Research Progress of Polyurethane Prepolymers
Abstract: Polyurethane prepolymerization
The research progress of polyurethane prepolymerization in recent years is summarized, the principle of polyurethane prepolymerization is introduced, the effects of isocyanate, polyol, chain extender, catalyst, temperature, moisture and other factors on polyurethane prepolymer are analyzed, and finally, it is pointed out that the improvement of the product performance and environmental protection is the key point of the research in the future.
Keywords.
Polyurethane; Prepolymer; Isocyanate; Polyol; Chain extender; Catalyst; Moisture
Preface.
Polyurethane sealant is prepared from isocyanate and hydroxyl-containing polyether/polyester polyol as the main raw materials, which contains -NCO and -NHCOO groups in the molecule, and is characterized by strong polarity, high activity, high temperature resistance, solvent resistance, and can be cured quickly. Polyurethane is usually synthesized by a two-step process, first by the polyol and isocyanate under certain conditions to produce polyurethane prepolymer, and then add fillers, adhesion promoters, curing agents and other raw materials to produce, due to the preparation of a wide range of raw materials for the preparation of pre-polymers, different formulas and conditions can be made into pre-polymers with different properties can be widely used in a variety of fields, such as polyurethane foams, coatings, adhesives, elastomers and fibers, and so on.
In order to further improve the quality and efficiency of polyurethane products, it is necessary to study in depth the effects of different factors on the preparation process and properties of prepolymers. The article analyzes the main raw materials, chain extenders, catalysts, temperature, moisture and other factors required for the reaction, and summarizes the research progress of polyurethane prepolymer preparation.
1 Preparation of polyurethane prepolymer
The main raw materials for the production of polyurethane prepolymers are isocyanate and polyether polyol/polyester polyol. It can be regarded as a block copolymer composed of hard and soft segments, the hard segment is composed of polyisocyanate or its and small molecule chain extender, and the soft segment is generally composed of polyester polyol or polyether polyol. After the polyol is dehydrated by high temperature and vacuum, isocyanate raw materials are added, and the polyurethane prepolymer can be obtained by reacting under certain conditions. During the reaction period, catalyst can be added to improve the reaction rate, chain extender can be added to improve the degree of cross-linking, and polymerization blocking agents such as p-benzenemethanesulfonic acid, benzoyl chloride, and phenylacetyl chloride can be added to seal the end and prolong the storage time of the prepolymer.
Polytetrahydrofuran diol, polyoxypropylene diol high temperature dehydration, add toluene diisocyanate, polyurethane prepolymer under certain conditions, add leveling agent, latent curing agent and defoamer, vacuum defoaming to get fast curing, non-foaming one-component polyurethane resin. Shi Yilun et al. used 4,4′-methylene di-p-phenylene diisocyanate and flame-retardant polyether polyol (FR-212) to prepare flame-retardant polyurethane prepolymer, which can effectively improve the flame-retardant and mechanical properties of phenolic foam.
2 Influence of main raw materials on the preparation of polyurethane prepolymerization system
Isocyanate and polyol are the main raw materials for the preparation of polyurethane prepolymer, and their types have a significant effect on the performance of the prepolymer. Isocyanates commonly used in the field of polyurethane include HDI, IPDI, TDI, MDI, etc. TDI (toluene diisocyanate) is a liquid at room temperature, which is more convenient to use and less costly, with the disadvantages of high vapor pressure and high toxicity. With MDI (diphenylmethane diisocyanate) molecule in the two benzene ring of the electron absorption ability is strong, NCO group activity is large, the polyurethane products produced by the mechanical properties of excellent. two NCO group in the MDI molecule is symmetric, the reaction activity is the same, so the MDI and water reaction to produce CO2 rate is relatively gentle, to prevent the formation of bubbles due to the local reaction of the violent. ipdi (isophorone diisocyanate) is an isophorone diisocyanate. IPDI (isophorone diisocyanate) is a non-yellow aliphatic isocyanate, reactivity is lower than the aromatic isocyanate, vapor pressure is also low, made of polyurethane sealant has excellent resistance to chemicals and optical stability, but there are shortcomings of the slow curing rate. Generally speaking, the reactivity of aromatic isocyanates is higher than that of aliphatic isocyanates, but the storability is not good.
Polyol has a greater impact on the performance of polyurethane, polyester-type polyol main chain contains ester bond (-COO-) or carbonate group (-OCOO-), the bond polarity, cohesion energy, adhesion, with higher strength and hardness, and its antioxidant property is also better than that of polyether-type polyurethanes, the disadvantage is that ester bond is easy to be hydrolyzed, and it is easy to reduce the hydrolysis resistance of polyurethane materials.
