9
Contemporary Machining Processes
Chemical blowing agents are further classified into inorganic and organic categories (Annicelli, 2020). The organic blowing agents have advantages such as good
dispersibility, stable gas output, and uniform bubbles (Jin et al., 2019). Depending
on the nature of the process, foaming agents can be endothermic and exothermic.
Exothermic blowing agents release energy during a reaction, which must be dissipated through a plasticization unit and tool. After activation, the reaction runs its
course without more energy being added and continues until the blowing agent completes its reaction (Rohleder and Jakob, 2016). The process is irreversible; it is not
possible to control or to stop it. The pressure in gas chambers may reach 1.2 and even
1.5 MPa. Moreover, the process may not be completed when the product is removed
from the mold as it requires a long time before finishing operations. The most often
used exothermic foaming agent is azodicarbonamide (ADCA), although it decomposes at a relatively high temperature of 230°C (Kmetty et al., 2018).
When heat must be continuously applied in order to initiate and propagate a reaction, substances are referred to as endothermic blowing agents. They usually dissociate water when reacting, which can lead to a hydrolytic degradation of polymer
chains (Rohleder and Jakob, 2016). The most commonly used endothermic foaming
agents are inorganic. They are normally a mixture of sodium bicarbonate and citric
acid, which can produce finer cells and reduce density from 1.24 to 0.645 g/cm 3 in
case of neat, extruded poly(lactic acid) (Kmetty et al., 2018). When sodium bicarbonate or ammonium bicarbonate are used as blowing agents, peak pressure in bubbles does not exceed 0.8–1.0 MPa. The heat is distributed steadily and consumed by
the polymer structure, so that the gas release effectively stops as soon as the heating
is reduced. In addition, decomposition products of the inorganic foaming agents are
harmful to neither humans nor the ozone layer and may therefore be applied in the
food industry, e.g., for packaging production.
Chemical foaming agents are generally low-molecular-weight compounds supplied as powder or pellet. Their advantage over physical (gaseous) foaming agents
lies in that they can be added to a solid polymer before heating, while physical foaming agents must be injected into an already fluidized polymer (Niaounakis, 2015).
Chemical foaming does not require design modification of the IMM; a standard
machine can be used, with the only exception of reduced power requirements. The
agents are mixed with the polymer at room temperature and atmospheric pressure.
The chemical blowing agents are mainly thermostable substances that do not require
additional equipment to be stored or transported.
However, chemical foaming does not provide for strict control of the foaming
process parameters, so that cell dimensions may vary from 10 up to 200 μm. Another
demerit is the ability of some chemical blowing agents to discharge active gases, e.g.,
ammonia, that may promote corrosion of low-alloy steels.
Addition of a chemical blowing agent to a polymer requires very precise dosing,
which means a more expensive apparatus. Otherwise, variation of agent proportions in
a polymer may cause the quality of manufactured parts to become unstable across lots.
Selection of a chemical foaming agent is mainly determined by a processed polymer type. Some agents may be applied quite universally, like ADCA, while others
are more specialized and dedicated to particular groups of plastics.
Contemporary Machining Processes
Chemical blowing agents are further classified into inorganic and organic categories (Annicelli, 2020). The organic blowing agents have advantages such as good
dispersibility, stable gas output, and uniform bubbles (Jin et al., 2019). Depending
on the nature of the process, foaming agents can be endothermic and exothermic.
Exothermic blowing agents release energy during a reaction, which must be dissipated through a plasticization unit and tool. After activation, the reaction runs its
course without more energy being added and continues until the blowing agent completes its reaction (Rohleder and Jakob, 2016). The process is irreversible; it is not
possible to control or to stop it. The pressure in gas chambers may reach 1.2 and even
1.5 MPa. Moreover, the process may not be completed when the product is removed
from the mold as it requires a long time before finishing operations. The most often
used exothermic foaming agent is azodicarbonamide (ADCA), although it decomposes at a relatively high temperature of 230°C (Kmetty et al., 2018).
When heat must be continuously applied in order to initiate and propagate a reaction, substances are referred to as endothermic blowing agents. They usually dissociate water when reacting, which can lead to a hydrolytic degradation of polymer
chains (Rohleder and Jakob, 2016). The most commonly used endothermic foaming
agents are inorganic. They are normally a mixture of sodium bicarbonate and citric
acid, which can produce finer cells and reduce density from 1.24 to 0.645 g/cm 3 in
case of neat, extruded poly(lactic acid) (Kmetty et al., 2018). When sodium bicarbonate or ammonium bicarbonate are used as blowing agents, peak pressure in bubbles does not exceed 0.8–1.0 MPa. The heat is distributed steadily and consumed by
the polymer structure, so that the gas release effectively stops as soon as the heating
is reduced. In addition, decomposition products of the inorganic foaming agents are
harmful to neither humans nor the ozone layer and may therefore be applied in the
food industry, e.g., for packaging production.
Chemical foaming agents are generally low-molecular-weight compounds supplied as powder or pellet. Their advantage over physical (gaseous) foaming agents
lies in that they can be added to a solid polymer before heating, while physical foaming agents must be injected into an already fluidized polymer (Niaounakis, 2015).
Chemical foaming does not require design modification of the IMM; a standard
machine can be used, with the only exception of reduced power requirements. The
agents are mixed with the polymer at room temperature and atmospheric pressure.
The chemical blowing agents are mainly thermostable substances that do not require
additional equipment to be stored or transported.
However, chemical foaming does not provide for strict control of the foaming
process parameters, so that cell dimensions may vary from 10 up to 200 μm. Another
demerit is the ability of some chemical blowing agents to discharge active gases, e.g.,
ammonia, that may promote corrosion of low-alloy steels.
Addition of a chemical blowing agent to a polymer requires very precise dosing,
which means a more expensive apparatus. Otherwise, variation of agent proportions in
a polymer may cause the quality of manufactured parts to become unstable across lots.
Selection of a chemical foaming agent is mainly determined by a processed polymer type. Some agents may be applied quite universally, like ADCA, while others
are more specialized and dedicated to particular groups of plastics.