Introduction and selection of polyether polyol
Polyether polyols are oligomers whose main chain contains ether bonds (—R—O—R—), and whose end groups or side groups contain more than 2 hydroxyl groups (—OH). It is formed by ring-opening polymerization with low-molecular-weight polyols, polyamines or compounds containing active hydrogen as initiators, and oxidized olefins under the action of catalysts. The alkylene oxides are mainly propylene oxide PO and ethylene oxide EO, among which propylene oxide is the most common. Polyol initiators include dihydric alcohols such as propylene glycol and ethylene glycol, trihydric alcohols such as glycerin and trimethylolpropane, and polyalcohols such as pentaerythritol, xylitol, sorbitol, and sucrose; amine initiators are diethyl alcohols. Amines, diethylenetriamine, etc. According to the number of active atoms contained in the initiator, polyether polyols with different functionalities can be prepared. The most commonly used polyethers in the preparation of polyurethane adhesives are polyoxypropylene diol and polyoxypropylene triol. tetrahydrofuran diol. The polyurethane resin adhesive formulated with polypolyol resin has good water resistance, impact resistance and low temperature resistance.
There are many imported and domestic polyether manufacturers in the market, with great differences in molecular weight, performance and use. They are widely used in polyurethane adhesives, elastomers and soft and hard foam products. Finding suitable materials among ether polyols to complete the formula design is a problem facing many polyurethane practitioners. Generally, according to the application, it is divided into soft foam polyether, hard foam polyether and elastomer polyether. After years of uninterrupted experiments and applications, the author found that this classification helps everyone to get familiar with and select polyethers as soon as possible on the macro level, business personnel can lock target customers faster, and users can choose the raw materials they need faster, but experience tells Me, this is not conducive to everyone's understanding of formula design and performance adjustment from a microscopic point of view, let alone thinking about the chain segment composition of polymers from the perspective of molecular structure.
The physical and chemical properties of polyether polyols are mainly determined by the type of initiator and oxyalkylene and molecular weight:
1. The amount of active hydrogen of the initiator determines the functionality of the polyether. The reactivity of the polyether synthesized by different initiators varies greatly. Generally speaking, when the molecular weight is not much different, the polyether of the same PO type Ether, ethylenediamine, and aniline are the most active polyethers, and pentaerythritol and sucrose polyols are also highly active, while ethylene glycol and propylene glycol are active polyethers. But relatively low.
2. For the same type of polyether, the higher the hydroxyl value, the smaller the molecular weight, the higher the reactivity, and the lower the viscosity.
3. Most of the polyethers of oxyalkylenes are mostly synthesized by PO or all of them are synthesized by PO, but there are also some varieties that first use PO to react with the initiator, and then use EO to block the end, and some varieties even use EO to polymerize. EO-terminated polyethers are usually more active than all-PO polyethers, and have better hydrophilicity. The higher the EO content, the more obvious this trend is.
According to the reactivity from low to high, we can simply distinguish polyether:
Low activity polyether, polyoxypropylene diol, triol, PPG
The functionality is 2 or 3, the initiator is 1,2-propanediol, ethylene glycol, trimethylolpropane or glycerin, PO ring-opening for polymerization, soft foam polyether and polyether for adhesive elastomers on the market Most of them are of this type, and they are also our most common general-purpose polyethers. They are generally numbered: 210, 220, and 330. The first number represents functionality, and the last two numbers represent molecular weight. Polyether 210 is polyoxypropylene diol with a functionality of 2 and a molecular weight of 1000.
Medium reactive polyether
The structure is similar to ordinary polyether, but part of the PO chain segment is changed to EO end capping. The difference in molecular structure leads to an increase in activity. The higher the EO content, the higher the activity and the stronger the hydrophilicity. Common high resilience polyether 330N and 550N are such polyethers.