Congcong Zhang et al. used MDI as the hard segment, FHTPB (terminal hydroxyl polybutadiene), HTPB (high cis-terminal hydroxyl polybutadiene), PTMG (polytetrahydrofuran glycol), PCL (polycaprolactone glycol) as the soft segment to produce four kinds of polyurethane elastomers (PUEs), and examined the influence of the molecular structure of the soft segment on the performance of the polyurethane elastomers, and the results showed that PCL-PUE has the best rigidity and the highest modulus of elasticity at low temperature and room temperature; HTPB-PUE has the best low-temperature flexibility and the best elongation at break at low temperature and room temperature.
Zhou Yifan et al. prepared castor oil-based polyurethane materials using HMDI, IPDI, and castor oil-based polyol as the main raw materials, and found that the heat-resistant performance and tensile strength of HMDI-type polyurethane were better than that of IPDI-type polyurethane, but the elongation at break of IPDI-type polyurethane materials was higher.
Liu Bo et al. used HMDI, TDI and IPDI as the raw materials with polyether polyol and 1,4 butanediol (BDO) respectively to produce three kinds of polyurethane elastomers, and investigated the effect of isocyanate type on the performance of the products. The results showed that HMDI type polyurethane elastomers had the best tensile strength, water resistance and heat resistance, while IPDI type polyurethane elastomers had the highest elongation at break.
Jin Yan et al. prepared polyurethane microporous elastomers using hydrogenated terminal hydroxybutadiene polyol (HLBH), polytetrahydrofuran ether polyol (PTMEG), p-phenylene diisocyanate (PPDI), BDO, and water as raw materials. The ammonia ester microporous elastomers prepared with HLBH showed better microphase separation and better low temperature modulus stability and lower fatigue thermogenesis.
Raw materials affect the prepolymerization reaction conditions, viscosity and storage period, and the groups contained in the raw materials also affect the final properties of the polyurethane products. Polyols of the same type with different molecular masses will produce polyurethane prepolymers with different properties.
3 The effect of chain extender on the preparation of polyurethane prepolymerization system
When polyfunctional, low molecular weight compounds containing amine or hydroxyl groups react with isocyanate, they act as cross-linking agent and chain extender, which will affect the relationship between soft and hard segments, thus affecting the performance of polyurethane. Chain extenders include amines and alcohols, amines include MOCA (3,3″-dichloro-4,4″-diamino-diphenylmethane) and modified MOCA, and alcohols include trimethylolpropane, glycerol, diethylene glycol, 1,4 butanediol, triethylene glycol, and so on.
Zheng Mengkai et al. synthesized a series of hard segments of different types and contents by prepolymerization method using PTMG-2000 (polytetrahydrofuran diol) as soft segments, MDI as hard segments, BDO (1,4-butanediol), BPDA (3,3″,4,4″-biphenyltetracarboxylic acid dianhydride), BTDA (3,3″,4,4″-benzylidene tetracarboxylic anhydride), and PMDA (1,2,4,5-phenylene tetracarboxylic acid dianhydride) as chain extender. A series of TPUs with different hard segment types and contents were synthesized by prepolymerization, and their mechanical and thermal properties were investigated. The results showed that the rigidity, thermal stability, flexibility and elasticity at room temperature and higher temperatures of arylimide-containing TPUs were significantly better than those of TPUs with BDO as chain extender.
Jianru Deng et al. synthesized UV-curable polyurethanes using poly(monoethylene glycol adipate)-2000 and IPDI as the main raw materials, BDO, ethylene glycol (EG), 1,2-propanediol (1,2-PG), 1,4-cyclohexanedimethanol (CHDM), and neopentyl glycol (NPG) as the chain extenders, and hydroxy ethyl acrylate (HEA) as the capping agent, respectively. The results showed that the CHDM chain-expanded polyurethanes had the best heat resistance and greater hardness; the higher reactivity of EG and BDO was attributed to the fact that they were straight-chain molecules, both with primary hydroxyl groups, high activity, and low site resistance when reacting with -NCO.
4 The effect of catalyst on the preparation of polyurethane prepolymerization system
Catalyst can reduce the reaction activation energy, accelerate the reaction rate to control side reactions, so in the preparation of polyurethane sealant prepolymerization system usually use catalysts, the requirements of the catalyst is: strong selectivity, high activity.
Alkaline catalysts are generally organic amines with large catalytic activity, and the ones with large alkalinity and small site resistance are more active. Tertiary amines have high efficiency in catalyzing the reaction between water and isocyanate, and are usually used in the preparation of polyurethane foams and are not suitable for the preparation of polyurethane prepolymers. Organometallic catalysts have high catalytic activity for -NCO and -OH, and the preparation of polyurethane prepolymers mostly adopts tin catalysts, such as dibutyltin dilaurate and stannous octanoate, etc.; and organolead, organomercurial, and organozinc compounds can catalyze the generation of urethane esters to produce gels, and such metal catalysts are mostly used in room-temperature curing polyurethane sealants.
