performance:
(1) The hardness range of ordinary rubber is generally between Shaoer A20-A90, while the hardness range of plastic is about Shaoer A95 Shaoer D100. Polyurethane elastomers can reach as low as Shaoer A10 and as high as Shaoer D85, without the need for filler assistance;
(2) High strength and elasticity can still be maintained within a wide range of hardness;
(3) Excellent wear resistance, 2-10 times that of natural rubber;
(4) Excellent resistance to water, oil, and chemicals;
(5) High impact resistance, fatigue resistance, and vibration resistance, suitable for high-frequency bending applications;
(6) Good low-temperature resistance, with low-temperature brittleness below -30 ℃ or -70 ℃;
(7) It has excellent insulation performance, and due to its low thermal conductivity, it has a better insulation effect compared to rubber and plastic;
(8) Good biocompatibility and anticoagulant properties;
(9) Excellent electrical insulation, mold resistance, and UV stability.
Polyurethane elastomers can be formed using the same processes as ordinary rubber, such as plasticization, mixing, and vulcanization. They can also be molded in the form of liquid rubber by pouring, centrifugal molding, or spraying. They can also be made into granular materials and formed using injection, extrusion, rolling, blow molding, and other processes. In this way, not only does it improve work efficiency, but it also improves the dimensional accuracy and appearance of the product
Post time: Dec-05-2023