Blowing Agent

Blowing agent refers to the formation of a bubble structure in the plastic, that is, a class of additives added to the manufacture of foam plastics. They can produce a large amount of gas under specific conditions, forming continuous or discontinuous pores (that is, open or closed holes), making plastics a porous structural material with a combination of gas and solid, which can reduce the density of plastics and enhance their sound insulation and heat insulation. According to the different ways of producing gas, blowing agents can be divided into two categories: physical blowing agents and chemical blowing agents.

Physical blowing agent
Physical blowing agent is mainly through the change of the physical state of blowing agent to form the bubble hole in the plastic.

The ideal physical blowing agent should have the following conditions:
① Inert, non-toxic;
② Compatible with resin;
③ The diffusion rate in the resin matrix is small;
④ When the resin reaction emits heat, or when placed to external heating should be easy to volatilize.

Generally, physical blowing agents are divided into three categories: (1) compressed gas; ② Soluble solid; ③ Volatile liquid with boiling point below 0℃.

In the process, when the pressure is eliminated, the compressed gas expands, or the liquid is expanded by heat evaporation, or the dissolved soluble solid substance sublimates to produce a gas.

There are many types of physical blowing agents, such as aliphatic hydrocarbons containing 5 to 7 carbons, chlorofluorocarbons, chlorofluorocarbons and carbon dioxide gases, since the 1950s, fluoro-trichloromethane (CFC-11) as the preferred foaming agent of polyurethane is widely used, because of its destructive effect on the atmospheric ozone layer, in order to protect the ecological environment of the earth, it is necessary to prohibit the use of CFCS compounds. Over the years, the world has been looking for and developing ideal alternative products, alternative blowing agent in addition to considering the nature of the blowing agent itself, generally also need to polyether polyols, foaming agent, catalyst and other raw materials for appropriate adjustment and improvement, so that the formula system to achieve optimization, so the key to physical blowing agent lies in the development and application of alternative products. So far, there are mainly four alternatives to blowing agent CFC-11.

(1) Carbon dioxide blowing agent
There are two types of carbon dioxide blowing agents, one is the reaction of isocyanate and water to produce carbon dioxide (water foam) as blowing agent, and the other is liquid carbon dioxide. Compared with CFC-11, the advantage of water foaming is that carbon dioxide ODP(ozone loss value) is zero, non-toxic, safe, no recycling problems, no need to invest in upgrading foaming equipment; The disadvantage is that the viscosity of polyol components is higher in the foaming process, the foaming pressure and foam temperature are higher, and the adhesion between foam and the substrate is poor, especially the thermal conductivity of hard foam products is high. Because carbon dioxide diffuses faster from the bubble hole, and air enters the bubble hole slowly, thus affecting the dimensional stability of the foam, although it can be improved by modification, it is still not as good as CFC-11 foam material.

At present, carbon dioxide blowing agent is mainly used in heating pipeline insulation, packaging foam and agricultural foam and other fields with low insulation requirements. The advantages and disadvantages of liquid carbon dioxide foam are the same as that of water foam, which is mainly used for polyurethane soft foam at present, and can be used for hard foam to overcome the shortcomings of water foam, such as increased consumption of isocyanate, brittle foam and poor adhesion to the substrate. However, the liquid foaming machine needs to be improved, and the storage and transportation cost of liquid carbon dioxide increases. At present, the liquid carbon dioxide foaming technology is still under continuous research and development.

(2) hydrochlorofluorocarbon blowing agent
Hydrochlorofluorocarbon (HCFC) blowing agents, which contain hydrogen in their molecules and are chemically unstable and easier to break down, have a much smaller ODP than CFC-11. Therefore, HCFC is used as a first-generation replacement for CFC blowing agents for temporary use during the transition period and should be replaced by chlorine-free compounds as soon as possible. At present, the European Union, the United States and Japan have banned the use of HCFC-type blowing agents.
Another HCFC replacement for CFC-11 is the 60:40 HCFC-22/HCFC-14LB mixture, which is the most commonly used solvent in industrial production and has mature production technology and is affordable. The disadvantage is that the HCFC-22/HCFC-141B system has a relatively low solubility in general polyols. Processing polyols containing HCFC-22 is relatively difficult. In addition, HCFC-124 has an ODP value of only 1/5 of HCFC-141B, allowing for a longer service life, and some foreign companies plan to use it in construction and refrigerator appliance foam to compete with higher-cost hydrogenated fluoroalkanes (HFC).

(3) hydrocarbon blowing agent
The hydrocarbons used for polyurethane foaming agents are mainly cyclopentane, especially the rigid foam system of cyclopentane has the advantages of low thermal conductivity, anti-aging performance, and zero ODP value, and is often used in the fields of refrigerators, cold storage and building insulation and insulation, and has become the first choice for hard foam CFC-11 substitutes in China.

In addition, with n-butane and isobutane as auxiliary foaming agents, the following two problems must be solved in the preparation of cyclopentane polyurethane rigid foam, explosion-proof equipment should be used to solve the problem of cyclopentane inflammability and explosive; Some measures such as n-pentane, isopentane and cyclopentane used together can improve the foam fluidity, so as to solve the problem of poor solubility of cyclopentane in polyether polyols.

(4) hydrogenated fluoroalkane (HFC) blowing agent
Hfc-type compounds with ODP value of zero are ideal substitutes for CFC-11 in the production of soft PU foam. The early HFC-type blowing agents are mainly HFC-134A and HFC-152A, which have low molecular weight and low boiling point, and the dosage is less than that of CFC-11 when the foam with the same density and similar physical properties is reached. And the performance is relatively stable, but their defect is that the thermal conductivity is relatively high, and the solubility in the general polyol is low, and the combined polyether phase containing HFC-134A and HFC-152A is processed.