1.Preface
As we all know, polyurethane slow rebound foam is also called memory foam and viscoelastic foam, and its unique feature is that it can conform to the shape of the bearer, which makes the contact area maximized and stress gradient minimized, and the stress concentration point can be relieved, and there is no local extrusion and tingling sensation. Therefore, slow rebound block foam is widely used as mattress, cushion, etc., which is beneficial to the blood circulation of human body and is a kind of high comfort health foam.
Theoretically, the resilience of polyurethane foam is related to its phase separation degree and glass transition temperature. Slow resilient foam can be obtained by selecting raw materials to adjust the glass transition temperature of the foam to the use temperature and at the same time reducing the phase separation degree of the foam. The usual process route is to mix low hydroxyl value soft foam polyether and higher hydroxyl value polyether to increase the cross-link density of foam, thus both increasing the glass transition temperature of foam and reducing the phase separation degree. For example, CN1606580, CN1572186, CN1229803 are based on this theory [1-3].
Traditionally, slow-rebound polyurethane block foams are prepared with TDI, and in recent years, due to the gradual attention to environmental protection, more and more attention has been paid to MDI, not only in the molding field, but also in the block foam field to try the MDI process [4-10]. In this paper, a modified MDI with the trade name wannate8107 is presented for the preparation of slow rebound block bubbles.
The reason why MDI prepolymerization technology is used is that the free foam density of MDI-based foam is higher than that of TDI-based, and in order to maintain the same foam density, the MDI system requires a higher amount of water. However, as the amount of water in the system increases, the urea group/carbamate ratio increases, which tends to separate the urea phase from the polyether soft segment microzone, resulting in unstable foam and leading to a stiff foam hand and poor aging performance. The preparation of MDI-based slow-rebound foam by prepolymer technology, i.e., using high molecular weight, polyhydroxy compounds to react with MDI to form prepolymers, can achieve control of the phase separation process during the foaming process, thus achieving the purpose of improving the final properties of the foam. Especially in the preparation of low-density foams, MDI prepolymer technology is more important. Secondly, the prepolymerization modification of MDI can also significantly improve the storage properties of MDI components.
2 Main chemical properties of MDI
The TDI used in the preparation of slow resilient block foam is usually 80% of 2, 4 bodies (mass fraction, same below) and 20% of 2, 6 bodies, which is a colorless or yellowish transparent liquid at room temperature with strong irritating odor and crystallization below 10℃. The MDI used in this experiment can be stored at 0℃ and in liquid state at room temperature, it has no irritating odor and the vapor pressure is about 1.33×10-3 Pa at 25℃, which is 1/2700 of TDI. It is welcomed by the production and construction personnel. From the molecular structure, MDI and TDI have similar molecular structure, both contain -NCO and benzene ring structure. The experimental results show that MDI and TDI have the following differences.
2.1. Reactivity differences
In the TDI system, TDI is mainly 2,4-TDI, due to the spatial site resistance, when the 4-position isocyanate group reacts preferentially, the reaction activity of the 2-position group is greatly reduced; while in the MDI system, it is mainly 4,4-MDI with symmetrical structure, which has less spatial site resistance, so the reaction activity is relatively high, which leads to the advantages of fast maturation and short debonding time of MDI system foam, which can reduce the proportion of catalyst, which reduces the atomization performance of the foam to a certain extent, and at the same time improves the production efficiency.
2.2 Difference of foaming multiplier
Since the NCO% content of MDI is lower than that of TDI, the free foaming density of MDI system is higher than that of TDI system under the same water volume. Figure 1 compares the difference of free foaming density between TDI and MDI systems under the same water volume. In order to ensure the same foaming multiplier, MDI system needs higher water volume, and the increase of water volume will obviously affect the basic physical properties such as foam resilience. Therefore, the development of slow-rebound foam of MDI system, especially the low density, puts higher requirements on the technical level of the formulation than TDI system.
The relationship between free foam density and water volume of TDI and MDI systems
2.3 Hardness difference
Compared with TDI, MDI has two benzene rings, higher cohesion energy, higher hardness of the prepared foam, which can reduce the isocyanate index, which can make up for the impact of density on cost on the one hand, and mitigate the impact of small organic molecules of polyether system on foam atomization performance on the other hand
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Another outstanding advantage of MDI-based slow resilient foam: the foam hardness is more obvious with isocyanate index, Figure 2 shows the different hardness trends of TDI and MDI systems with the change of isocyanate index.
3. Experimental part
3.1 Main raw materials
Soft foam polyether: hydroxyl value 56 mg(KOH)/g; slow rebound polyether: hydroxyl value 240 mg(KOH)/g; amine catalyst; tin catalyst; organosilicon foam leveling agent; porosity-opening agent; pure water; Beijing Keju Modified MDI wannate8107, NCO content about 31-32%
3.2 Main equipment
Electronic universal material testing machine, Zwick Z005, Zwich Instruments, Germany; horizontal combustion instrument, SPF-01, Nanjing Jiangding County Fangshan Instrument and Equipment Factory; DMA (dynamic thermomechanical instrument), Q800, TA Instruments, USA; optical microscope, S8APO, Beijing Lubrizol Mingguang Science and Technology Co.
