It's really "a wave of riots, and another wave." Following blowing materials and mask materials, Ministry of Public Security recently notified entire country that it would conduct a "one helmet and one belt" security operation, and many places said that it would begin on June 1st. Riding a motorcycle or electric vehicle without a safety helmet and driving without a seat belt will be severely investigated! After notice was issued, electric car helmet, which had previously received little attention, has become a hot item online and offline.
The production process of helmet can be roughly divided into 6 parts: shell production, hat core production, comfort layer lining production, other accessories, assembly, random inspection. Shell production mainly includes helmet shell and inner shell materials. The inside of helmet is a buffer layer, mainly made of EPS (expanded polystyrene) or EPP (expanded polypropylene) and other materials. EPS is more common, and EPP material is obtained by modifying PP. It is widely used. EPP helmets Mostly high-end helmets. Combined with application of EPP materials in a light vehicle, today I will introduce you to the knowledge of EPP materials.
Fig. 1. Application of EPP material in cushioning layer of helmet and car
1. Expanded polypropylene method
The preparation of foam materials from polypropylene usually consists of introducing cells of same size into a polypropylene matrix, and more common method is to add a blowing agent to polypropylene. According to different foaming agents, PP foaming methods can be divided into physical foaming method and chemical foaming method, among which chemical foaming agents can be divided into exothermic and endothermic foaming agents.
Second, internal factors affecting expanded polypropylene
2.1 Melt strength
It is very difficult to use ordinary polypropylene for foaming, since there are crystalline regions in structure of PP polymer, and melting point is in range of 165-170°C. After temperature exceeds melting point, viscosity of polypropylene itself is not enough, and it will. The gas generated by foaming goes out all over, and gas cannot be effectively wrapped, and merging and collapsing hole phenomenon will be formed, so that a good foaming product cannot be formed.
Figure 2. Relationship between polypropylene melt strength and temperature
Roughly speaking, high melt strength polypropylene (HMSPP) has higher melt strength and has a significant work hardening effect during processing, which has more "expansibility" than ordinary polypropylene. The main manufacturers of HMSPP in market are Basell and Borealis. Manufacturers with PP foaming technology are mainly Kaneka, BASF and Berstorff. The main production methods of HMSPP include blending method, direct synthesis method, micro-crosslinking method, and four beam radiation method. categories.
2.2 Polypropylene crystallinity and crystal area size
As mentioned earlier, polypropylene is a crystalline polymer that can be divided into crystalline and amorphous regions. Among them, crystalline region has a strong bonding ability, and blowing agent cannot penetrate into it, so it can only be foamed into amorphous region of items, so that foaming will be uneven and amount of foaming will be small, and it will not be possible to obtain a foamed material with uniform cells. In addition, a small crystal area can promote cell nucleation and reduce their size; a large crystal area does not contribute to nucleation of cells and even leads to a violation of foaming.
In addition to main factors, it is also related to other factors such as foaming agent, processing technology, etc. Internal factors can be considered from these two aspects: one is modification of polypropylene to obtain polypropylene with high melt strength. (HMSPP); Second, change crystallization behavior of PP.
3. Method for preparing expanded polypropylene
Foaming is a complex kinetic process, usually occurring in several stages. There are four methods for making PP foam materials: extrusion foaming method, kettle pressure foaming method, pressure foaming method and molded foaming method.
Extrusion foaming method: is continuous production of sheets, plates, pipes and rods from expanded polypropylene using physical or chemical blowing agents.
Pressure Foaming Method: A physical blowing agent is injected near molten state of polypropylene resin. After holding pressure for a certain time, PP melt and foaming agent form a homogeneous system. After that, foamed polypropylene material is obtained by means of a momentary pressure drop or temperature change.
Pressure foaming method: it uses rapid temperature changes to form a homogeneous PP/foamer system, and melt is injected into mold at rapidrum heating.
Figure 3. Map of injection foam cells
Shaping and foaming method: it is to raise temperature of molding equipment to a certain foaming temperature and put it into a polypropylene mold to close mold. When polypropylene reaches foaming temperature, inject physical blowing agent or chemical blowing agent into mold, and then maintain pressure to make blowing agent diffuse into melt. A pressure differential is then instantly generated by opening mold to produce a foamed product.
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