Discussion on foaming molding technology of plant fiber materials

02
Mechanism characteristics of foaming and molding of plant fibers

The traditional polymer foaming molding process generally needs to go through three stages: bubble nucleation, bubble nucleus growth and bubble stable curing
Compared with the molecular structure of general plastic polymers, plant fibers are macromolecular structures, so there are certain differences between the two in terms of foaming process and foaming mechanism.
Plant fiber foam materials have the following characteristics:
(1) Plant fiber raw materials are an internal cross-linked structure formed by hydrogen bonding on the fiber surface, and there are large gaps between fiber molecules, which is not conducive to the preservation of gas in the foaming process; (2) The length and size of fibrous macromolecules are uneven; (3) There are many impurities, such as waste paper fibers containing chemicals added in the papermaking process and inks remaining on the surface of the fibers during the printing process; (4) Because plant fiber is a hydrophilic substance, its hygroscopicity is large.
From the above characteristics, it can be seen that the necessary conditions for foaming molding are:
(1) When plant fiber raw materials are used as foam matrix materials, their particle size should not be too fine. For example, in the chemical foaming process, if the particle size is too small, the entanglement of plant fibers and adhesives is weakened, which is not conducive to the preservation of gases. (2) It must be in the fluid or melt state in order to make the foaming agent as a bubble nucleus evenly dispersed in the system. (3) Adhesive is an indispensable raw material for increasing the strength of plant fiber foaming materials, and is the main condition for molding, so the adhesive should be carefully selected.
03

Plant fiber foaming molding process

Foaming technology
At present, the foaming technology of plant fiber foaming materials can be mainly divided into two types: those that do not use chemical foaming agents and those that use chemical foaming agents. The foaming technology, which does not use chemical blowing agents, is mainly foamed by water vapor, which has excellent biodegradability and can be recycled and reprocessed. The use of chemical foaming agent is the use of chemical foaming agent heating gas foaming, the method is easy to control, commonly used foaming agents are: sodium bicarbonate, azodiisobutycyanide, urea, toluenesulfonylhydrazide, etc.
Molding process
a) One-step molding process: mainly for the overall pouring foaming molding, specifically through the plant fiber raw materials are crushed and fully mixed with foaming agents and other additives and refined, and finally poured in a special mold, because the mold has a limited form, and then the relevant model can be finalized after foaming. Make cushioning packaging products with smaller volume and lighter weight.

Foaming technology
At present, the foaming technology of plant fiber foaming materials can be mainly divided into two types: those that do not use chemical foaming agents and those that use chemical foaming agents. Foaming technology, which does not use chemical blowing agents, mainly uses water vapor for foaming
Two-step molding process: the plant fiber is crushed and milled into a fibrous form of less than 5mm2, so that it is mixed with starch in a certain proportion to mix and granulate with starch. The mixed particles are fed into the extruder to make cylindrical pellets. In the granulation process inside the extruder, the raw material is foamed by the action of water vapor, forming foamed particles. The foamed particles are fed into a special metal mold of the thermoforming machine, and foaming is carried out under pressure in the metal mold. Finally, according to the different metal mold shapes manufactured, the corresponding packaging material products of different shapes can be obtained. It is suitable for making cushion packaging products with large volume and heavy quality.
04

Prospect of foaming and molding process of plant fibers

The research on foaming and molding methods of plant fibers has made great achievements in recent years, but there are also many problems, especially in the foaming process and technology.
1) Focus on the development of foaming technology with water vapor as foaming agent, and completely eliminate the impact of the production and application of foaming materials on the environment.
2) A series of process test studies should be carried out to determine the best production process and method; At the same time, we should strengthen the research and development of plant fiber foaming production and molding technology and equipment, especially in achieving breakthroughs in meeting the requirements of continuous and automated production.
3) A comprehensive and systematic study should be carried out on the characteristics of plant fiber foam packaging materials, so as to make targeted improvements to improve the comprehensive performance of plant fiber foam packaging products.
Generally speaking, in the case of increasingly scarce human resources and increasing awareness of environmental protection, the research of plant fiber foam cushioning packaging materials is the general trend of the development of cushioning packaging materials. It is foreseeable that in the near future, plant fiber cushioning packaging materials will replace traditional plastic cushioning packaging materials.