Cold-cured high-resilience polyurethane foam is an excellent seat cushion material, which has the advantages of good resilience, good flame retardancy and low cost. However, in the actual production process of high-resilience foam, a series of defects such as foam shrinkage, foam hollow collapse, residual odor, poor surface and pores, and poor moisture-heat aging performance are often encountered. In recent years, the author has carried out some explorations on practical problems in production.
1. Foam shrinkage
In actual production, the most common and difficult problem to solve is foam shrinkage. There are two main reasons for the shrinkage phenomenon, tooling molds and raw materials, and the two complement each other.
1.1 Tooling and mold aspects
In the case of poor mold sealing, it is easy to cause leakage, so that the foam body cannot reach the designed density, resulting in foam shrinkage. While shrinking, the foam product will produce a hard edge phenomenon near the corresponding parting line. It can be solved by improving the tightness of the mold mouth or properly increasing the mold clamping force.
1.2 Raw materials
If the bubble film wall is more elastic during the foaming process, and when a large amount of gas occurs and causes volume expansion, the cells also expand without breaking, and most of the obtained bubbles are closed cells, that is, the closed cell ratio is high, then when the foam is When the body cools, the gas pressure in the bubble drops, causing the foam to shrink and deform. The author believes that there are four main solutions for this closed-cell phenomenon.
(1) The pore size and open porosity of the foam can be controlled by adjusting the amount of catalyst. Usually, amine catalysts mainly catalyze the reaction of isocyanate and water (ie, foaming reaction), and triethylenediamine or organotin catalysts are mainly used to catalyze the reaction between isocyanate and polyol (ie, gel reaction). If the catalyst that promotes gelation is excessive, the foam will gel prematurely, and the cell wall membrane has good toughness and is not easy to rupture, forming closed cells. In order to control the pore size and open cell ratio of the foam, the amount of gel catalyst can be appropriately reduced to reduce the growth rate of molecular chains, so that the elasticity of the bubble film wall is reduced at the peak of gas generation, and the closed cell ratio is reduced.
2) The formation of closed cells is also related to the degree of polymerization and branching of polyether polyols. This is because in the NCO/OH reaction, the polyether with high functionality forms a network structure faster, that is, the formed cell membrane The wall elasticity is larger, increasing the closed cell rate. The average functionality of the polyether can be lowered to reduce the foam closed cell rate.
(3) The amount of foam stabilizer is too high, which will cause the cells to be too stable and not open, resulting in shrinkage. Therefore, the amount of foam stabilizer in production should be appropriate.
(4) When the isocyanate index is too high, it may cause the phenomenon of foam closed cells to be aggravated, resulting in shrinkage. The isocyanate index should be controlled during production.
2. Partially hollow and collapsed foam inside
There are two main reasons for the phenomenon of partial hollowing and collapse of the foam in the production process of high-resilience polyurethane foam.
2.1 Unbalanced reaction rates of gel and foam
In the foaming process, in the final stage of the generation of a large amount of gas, the viscosity of the bubble film wall is relatively large, but the elasticity is poor. In this way, when the gas in the bubble continues to increase, it cannot withstand the stretching of the film wall, resulting in the rupture of the bubble. To allow the gas to escape, the hole is opened. If the wall of the foam film ruptures when a large amount of gas occurs, the meridians and skeletons of the cells do not have enough strength to prevent this rupture, and the rupture will spread further, which will cause the entire foam to collapse; if the rupture spreads to a small part, it will do If it stops, it will also cause the foam to be partially hollowed out or cracked. In this case, if the gel catalyst in the raw material is increased or the amount of foaming catalyst is reduced to improve the balance between gelation and foaming reaction, the strength of the bubble film wall can be increased when a large amount of gas occurs, and the amount of gas generated can be appropriately reduced, thereby reducing the Or improve the phenomenon of hollow or collapsed foam. This phenomenon is exactly the opposite of the closed-cell shrinkage phenomenon. When the foaming catalyst is unchanged and the amount of gel catalyst is low, it is easy to cause excessive opening and collapse of the foam.
2.2 The amount of foam stabilizer is low
Silicone foam stabilizer is one of the indispensable raw materials in the polyurethane foaming process. It can reduce the surface tension of each raw material component in the foam system, stabilize the foaming process, and make the cells fine and uniform. When the system is in the low viscosity stage, it enables the stomata wall film to grow to a thickness suitable for opening, creating conditions for the final opening. If the amount of foam stabilizer is too low, the stability of the foam pores will be poor, and the pores will be opened prematurely, resulting in collapsed foam or partial hollowing.
Appropriate foam stabilizers can coordinate the time period of cell opening, which is a major process in the foaming process of high resilience foam, otherwise closed cell shrinkage will occur. However, the opening must appear when the foaming reaction and the gelling reaction are basically completed and reach equilibrium, that is, when the foam reaches the highest point and the foam strength can support its own weight, otherwise the foam will collapse or become hollow.
