Casting polyurethane elastomer (CPU elastomer), commonly known as liquid rubber, is a product produced by pouring a liquid reaction mixture into a mold cavity. The synthesis method mainly uses the prepolymer method (that is, the two-step method), because the comprehensive properties of the products produced by the prepolymer method are better than the one-step method [1-3]. However, adverse phenomena and product defects often occur in the synthesis of prepolymers, such as air bubbles due to incomplete degassing of the material, the curing temperature is too high, the surface becomes yellow, or the depression occurs when too little material is poured into the cavity. The prepolymer itself has a large number of bubbles, high viscosity and poor fluidity, which will also cause defects in the final product. At the same time, during the storage of the prepolymer, there will also be problems of changes in moisture and NCO values. Often due to the above reasons, the performance of its products is reduced, and the rate of defective products and scrap products is high. In order to make the product reach a higher qualification rate, the secondary and waste products are analyzed, and the reason is found from its synthesis and curing molding process, and a solution is proposed. In this paper, the prepolymer method is used to synthesize cast polyurethane elastomer products. Based on the analysis of the undesirable phenomena and the defects in the products during the synthesis process, corresponding solutions are proposed.
Prepolymer Bubble Problem
05%, During the preparation of the prepolymer, the reasons for the generation of bubbles are as follows: first, the increase in the viscosity of the material leads to increased bubbles; second, the moisture content in the raw materials (oligomeric polyols and polyisocyanates) exceeds 0.05%, As a result, it reacts with the polyisocyanate to produce CO2 bubbles; the third is the presence of moisture in the reaction device; the fourth is the excessive negative pressure in the reactor, which causes the boiling point of the material to decrease and the material to vaporize, and a large number of bubbles are generated; the fifth is to add Too much filler, and it is easy to produce more bubbles when stirring. The above five kinds of bubble generation sources can be solved one by one. For example, to extend the dehydration time of raw materials and control the moisture content below 0.05%; thoroughly dry the reaction container; increase the negative pressure value as much as possible to ensure that the material does not vaporize, and find a suitable vacuum to stabilize the production process. Reduce or avoid vaporization of materials. When synthesizing the prepolymer, the generation of air bubbles is extremely unfavorable. A large number of air bubbles in the liquid will cause the temperature of the reaction system to fluctuate greatly, making the reaction difficult to control. The generation of air bubbles should be minimized to provide convenience for the prepolymer degassing to be completed smoothly .
Prepolymer viscosity issues
Viscosity is an important indicator that reflects the degree of polymerization of materials in a macroscopic view. It is also a concrete manifestation of the degree of reaction in the polymerization process. During the prepolymer synthesis process, when the oligomer polyol is polymerized with the polyisocyanate, as the reaction time continues, the degree of reaction gradually deepens, and the viscosity of the reaction material will follow. Increase, which brings difficulties to production operations. The main reasons for the increase in the viscosity of the material are: First, the choice of raw materials or process must be clear, because the fluidity of different raw materials is different, and the fluidity of polyester is much worse than that of polyether, so synthetic polyester prepolymers It also has poor maneuverability; if a polyester prepolymer must be selected for the reaction, an unsaturated polyester can be used, and the effect will be better; of course, the semi-prepolymer process can also be used to change the polyester type Disadvantages of prepolymers. The second is the determination of the raw material ratio. In the raw material ratio, the increase in the content of the hard segment will speed up the reaction and the viscosity of the material. Therefore, the ratio of the mass of the hard segment to the soft segment is adjusted by experiments. The third is the choice of reaction temperature and time. The higher the reaction temperature or the longer the reaction time, the stronger the degree of polymerization of the materials and the larger the viscosity of the materials. The balance point of the reaction temperature and time must be adjusted through experiments.
A method of adjusting the viscosity was obtained during multiple tests. Specifically, when the material reacted to a certain degree, the reaction material was sampled to determine its NCO value, and compared with the pre-designed NCO value to see if it was close. If the NCO value is close to the design value, the temperature should be reduced to stop heating, and the material should be discharged and stored. The best experimental method is to make a graph of the relationship between the NCO value and the viscosity of the reaction material to facilitate the experiment. This method is suitable for small-scale or pilot-scale synthetic tests.
In addition, the relationship between the viscosity and the NCO value was determined from the pilot and pilot tests. According to their relationship ratios, and under several specific ratios, the rotational speed of the agitator was measured to make a graph of the relationship between the viscosity / NCO ratio and the rotational speed of the agitator. Determine whether to proceed to the next step. This method is suitable for mass production.
