Discussion on application of catalyst in polyurethane rigid foam for refrigerator
preface
In the foaming process of polyurethane rigid foams for refrigerators, the one-step method is generally adopted. For refrigeration equipment such as refrigerators, factors such as whether foam is evenly distributed in large cavities with complex structures, the degree of uniformity of foam pores, and the high or low closure rate of foam have a great impact on the overall performance of refrigerators. These factors are related to the degree of bonding between foam and cavity wall, the stability of foam size, etc. With the continuous emergence of new varieties of rigid polyurethane foam materials, and in view of the fact that refrigerator manufacturers are eager to speed up production in order to improve production efficiency and reduce costs, we have consulted relevant information, screened and improved a variety of amine catalysts, carried out a series of contrast application tests, and applied them in the rigid polyurethane foam for refrigerators with HCFC-14b1 as foaming agent. These situations are discussed below.
Main raw materials and specifications: polyether A, OH value 430mgKOH · g-1 ‘, moisture content 0.1%; polyether B, OH value 400mgKOH · g-1’, moisture content 0.1%; C polyether, OH value 360mgKOH · g-1 ‘, moisture ` 0.1%: D polyether, OH value 480mgKOH · g-1’, moisture sensitivity 0.1%; A. Tetrapolyether B, C and D are all produced by Shanghai Gaoqiao Petrochemical Plant No. 3; Foam homogenizer, acid value ` 0.1%; Moisture 0.1%; Water, distilled water; HCFC-141b, purity) 99.5%, imported: P21NCO31.5%, imported; Catalytic system a, b, e, d, e, self prepared.
Methods In the experiment, we investigated the influence of several catalyst systems in the formula on the foaming process. The experiment was carried out at the ambient temperature of 20 ℃ and 1 ℃. The mixing speed is 3000 rpm. Weigh all components except PAIP according to the formula and put them into a plastic cup. Stir and mix evenly, adjust the temperature to 20 ℃, and then add IP with the material temperature of 20 ℃ measured in advance,
Quickly stir for seconds, then put a cylindrical transparent mold with elevation scale lined with transparent plastic film on the plastic cup, time it, and record the milky white time, fiber time, non stick time and post curing time of the foaming process, as well as the foaming height at regular intervals. In order to judge the differences in the demoulding characteristics of foam materials composed of various catalytic systems, we mixed the different materials according to the above method and poured them into the square mold used for the test (the length x width x height are 2c5m, the mold temperature is 45 ℃, and the transparent plastic film is lined), and carried out several tests. The demoulding tests were carried out 150 seconds, 180 seconds, 210 seconds, 240 seconds, 270 seconds, 300 seconds, 330 seconds, and 360 seconds after the start of mixing, respectively, The thickness increase rate of foam formed by each catalyst system during demoulding at different times was measured. Similarly, use the above method to mix the materials and pour them into the L mold, measure the physical properties of foam at different parts of the L mold, and conduct comparative tests on different catalytic systems
When the fiber time is the same, the opalescence time of different catalytic systems is different. If the length of the time interval between the opalescence time and the fiber time is used to measure the foam fluidity, a long interval means that there is sufficient time before the fiber time, the materials can fill the large and complex shaped cavity calmly, the foam is evenly distributed, and the density gradient is small. If the foam fluidity is evaluated according to the time interval, Then the order from superior to inferior is d one boc one e. When the fiber time is the same, the non sticking time and post curing time of each catalytic system are different. Generally, the curing speed is measured by the intervals between the fiber time and the non sticking time and the post curing time. Short intervals mean fast curing reaction, and the demoulding time can be shortened accordingly. If the relative speed of curing reaction is evaluated according to this time interval, the order from fast to slow is: hour coh, d, e. 3.2 Analyze the fluidity of foam based on the foaming speed curve, rising height and flow index. As can be seen from Table 3 and the figure, the d catalytic system foam has a fast rising speed, especially at the initial stage. The curing height and final height are good and high, respectively. The flow index is large, and the average density of foam
In the foaming process of polyurethane rigid foams for refrigerators, the fluidity of foam and the curing speed of foam are the main indicators of foaming process performance. It can be seen from Table 5 that the material fluidity is good, the foam is evenly distributed everywhere, the foam density gradient is small, the cell structure is nearly uniform, small and spherical, the closed cell rate is high, the thermal conductivity is low, the change rate of foam after forming is small, and the foam curing speed is fast, which can shorten the demoulding time during refrigerator production and improve production efficiency. The key to improve the fluidity and curing speed of foam used in refrigerators with HCFC-1l4b as foaming agent is to select an appropriate amine catalytic system. Different catalytic systems play different roles.
(l) The early onset catalytic system promotes the nucleation of foam when it starts, and is conducive to improving the thermal conductivity and dimensional stability.
(2) The delayed catalytic system can delay the fiber time, so that the foam can fill the cavity calmly with good fluidity. The density gradient of products in various parts of the mold is small and evenly distributed, which is conducive to the bonding of foam body and cavity wall.
(3) The promotion of post curing catalytic system can shorten the interval from fiber time to post curing time of foam, and can demould as soon as possible, which is conducive to improving the strength and dimensional stability of foam system.
(4) The combination system with early onset characteristics, superior foam fluidity and strong post curing speed can be obtained by selecting appropriate catalytic system.