3. Foam has residual odor
Residual odor in foam may originate from three sources.
(1) When the isocyanate is excessive, there will be residual toluene diisocyanate in the formed foam, resulting in a pungent odor.
(2) If the polyether selected in the raw material formula contains a lot of volatile matter, there may be a "polyether smell" after foaming.
(3) The amine odor caused by the residual amine catalyst in the foam is relatively large. There are two ways to solve this odor. First, the foam can be stored at a high temperature for a period of time to volatilize the residual catalyst in the foam, but it is difficult to operate in practice. Second, adding an amine catalyst that can participate in the chemical reaction of the foam system can reduce the amine odor caused by conventional amine catalysts, but at the same time, the foam cost will be increased accordingly.
4. There are pores on the surface of foam products
There are air holes on the surface of foam products, or dark holes inside, these phenomena may have the following five reasons.
(1) The surface finish of the mold is not enough, which affects the fluidity of the material system, making the foam surface rough and porous. This mainly depends on improving the surface finish of the mold, careful operation, and the use of a better release agent.
(2) If the viscosity of the material system is too high and the fluidity is poor, it will cause residual bubbles on the surface of the foam product. This is mainly solved by reducing the viscosity of the combined polyether. The more suitable viscosity in practice is 1500-1800mPa·s.
(3) If the gel speed is too fast and the time is too short during the foaming process, the viscosity of the material system will increase rapidly, and the fluidity will become poor, which may cause pores on the surface. The gel time is generally controlled at 55-65s. But the gel time should not be too long. Otherwise, if the tightness of the mold does not meet the requirements, it will cause waste of raw materials.
(4) The initial foaming speed is too fast. Generally speaking, after the raw material is more evenly covered on the inner surface of the bottom of the mold, and then rises quickly, the foam will have a better surface quality; if the raw material does not flow naturally to the surface of the mold and then foam, Lifting expands the raw material to this point, where bubbles or dark holes are more likely to be generated. Therefore, the lifting time should be appropriately extended. Generally controlled in 10-15s. However, this time is greatly affected by the amount of catalyst and material temperature and mold temperature in actual production. Therefore, the material temperature and mold temperature should be strictly controlled during production. Generally, the material temperature should be controlled at 22-24 °C.
(5) The design of the mold exhaust hole is not suitable. Generally speaking, the vent holes of the mold should be as small and large as possible, and the positions should be distributed on the highest point of the foaming mold and the clamping line. The vent hole can guide the material system. A reasonable distribution of vent holes can minimize air bubbles or dark holes. At the same time, in actual production, the design of the pouring route should also match the distribution of the exhaust holes. In the production of large seat cushions, if raw materials are poured in two places at the same time, vent holes should be set above the confluence of the two raw materials as much as possible to avoid the generation of dark holes.
5. Poor damp heat aging performance
The damp heat aging performance of seat cushion foam is a more demanding test required by Volkswagen's VW50180 standard. Previously mainly used for BORA A4 seat foam testing, this test is now being rolled out on JETTA seat foam. This test is to store the foam at a relative humidity of 95%-100% and 90°C for 200 hours, then compress the foam by 50% in a 70°C oven, store it for 22 hours, and then take it out and measure it after placing it for 0.5 hours. greater than 15%.
The reason that affects the damp-heat aging performance is mainly related to the isocyanate index.
(1) In actual production, when the isocyanate index is low, the damp heat performance of the foam can be deteriorated.
Under normal circumstances, the general amount of isocyanate should be slightly higher than that in the theoretical total reaction, and the isocyanate index is 1.05, so the end group of the final product of the chain extension reaction should be NCO.
That is, nOCN-R-NCO+(n-1)HO-R'-OH→OCN-R-NHCOO-R'-OCONH-R-NCO
When the amount of isocyanate is lower than the theoretical amount, the end of the macromolecule that should be obtained by the chain extension reaction is a hydroxyl group. Hydroxyl groups have strong hydrophilicity, which leads to a decrease in foam resilience under damp-heat state, that is, a decrease in damp-heat aging performance. This is also the reason why the foam tends to become soft and deformed in the rainy summer, or in areas with high humidity and high temperature in the south.
(2) If the isocyanate index is higher than the normal 5% or more, due to excessive NCO, NCO can react with water in the air, and there are too many urea groups in the foam, resulting in a stiff foam feel and reduced resilience, which can also lead to The damp heat aging properties of the foam deteriorate.
6. Conclusion
The generation of foam defects is mainly affected by factors such as raw material formula, tooling and mold state, and production process parameter control. It is necessary to comprehensively consider various factors to effectively reduce foam defects.
