Research on the application of MDI in slow resilient polyurethane block foam

Research on the application of MDI in slow resilient polyurethane block foam
1. Preface
As we all know, polyurethane slow rebound foam is also called memory foam and viscoelastic foam, which is unique in that it can conform to the shape of the load-bearing object, so that the contact area can be maximized, the stress gradient can be minimized, and the point of concentration of the force can be relieved, and there is no local squeezing and stinging sensation. Therefore, slow rebound block foam is widely used as mattresses, cushions, etc., which is beneficial to the blood circulation of the human body and is a highly comfortable health care foam.
Theoretically, the resilience of polyurethane foam is related to its phase separation degree and glass transition temperature. 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 slow resilience foam can be obtained. The usual process route is to mix low hydroxyl value soft foam polyether and higher hydroxyl value polyether to improve the crosslinking density of the foam, thereby both increasing the glass transition temperature of the foam and reducing the degree of phase separation. 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, people are paying more and more attention to MDI, not only in the field of molding, but also want to try the MDI process in the field of block foams [4-10]. In this paper, a modified MDI, trade name wannate8107, is introduced for the preparation of slow-rebound block foams.
The reason why MDI prepolymerization technology is used is that the free foaming density of MDI-based foams is higher than that of TDI-based, and in order to maintain the same foaming density, the MDI system requires a higher amount of water. However, as the amount of water in the system increases, the ratio of urea group/carbamate increases, which tends to separate the urea phase from the microregion of the polyether soft segments, resulting in unstable foams and leading to a stiff foam feel and poor aging properties. The use of prepolymer technology to prepare MDI-based slow resilient foams, i.e., the use of high molecular weight, multi-hydroxyl compounds to react with MDI to form prepolymers, can realize the control of the phase separation process in the foaming process, thus achieving the purpose of improving the final properties of foams. Especially in the preparation of low density foam, MDI prepolymer technology is more important. Secondly, the prepolymerization modification of MDI can also significantly improve the storage performance of MDI components.
2 Main chemical properties of MDI
Preparation of slow rebound block bubble TDI is usually 2, 4 body accounted for 80% (mass fraction, the same below), 2, 6 body accounted for 20%, colorless or yellowish transparent liquid at room temperature, with a strong irritating odor, in 10 ℃ below the emergence of crystals, is a highly toxic and dangerous goods, the transportation has strict requirements. MDI used in this experiment can be stored at 0 ℃, room temperature is liquid. MDI no irritating odor, 25 ℃ vapor pressure of about 1.33 × 10-3 Pa, 1/2700 for TDI, volatility is small, relatively small toxicity to the human body, is a non-hazardous product, no special requirements for transportation, in the production, foaming process without pollution, conducive to industrial safety protection, by the production, It is welcomed by production and construction personnel. From the molecular structure, MDI and TDI molecular structure is similar, both contain -NCO and benzene ring structure. Experimental results show that MDI and TDI have the following differences:
Reactivity difference
In the TDI system, TDI is mainly 2,4-TDI, due to the spatial resistance, when the 4-position isocyanate group reacts preferentially, the reactivity of the 2-position group is greatly reduced; whereas in the MDI system, the structurally symmetric 4,4-MDI is mainly, with less spatial resistance, so the reactivity is relatively high, which results in the foam of the MDI system with the advantages of fast maturation speed and short debonding time, and the proportion of catalyst is reduced, and a certain degree of catalyst is reduced, so the foam of MDI system has the advantages of fast maturation speed and short debonding time, and the catalyst is reduced. Reducing the proportion of catalyst reduces the atomization performance of the foam to a certain extent, and at the same time improves the production efficiency.

Difference in foaming multiplicity
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 with the same amount of water. Figure 1 compares the same amount of water, TDI and MDI system free foaming density difference. In order to ensure the same foaming ratio, MDI system needs higher water quantity, and the increase of water quantity will obviously affect the basic physical properties such as foam resilience. Therefore, the development of slow resilient foam of MDI system, especially the low density, puts forward higher requirements on the technical level of formulation than TDI system.
Relationship between free foaming density and water quantity for TDI and MDI systems
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, on the one hand, it can make up for the impact of the density on the cost, on the other hand, it alleviates the impact of small molecule organic matter in the polyether system on the atomization performance of the foam.
Another outstanding advantage of MDI-based slow resilient foam: the foam hardness is more obvious with the isocyanate index, Figure 2 shows the different hardness trends of TDI and MDI systems with the change of isocyanate index.

