By adding some fillers to the plastic, some properties of the plastic can be improved, so that it is more suitable for some special uses. In order to reduce the density and hardness of plastic, or enhance its heat insulation or sound insulation, the most ideal filler is the void. Plastics containing voids or bubble holes are classified as foam, and with the degree of foaming, that is, the difference in the volume share of foam caused by the void, the performance of foam may be quite different from the base plastic. Blowing agent is a chemical that can be added to plastic, and at the right time during processing, it will release gas, causing the formation of bubble holes in the plastic. The formation of plastic foam can generally be divided into four stages.
·Four stages of the foaming process·
【 First stage 】
The blowing agent must be completely evenly dispersed within the polymer, which is usually in a liquid or molten state. The blowing agent can then form a true solution in the polymer, or it can just be uniformly dispersed in the polymer, forming a two-phase system.
【 Second stage 】
After a large number of individual bubbles are formed, the system is transformed into a system of gas dispersed in a liquid. At this time, nucleating agents are often needed to promote the formation of a large number of small bubbles. Nucleators are generally very fine, inert particles that provide sites for the formation of new gas phases.
【 Third stage 】
The pores that were originally formed continue to grow as more gas diffuses through the polymer and enters the pores. If this period is long enough, the individual cells will come into contact with each other. If the wall separating a single cell breaks, a larger cell is formed through this coalescence. If the foam is formed mainly through the interconnect of bubbles, it is called open-cell foam. If the foam is formed by unconnected bubble holes, it is called closed cell foam. If cell coalescence is allowed to continue unchecked, the foam collapses because the gas is automatically separated from the polymer.
【 Fourth stage 】
When the viscosity of the polymer increases and the cell can no longer grow, the foam stabilizes. The viscosity of the polymer can be increased by cooling, crosslinking, or other methods.
It can be seen that the last three stages of the foaming process, from the point of view of time, can be as short as a fraction of a second, the longest will not exceed a few seconds. The formation of foam requires the polymer to be liquid. This allows the polymer to be dissolved or plasticized by heating. The production process of foam is almost the same as any ordinary plastic production process, usually through extrusion, rotomolding and injection molding, as well as plasticizing paste processing and thermoforming. For the same reason, virtually any kind of plastic can be made into foam. PVC (hard and soft), polystyrene, polypropylene, ABS and polyethylene have all been made into foam on an industrial scale. The same is true of heat-resistant engineering plastics and thermosetting polymers.
·AC Foaming Agent·
AC blowing agent is prepared by the oxidation of diurea, light yellow or orange yellow crystalline powder. Its molecular weight is 116, decomposition heat 359.9J/g℃, decomposition gas is mainly nitrogen (65%), carbon monoxide (32%) and a small amount of carbon dioxide (3%). The decomposed solid residues are mainly biurea, cyanuric acid and urazole. Decomposition slightly ammonia flavor, non-flammable, self-extinguishing. Stable storage at room temperature. This product can be considered non-toxic.
AC blowing agent has good performance. Therefore, it is widely used, especially in hard PVC foam profiles.
·Effect of each component of rigid PVC foam profiles on AC foam·
1.1PVC resin:
PVC resin with K value in the range of 58-65 is selected for general profiles. PVC resin with low K value is often used for foaming materials, which is conducive to rapid gelation during processing and ensures that the melt has a homogeneous structure at foaming temperature.
1.2 Stabilizer:
In addition to having a heat stabilizing effect in the formulation, the heat stabilizer also plays a role in reducing the decomposition temperature of the initiator, acting as an activator, promoting the decomposition of AC, and making the foaming temperature suitable for the processing temperature.
The experiment showed that the decomposition temperature of AC was decreased significantly with the increase of trisalt dosage, and the gas output was increased. The hard barium and hard lead also have the effect of reducing the decomposition temperature of AC, but they affect the reduction of AC gas output. Therefore, a comprehensive consideration should be taken to reasonably select the amount of stabilizer.
1.3AC Blowing agent:
With the increase of the amount of AC blowing agent, the gas volume increased and the product density decreased. When the amount of AC is at a certain value, the density of PVC products is low, and the surface is smooth. However, as the amount of AC continues to increase, the amount of gas is too much, so that the strength of PVC melt can not cover the bubble hole, causing the bubble hole to break, the appearance of the product appears to collapse, and the product density increases. Anti-ac blowing agent dosage must be selected according to different products an optimal dosage.
1.4 Modifiers:
There are many kinds of hard PVC modifiers, among which CPE and ACR are the main modifiers. The addition of ACR can improve the melt strength of PVC resin, so that the bubble wall can withstand the pressure of the gas in the bubble during the foaming process without breaking and causing large holes. As the amount of ACR increases, the melt strength increases, and the bubble pores become smaller and more. Therefore, ACR not only affects the cell structure, but also affects the product density.
The main function of CPE is to improve the impact strength of hard PVC materials. The selection of different types of CPE and its dosage have great influence on the performance of PVC system. Generally, CPE-135A is good, and the amount of addition is 5%-15%.
1.5CaCO3 Usage:
In the low foaming system, the amount of CaCO3 is small, and it has the role of nucleating agent, which helps to form suitable bubble holes. However, excessive addition of CaCO3 makes the cell irregular, and the material density increases correspondingly, which is also unfavorable to the impact strength. Generally, the dosage of 10%-15% is better.
·Attention·
① In the formulation of rigid PVC foam profiles, the PVC resin type, the amount and selection of stabilizer, the selection of modifier, and the selection of CaCO3 all have a significant impact on the AC foam system, and the best formula must be selected through experiments.
② Extrusion pressure, extrusion temperature, die temperature, retention time and screw speed in the extrusion molding process all have an effect on the AC foaming system, so the conditions of the extrusion process must be strictly controlled.