Prepolymer mobility issues
The fluidity is an important index reflecting the processability of the material, and the fluidity of the material has a great relationship with its viscosity. In most cases, the fluidity of the material decreases as its viscosity increases. Good or bad liquidity plays an important role in whether the material can be discharged smoothly. Therefore, it is particularly important to solve the problem of material fluidity. The following are three methods to improve the material fluidity: one is to reduce the viscosity of the material as much as possible; the other is to add a toughening agent, but not too much, generally less than 10% because of toughening Adding more additives will increase the flexibility of the product and reduce the strength; the third is to adjust its fluidity through temperature, but the temperature should not be too high. The excessive temperature will cause the material to accelerate polymerization and even gel, which will greatly increase the fluidity of the material Decrease, and degradation reaction may occur, generally do not exceed 100 ℃. The fluidity of the prepolymer determines the success of the next chain extension reaction, so solving the fluidity of the prepolymer is the focus of the whole experiment.
Yellowing of prepolymers
Studies by Liu Liangbing and Lee DK [4-6] et al. Show that polyurethane yellowing mainly has several aspects: one is the effect of polyisocyanate types, and aromatic polyisocyanates will generate quinone structure under thermal oxidation and ultraviolet radiation, Eventually leads to yellowing of the prepolymer. Aliphatic polyisocyanates can be used to solve them, such as: IPDI and HMDI, but because the activity of aliphatic polyisocyanates is lower than that of aromatic polyisocyanates, the reaction time will be longer when using aliphatic polyisocyanates; the second is oligomerization. Of the molecular weight of the polymer polyol, the smaller the molecular weight of the oligomeric polyol, the more easily the prepolymer turns yellow, because when the molecular weight of the oligomeric polyol is smaller, the NCO on the benzene ring of the prepolymer is susceptible to thermal oxidation. Yellow, the solution is to adjust the molecular weight of the oligomeric polyol, and it should be more appropriate to choose a molecular weight of 1 000 to 3 000; the third is the storage environment of the prepolymer, which may be due to the aromatic polyisocyanate easily oxidized, leading to prepolymer The polymer turns yellow. The solution is to store it in airtight and sealed. It is best to inject nitrogen to protect it. You can also add antioxidants and ultraviolet absorbers, such as antioxidant 1010 and ultraviolet absorber UV-360.
Problems with storage of prepolymers
Prepolymers often absorb moisture during storage, and the presence of moisture can lead to unfavorable results: it reacts with NCO in the prepolymer to form urea groups and increases the viscosity of the prepolymer, which in turn is based on urea groups. The chemical point can further react with the NCO group to form biuret branches or crosslinks, which can reduce the storage stability of the prepolymer or even gel. There are two possible sources of moisture: one is that the humidity in the storage environment is too high; the other is that the storage container is not well sealed and the moisture content itself. The solution should be to reduce the humidity of the place, and to seal the prepolymer from air, and it is best to fill it with nitrogen to protect it.
Changes in the NCO value of prepolymer storage
During the storage of the prepolymer at room temperature, the following situations often occur: one is that the polyisocyanate in the prepolymer continues to react with the oligomeric polyol to reduce the NCO value; the other is that the trace moisture in the mixture reacts with the polyisocyanate To reduce the NCO value. There are several hazards to the reduction of NCO value: first, it will increase the viscosity of the prepolymer, poor fluidity, and difficult to defoam; second, it is lower than the NCO value set before the experiment, resulting in the addition of polyisocyanate and increased operation; three It is the reaction of trace moisture and polyisocyanate to form byproducts such as biuret and urea group, which will also affect the product performance. In order to make the NCO value reach the required set value, the following solutions can be taken: make the NCO value in the stored prepolymer slightly higher than the set value, so that the storage time can be extended. If the NCO value is lower than the set NCO value, the required amount of polyisocyanate can be added by calculating the prepolymer formula. It is necessary to pay attention to the change of NCO value in the prepolymer at any time to prevent it from increasing in viscosity and poor fluidity. It is best to use up the prepolymer in a few days.
There are many factors that affect the molding of castable polyurethane elastomers. This article mainly analyzes the two major aspects of product defects caused by the prepolymer synthesis process and storage, and proposes corresponding solutions, but in production practice Different situations often occur, and different solutions should be found for different situations.