Gong Tao synthesized eight organometallic catalysts, including stannous octanoate, dibutyltin dilaurate, zirconium acetylacetonate, nickel acetylacetonate, MB20 (19% Bi ions), BiCAT Zn (19% Zn ions), neodymium neodecanoate, and cerium neodecanoate, using polybutyleneglycol adipate (PBGA), IPDI, BDO, and isophorone diamine (IPDA) as the hard section, respectively. , synthesized aliphatic polyurethane resins in weak solvent system, and found that the used tin and neodymium, cerium, bismuth and zinc curing agents can effectively catalyze the controlled polymerization of aliphatic isocyanates with hydroxyl and amine groups, Ni has a strong gel-promoting effect, which makes it difficult to control the synthesis rate, and zirconium has strong degradation-promoting effect, which makes the resin viscosity decrease rapidly.
Liu Bingling prepared polyether-type polyurethane (PEPU) elastomers by ester-exchange polycondensation using methyl 1,6-hexamethylene dicarbamate (HDC) and poly(tetrahydrofuran ether glycol) (PTMG) as raw materials, BDO as chain extender, and tetraethyl titanate and dibutyltin oxide as catalysts, respectively, and both catalysts were able to efficiently catalyze the synthesis of high molecular weight PEPUs by ester-exchange polycondensation under the same process conditions. Under the same process conditions, both types of catalysts can effectively catalyze the ester exchange polycondensation to synthesize high molecular weight PEPU, the thermal properties of PEPU made from tetraethyl titanate are better, and the mechanical and optical properties of PEPU made from dibutyltin oxide are better.
Gogoi investigated the effects of two catalysts, DBTDL and DMDDE, on the synthesis of polyurethane prepolymers. It was found that the NCO content of the prepolymer decreased with the increase of catalyst dosage, indicating that both catalysts could promote the cross-linking reaction of the prepolymer. The catalyst affects the viscosity of the prepolymer, but the type and dosage do not have much effect on the viscosity of the prepolymer; the molecular mass of the prepolymer shows a tendency of decreasing and then increasing after the dosage of the two catalysts is increased respectively, and the distribution of the molecular mass becomes broader.
5 The influence of other factors on the preparation of polyurethane prepolymerization system
In addition to the above factors, there are some other factors that have a greater impact on the preparation and performance of polyurethane prepolymer, such as: temperature, moisture content, NCO content, cross-linking degree and so on.
Within a certain temperature range, with the increase of reaction temperature, the nucleophilic addition reaction rate of NCO group in isocyanate and polyol increases, which can shorten the reaction cycle; when the temperature is too high, the NCO group will self-polymerize, or react with urethane to generate urethane ester, or even gel, so the temperature of prepolymer production should be strictly controlled. Ai Qingsong et al. prepared polyurethane elastomers using PTMG and MDI, investigated the effects of different temperatures (75°C, 80°C and 85°C) on the properties, and performed structural analyses by high performance liquid chromatography (HPLC), infrared spectroscopy (IR) and nuclear magnetic resonance hydrogen spectroscopy (1H-NMR). The results showed that the internal structures of the products synthesized at 75 °C and 80 °C were the same, but the latter had relatively higher molecular weights and optimal properties; the products synthesized at 85 °C showed branched structures internally, and the HPLC results indicated that the branching reaction occurred at the prepolymerization stage.
In the preparation of prepolymer, the polyol is usually dehydrated by vacuum heating, and the charging or reaction period should be carried out under the protection of nitrogen to avoid the influence of moisture as much as possible, which will cause the carbamate group to react with urea group to generate diuretion, and the urea group to react with NCO group to generate urethane carboxylic acid ester, so as to make the viscosity of the prepolymer increase, and even the gel occurs. Liu Keshuo et al. prepared polyurethane prepolymers using hydroxy-terminated polybutadiene (HTPB) and MDI as raw materials, and added different ratios of moisture to PPU during the reaction process, and examined the effect of moisture on the properties of prepolymers. The results showed that a small amount of moisture plays the role of chain expansion, and a larger amount of moisture causes molecular cross-linking, promotes gelation, raises the glass transition temperature of the product, and reduces the mechanical properties of the product. The increase of moisture can form more urea groups, the hydrogen bonding between soft and hard segments is weakened, the hydrogen bonding between hard segments is strengthened, and the hydrogen bonding of carbonyl group tends to be disordered.
6 Conclusion
At present, there is an increasing demand for polyurethane products in various industries, especially in automobile, rail transportation, aerospace, electronics and other high-precision industries, which require higher quality of the products, and polyurethane prepolymer plays a decisive role in the performance of polyurethane products, so it is necessary to carry out more in-depth research on the preparation of polyurethane prepolymers to improve the performance of the products continuously.
When exploring the preparation process of prepolymerization, it can be combined with the characteristics of the raw material and the target performance of the product. The future direction of polyurethane development should tend to be more excellent performance and better environmental protection, which requires researchers to fully control the factors affecting the preparation of prepolymerization system and process conditions, to improve product fatigue and aging resistance; the use of more environmentally friendly raw materials, such as low-toxicity, low-odor, low volatility of the isocyanate, polyols, catalysts; as far as possible to avoid the use of solvents to reduce the VOC content of the product.
