The principle and characteristics of chemical blowing agents

Chemical blowing agents Chemical blowing agents can also be divided into two main types: organic chemicals and inorganic chemicals. There are many types of organic chemical blowing agents, while inorganic chemical blowing agents are limited. The earliest chemical blowing agents (circa 1850) were simple inorganic carbonates and bicarbonates. These chemicals emit CO2 when heated, and they are eventually replaced by a mixture of bicarbonate and citric acid because the latter has a much better prognostic effect. Today’s more excellent inorganic foaming agents have basically the same chemical mechanism as above. They are polycarbonates (the original is Poly-carbonic
acids) mixed with carbonates.

The decomposition of polycarbonate is an endothermic reaction, at 320°F
About 100cc per gram of acid can be released. When the left and right CO2 is further heated to about 390°F, more gas will be released. The endothermic nature of this decomposition reaction may bring some benefits, because the heat dissipation during the foaming process is a big problem. In addition to being a gas source for foaming, these substances are often used as nucleating agents for physical foaming agents. It is believed that the initial cells formed when the chemical blowing agent decomposes provide a place for migration of the gas emitted by the physical blowing agent.

Contrary to inorganic foaming agents, there are many types of organic chemical foaming agents to choose from, and their physical forms are also different. In the past few years, hundreds of organic chemicals that may be used as blowing agents have been evaluated. There are also many criteria used to judge. The most important ones are: under the conditions of controllable speed and predictable temperature, the amount of gas released is not only large, but also reproducible; the gases and solids produced by the reaction are non-toxic, and it is good for foaming polymerization. Objects must not have any adverse effects, such as color or bad smell; finally, there is a cost issue, which is also a very important criterion. Those foaming agents used in the industry today are most in line with these criteria.

The low-temperature foaming agent is selected from many available chemical foaming agents. The main problem to be considered is that the decomposition temperature of the foaming agent should be compatible with the processing temperature of the plastic. Two organic chemical blowing agents have been widely accepted for low-temperature polyvinyl chloride, low-density polyethylene and certain epoxy resins. The first is toluene sulfonyl hydrazide (TSH). This is a creamy yellow powder with a decomposition temperature of about 110°C. Each gram produces approximately 115cc of nitrogen and some moisture. The second type is oxidized bis(benzenesulfonyl) ribs, or OBSH. This foaming agent may be more commonly used in low-temperature applications. This material is white fine powder and its normal decomposition temperature is 150°C. If an activator such as urea or triethanolamine is used, this temperature can be reduced to about 130°C. Each gram can emit 125cc of gas, mainly nitrogen. The solid product after decomposition of OBSH is a polymer. If it is used together with TSH, it can reduce odor.

High-temperature foaming agent For high-temperature plastics, such as heat-resistant ABS, rigid polyvinyl chloride, some low-melt index polypropylene and engineering plastics, such as polycarbonate and nylon, compare the use of blowing agents with higher decomposition temperatures Suitable. Toluenesulfonephthalamide (TSS or TSSC) is a very fine white powder with a decomposition temperature of about 220°C and a gas output of 140cc per gram. It is mainly a mixture of nitrogen and CO2, with a small amount of CO and ammonia. This blowing agent is commonly used in polypropylene and certain ABS. But due to its decomposition temperature, its application in polycarbonate is limited. Another high-temperature blowing agent-5-based tetrazole (5-PT) has been successfully used in polycarbonate. It starts to decompose slowly at about 215°C, but the gas production is not large. A large amount of gas will not be released until the temperature reaches 240-250°C, and this temperature range is very suitable for the processing of polycarbonate. The gas production is approximately
175cc/g, mainly nitrogen. In addition, there are some tetrazole derivatives under development. They have a higher decomposition temperature and emit more gas than 5-PT.

The processing temperature of most major industrial thermoplastics of azodicarbonate is as described above. The processing temperature range of most polyolefin, polyvinyl chloride and styrene thermoplastics is 150-210°C
. For this kind of plastic, there is a kind of blowing agent that is reliable to use, that is, azodicarbonate, also known as azodicarbonamide, or ADC or AC for short. In its pure state, it is a yellow/orange powder at about 200°C
Start to decompose, and the amount of gas produced during decomposition is
220cc/g, the gas produced is mainly nitrogen and CO, with a small amount of CO2, and also contains ammonia under certain conditions. The solid decomposition product is beige. It can not only be used as an indicator for complete decomposition, but also has no adverse effect on the color of the foamed plastic.

AC has become a widely used foam foaming agent for several reasons. In terms of gas production, AC is one of the most effective foaming agents, and the gas it releases has high foaming efficiency. Moreover, the gas is released quickly without losing control. AC and its solid products are low-toxic substances. AC is also one of the cheapest chemical blowing agents, not only from the gas production efficiency per gram, but also from the gas production per dollar is quite cheap.

In addition to the above reasons, AC can be widely used because of its decomposition characteristics. The temperature and speed of the released gas can be changed, and it can be adapted to 150-200°C
Almost all purposes within the scope. Activation, or action additives change the decomposition characteristics of chemical blowing agents, this problem has been discussed in the use of OBSH above. AC activates much better than any other chemical blowing agent. There are a variety of additives, first of all, metal salts can reduce the decomposition temperature of AC, and the degree of decrease depends mainly on the type and amount of additives selected. In addition, these additives also have other effects, such as changing the rate of gas release; or creating a delay or induction period before the decomposition reaction starts. Therefore, almost all gas release methods in the process can be designed artificially.

The size of AC particles also affects the decomposition process. Generally speaking, at a given temperature, the larger the average particle size, the slower the gas release. This phenomenon is particularly obvious in systems with activators. For this reason, the particle size range of commercial AC is 2-20 microns or larger, and the user can choose at will. Many processors have developed their own activation systems, and some manufacturers choose various pre-activated mixtures provided by AC manufacturers. There are many stabilizers, especially those used for polyvinyl chloride, and certain pigments will act as activators for AC. Therefore, you must be cautious when changing the formula, because the decomposition characteristics of AC may change accordingly.

AC available in the industry has many grades, not only in terms of particle size and activation system, but also in terms of fluidity. For example, adding an additive to AC can increase the fluidity and dispersibility of AC powder. This type of AC is very suitable for PVC plastisol. Because the foaming agent can be fully dispersed into the plastisol, this is a key issue for the quality of the foamed plastic final product. In addition to using grades with good fluidity, AC can also be dispersed in phthalate or other carrier systems. It will be as easy to handle as liquid.


Post time: Jan-13-2021