TDI and MDI system hardness change curve with index
3. Experimental part
Main raw materials
Soft foam polyether: hydroxyl value 56mg(KOH)/g; slow rebound polyether: hydroxyl value 240 mg(KOH)/g; amine catalyst; tin catalyst; organosilicone leveling agent; pore opener; purified water; Beijing Kejiu Modified MDI wannate8107, the NCO content of about 31-32%.
Main equipment
Electronic universal material testing machine, Zwick Z005, Zwich Instruments, Germany; horizontal combustion meter, SPF-01, Fangshan Instrument and Equipment Factory, Jiangding County, Nanjing; DMA (Dynamic Thermomechanical Meter), Q800, TA Instruments, USA; Optical microscope, S8APO, Beijing BORUN MINGGUANG Science and Trade Co.
Test methods
Table 1 Foam test content and cited standards
Manual foaming
Foaming conditions: room temperature is 20~30℃, material temperature is 22℃.
Foaming formula: A component and B component are weighed according to the index of isocyanate index 0.8, pour the measured A component and B component into the plastic beaker, stir under the stirrer speed of 3000 r/min for 5~6 s, then pour it into the homemade square box for free foaming, and the foam molding is labeled as MDI foam, and the foam is labeled as TDI foam by using the same process and suitable formula. TDI foam.
Box foaming
The main process parameters are as follows:
Figure 3 Box foaming process flow
4 Results and discussion
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.
According to the traditional sense, the mechanical properties of soft foam prepared by MDI are not as good as those of TDI, which is because: firstly, the functional degree of TDI itself is lower than that of traditional modified MDI; secondly, the thermodynamic phase separation of the foam prepared by TDI is better, while the soft and hard phases of the foam prepared by MDI are more easily dissolved.
The wannate 8107 prepared in this paper is based on the drawbacks of traditional modified MDI in soft foam, targeted modification. Due to its asymmetric structure and relatively low reactivity, it reduces the crystallization tendency of the hard phase in the foam, enhances the flexibility of the foam, improves the elongation, and gives the final product good mechanical properties.
In addition, due to the asymmetry of the molecular structure, the crystallization property of pure 2,4’MDI is greatly reduced, the freezing point is 34℃, and then there is the spatial resistance effect of methylene so that the activity of the group on the 2-position of the benzene ring is lower than that on the 4-position of the benzene ring, and the generation of dimer impurities is slow in the process of storage, so wannate8107 can be stored above 0℃, which can greatly reduce the storage time. Therefore, wannate8107 can be stored above 0℃, which greatly reduces the disadvantage of TDI crystallization below 10℃.
The slow rebound block foam of different systems prepared with the same formula, the rebound of the foam of TDI system is slightly slower than that of MDI system, which can be explained from two aspects: one is the difference in damping caused by the molecular structure, which can be referred to in Fig. 4; the other is the morphology and structure of the foam’s bubble holes, which can be referred to in Fig. 5. In the MDI system, the rebound time of the foam can be adjusted by adjusting the ratio of the polyether.
Generally polyurethane materials are flammable. Polyurethane flexible foam is easy to burn when there is sufficient oxygen supply and a certain temperature is reached because of its small density, high open cell ratio, low thermal conductivity and thermal decomposition temperature. As a block foam commonly used for furniture, its flame retardant property is what we must pay attention to. Table 3 shows the flame retardancy data of the tested TDI and MDI systems without flame retardant. From Table 3, it can be found that in terms of horizontal burning, MDI and TDI are close to each other, and in terms of flame retardancy, MDI can be completely used in furniture soft foam.
DMA test
Fig. 4 Comparison of DMA profiles of TDI and MDI
In the slow rebound foam, both TDI and MDI systems, the degree of phase separation is poor, that is, the soft and hard chain segments mixed tolerance is better, the transformation peak of the polyether soft chain segment enrichment area does not appear, TDI does not reflect the degree of phase separation is good, and the soft and hard segments of the mixed tolerance area of the glass transition temperature are about 18 ℃, relatively close. Only the transformation peak of TDI system is stronger, that is, the hysteresis of the chain segment movement is more powerful, the corresponding energy storage modulus decays faster, and the damping property of TDI is better, and the performance of rebound at room temperature is slower.
Micrographs

Figure 5 Micrographs
The two micrographs reflect the open and closed cell condition of the foam, the bubble wall of MDI foam is almost completely ruptured, which also confirms the result of the lower closed cell ratio (2.0%), the bubble holes of the MDI system are more regular, and the air permeability is higher, which is more comfortable. The TDI system has messy pores and is more closed cell, which is one of the main reasons for its slower rebound.
5 Future Prospects
In today’s era of environmental protection and high quality, MDI-based slow resilience foam is accepted by more and more downstream customers because of the advantages of good production environment and excellent comprehensive performance. The Wannate8107 modified MDI product developed by Beijing Kejiu Chemical New Material Co., Ltd. for slow-rebound block foam can fully meet the performance requirements of customers for MDI-based slow-rebound mattresses.
Analyzed from the process, with the various modified MDI technology advances, the whole MDI foam technology can basically completely replace the TDI system; from the national policy, TDI due to its product characteristics, is classified as a highly toxic products, MDI to replace TDI is also inevitable. Therefore, from the perspective of the future, modified MDI for slow rebound block soft foam has a bright future.
6 Conclusion
1、Beijing Keju developed wannate8107, the vapor pressure is very low, 1/2700 of TDI at 25℃, which greatly improves the foaming environment; it can be stored at 0℃ without crystallization, which greatly improves the transportation and storage conditions;
2, at present, the process of slow rebound polyurethane block foam is mainly TDI process, Wanhua’s modified MDI process can be operated widely, and its mechanical properties and flame retardant properties are close to those of TDI foam, which are far more than the requirements of the national standard, so it can be completely used for the preparation of slow rebound block foam;
3, TDI poor permeability, poor air circulation, good damping is the main reason for slow rebound, while the foam prepared by MDI has higher air permeability and is more comfortable.