Highly active polyether
1. The polyether synthesized with ethylenediamine, aniline, etc. as the initiator, the most common ones are polyether 403 and 4110, with four functionalities, often used in rigid foam. Due to the existence of amino groups, this kind of polyether has a certain self-catalysis effect, and is rarely used alone. It is generally used in combination with the above two polyethers.
2. The multifunctional polyether with sorbitol and sucrose as the starting agent has high reactivity and is basically only used in rigid foam.
Distinguishing polyethers from the perspective of reactivity can better allow us to understand the formula from a molecular perspective, which helps us improve experimental efficiency and avoid detours. Most of the time, we seldom use only one polyether in the design formula, and mostly use several polyethers in combination to achieve a balance of performance. The closer the reactivity gradient is to the polyether, the more consistent the curing effect can be achieved in the later stage of curing. , the performance can be reflected more. However, if the curing gradient has a large difference, the problem of multiple catalysis must be considered. The greater the difference, the easier it is to cause incomplete curing and result in failure of formulation design. Example: In many cases, several polyethers are mixed and solidified into cloudy or tofu lumps. The reason is that the polyether with high reactivity solidifies first, and the formed three-dimensional structure hinders the polyether with low reactivity from continuing to solidify, resulting in poor reaction. Thorough, inconsistent cure.
The above is the polyether in the traditional sense. In practice, we still have some varieties that are also used as polyether. Let’s make a brief introduction below.
Polymer polyol, POP
Vinyl polymer grafted polyether polyol is commonly known as "Polymer Polyol" (Polyether Polyol), or POP for short. Polymer polyol is polyether based on general-purpose polyether polyol (generally general-purpose soft foam polyether triol, high-activity polyether), plus acrylonitrile, styrene, methyl methacrylate, vinyl acetate, chlorine Vinyl monomers such as ethylene and initiators are formed by free radical grafting polymerization at about 100 degrees under the protection of nitrogen. POP is an organically filled polyether polyol, which is used to prepare high-load or high-modulus soft and semi-rigid polyurethane foam products. Partially or completely adopting this kind of organic filled polyether to replace the general-purpose polyether polyol can produce foam plastics with low density and high load-bearing performance, which not only meets the hardness requirement, but also saves raw materials. The appearance is generally white or light milky yellow, also known as white polyether.
Polytetrahydrofuran ether, PTMG, PTMEG
PTMG is a linear polyether diol with terminal primary hydroxyl groups, which is formed by ring-opening polymerization of tetrahydrofuran. This type of polyurethane material has good low temperature performance, wear resistance, aging resistance, hydrolysis resistance and mildew resistance, and is also very flexible and elastic. Well, non-toxic, it is the "noble" among polyethers, and it is often used in the production of high-performance polyurethane elastomers and polyurethane elastic fibers (spandex).
There are three specifications of PTMG in the market based on molecular weight: PTMG650, PTMG1000 and PTMG2000, among which PTMG650 is liquid at room temperature, while PTMG1000 and PTMG2000 are white waxy solids, which melt into colorless transparent liquids when heated to about 50°C.
Castor oil is a good vegetable oil containing hydroxyl groups. It has good compatibility and similar properties with polyethers, and is often used as a polyether. According to analysis, castor oil contains about 85%-90% ricinoleic acid (9-enyl-1,2-hydroxy octadecanoic acid) and 10%-15% oleic acid and linoleic acid without hydroxyl.
The hydroxyl value of castor oil is between 150 and 165mgKOH/g. Based on this calculation, it can be considered that it contains about 70% trifunctional components and 30% difunctional components, and a small amount of multifunctional components. The average functional The degree is 2.7, the average molecular weight is about 1000, and the reactivity is high.
Since castor oil contains 10%-15% oleic acid and linoleic acid without hydroxyl, the presence of these substances acts like a plasticizer, and castor oil contains certain ester bonds, so its low temperature performance and Mechanical properties are superior to polyoxyalkylene ethers.