3.3 Test Methods
Table 1 Foam test content and cited standards
3.4.2 Manual foaming
Foaming conditions: room temperature is 20~30℃, material temperature is 22℃.
Foaming formula: component A and component B are weighed according to the index of isocyanate index 0.8, and the measured component A and component B are poured into the plastic beaker, stirred for 5~6 s under the stirrer speed of 3000 r/min, then poured into the homemade square box for free foaming, and the foam is marked as MDI foam after forming, and the same process and suitable formula are used for TDI foaming, and the foam is marked as TDI foam.
3.4.3 Box foaming
The main process parameters are as follows.
4 Results and Discussion
4.1 Mechanical and flame retardant performance test
Table 2 Slow rebound block foam formulation
As can be seen from Table 3, the mechanical properties of TDI and MDI systems are relatively close to each other.
According to the traditional sense, the mechanical properties of soft foam prepared by MDI are not as good as those of TDI, because: firstly, the functional degree of TDI itself is lower than that of traditional modified MDI; secondly, the thermodynamic degree of phase separation of foam prepared by TDI is better, while the soft and hard phases of foam prepared by MDI are more easily intercalated.
And the wannate8107 prepared in this paper is precisely based on the disadvantages of traditional modified MDI in soft foam, to carry out targeted modification. Due to its asymmetric structure and relatively low reactivity, it can reduce the crystallization tendency of hard phase in the foam, enhance the flexibility and elongation of the foam, and give the final product good mechanical properties.
In addition, due to the asymmetry of molecular structure, the crystallization property of pure 2,4’MDI is greatly reduced, and the freezing point is 34℃, and the spatial site resistance effect of methylene makes the group activity at the 2 position of benzene ring much lower than that at the 4 position of benzene ring, and the generation of dimer impurities in the storage process is slow. Therefore, wannate8107 can be stored above 0 ℃, which greatly reduces the disadvantage of crystallization of TDI below 10 ℃.
The slow rebound block foam of different systems prepared by the same formula, the foam rebound of TDI system is slightly slower than that of MDI system, which can be explained from two aspects: one is the damping difference caused by the molecular structure, which can be referred to Figure 4; the second is the bubble pore morphology structure of the foam, which can be referred to Figure 5. In MDI system, the rebound time of the foam can also be adjusted by adjusting the ratio of polyether.
Generally polyurethane materials are flammable. Polyurethane flexible foam is easy to burn when it has sufficient oxygen supply and reaches a certain temperature because of its small density, high open pore rate, low thermal conductivity and thermal decomposition temperature. As a block foam commonly used in furniture, its flame retardant performance is what we must pay attention to. Table 3 shows the flame retardant performance data of TDI and MDI systems tested without adding flame retardant. From Table 3, we can find that MDI and TDI are close to each other in terms of horizontal combustion, and in terms of flame retardant, MDI can be used in furniture soft foam.
4.2 DMA test
Figure 4 Comparison of DMA profiles of TDI and MDI
In the slow resilient foam, both TDI and MDI system, the degree of phase separation is relatively poor, that is, the soft and hard chain segments are better mixed, the transformation peak of the soft chain segment enrichment area of polyether does not appear, and TDI does not reflect the good degree of phase separation, while the glass transition temperature of the soft and hard segment mixing area is around 18℃, which is relatively close. Only the transition peak of TDI system is stronger, that is, the chain segment movement hysteresis is more powerful, the corresponding energy storage modulus decay is faster, TDI damping is better, and the performance of rebound at room temperature is slower.
4.3 Micrographs
The two micrographs reflect the open and closed pore of the foam, the wall of MDI foam pore is almost completely broken, which also confirms the result of lower closed pore rate (2.0%), the pore of MDI system foam is more regular, the air permeability is higher and more comfortable. In contrast, the TDI system has messy bubble pores and closed pores, which is one of the main reasons for its slow rebound.
5 Future prospects
In the era of environmental protection and pursuit of high quality, MDI-based slow resilient foam has the advantages of good production environment and excellent overall performance, which is accepted by more and more downstream customers. Ltd. has developed Wannate8107 modified MDI products for slow resilient block foam, which can fully meet the performance requirements of customers for MDI-based slow resilient mattresses.
From the process analysis, with various modified MDI technology progress, all MDI foam technology basically can completely replace TDI system; from the national policy, TDI is classified as highly toxic products due to its product characteristics, MDI replacement of TDI is also inevitable. Therefore, from the prospective, the prospect of modified MDI for slow resilient lumpy soft foam is bright.
6 Conclusion
1. wannate8107 developed by Beijing Keju has a very low vapor pressure, which is 1/2700 of TDI at 25℃, greatly improving the foaming environment; it can be stored at 0℃ without crystallization, greatly improving the transportation and storage conditions.
2、At present, the process of slow-rebound polyurethane block foam is mainly TDI process, but Wanhua’s modified MDI process is widely operable, and its mechanical properties and flame retardant properties are close to those of TDI foam, far exceeding the requirements of the national standard, which can be fully used to prepare slow-rebound block foam.
3, TDI poor air permeability, poor air circulation, good damping is the main reason for slow rebound, while the air permeability of the foam prepared by MDI is higher and more comfortable.
