Preparation technology of supercritical foaming material

Bubble foam plastic is formed in the gas dispersion in solid polymer micelles, foam plastic is qualitative light, adiabatic, buffer, insulation, anticorrosion, price is low wait for an advantage, so in daily necessities, packaging, industry, agriculture, transportation, industry, military industry, aerospace industry is widely used, such as the U.S. market in recent years, the demand for plastic foam continued to grow at an annual rate of 4.2%. The development of foamed plastics in China has been very rapid since the 1990s, among which the main varieties are polyurethane (PU) soft and hard foamed plastics, polystyrene (PS) foamed plastics and polyethylene (PE) foamed plastics. But the above three types of materials have certain limitations.

(1) in the process of polystyrene (PS) foaming, fluorochlorinated hydrocarbon compounds or butane are commonly used, which have adverse effects on the environment. As the product degradation is difficult and easy to form "white pollution", the United Nations environmental protection organization has decided to stop using PS foaming products by 2005. According to the national act on the phasing out of ozone depleting substances in China, China has decided to eliminate all fluorocarbons used in foam production by December 31, 2005.

(2) there are isocyanate residues harmful to human body in the foaming process of polyurethane foam sheet, and the foaming material cannot be recycled.

(3) crosslinked polyethylene foam with low rigidity, the highest temperature is 80 ℃, likewise, polystyrene foam is heated to its softening deformation when the glass transition temperature of 105 ℃.

Therefore, foamed polypropylene materials came into being. Polypropylene resin has qualitative light, rich raw material sources, superior price performance ratio and excellent heat resistance, chemical corrosion resistance, easy recycling, etc, is the world's fastest growth in output common thermoplastic resin, the research development of polypropylene foam also became a hot spot in recent years, some developed countries are developing and instead expanded polystyrene green packaging materials, including pp foam particles (EPP) and foamed sheet is the most typical one of the largest varieties and dosage.

Compared with the above three types of foam plastics, polypropylene foam materials have many advantages:

(1) the rigidity of polypropylene (PP) is better than that of PE, the bending modulus of PP is about 1.52Gpa, PE is only 207Mpa, and the chemical resistance is similar to PE. Its compression strength is slightly lower than hard PUR and foaming PS, but higher than soft PUR and PE.

(2) the vitrification temperature of polypropylene is lower than room temperature, has better impact resistance than PS, and compared with the difficulty of recycling PS foam, polypropylene foam is an environmentally friendly material.

(3) polypropylene has high thermal deformation temperature and can be applied in some high temperature fields. Foamed polypropylene usually can withstand temperatures up to 130 ℃, the highest use temperature 80 ℃ than polyethylene foam is much higher. The closed bubble structure makes its thermal conductivity not affected by humidity, so it can be used as insulation material.

(4) good low temperature characteristic: products even when - 30 ℃ show good performance.

(5) energy absorption: due to the excellent energy absorption characteristics of PP foam products, they have excellent anti-pressure and energy absorption performance. It is widely used in the energy handling system of automobile bumper and other anti-collision energy absorber.

(6) stability restoration of size and shape: PP foam products will be quickly restored to original shape after repeated impact and deflection deformation without permanent deformation.

(7) lightweight and reusable: the density of PP foam products can reach a very low level, so the weight can be greatly reduced. Easy to recycle and reuse, easy to decompose, products do not contain toxic and harmful components to human body, combustion does not produce toxic substances.

(8) good surface protection and sound insulation performance: PP foam products are semi-rigid, with moderate hardness and softness, and do not scratch or bruise objects in contact with them. They have good surface protection.

At present, there are more than 70 PP resin manufacturers in China, with more than 100 sets of various devices, and the production capacity is about 4.5 million tons/year. It is predicted that the total production capacity of PP resin in China will exceed 6.5 million tons/year by the end of 2006. However, the application of PP resin in China is mainly knitting products and agricultural films. PP resin industry is faced with the problem of single consumption structure, weak demand and low product grade. All resin production enterprises are actively adjusting the production thinking road, developing marketable, high-quality, high value-added special materials, so that PP has greater application value and economic benefits.

It can be seen that polypropylene foaming products have good performance and application prospect, but the development of polypropylene foam is very difficult. At present, China has not mastered its related and industrialized core technology. In terms of abroad, the core of the foamed polypropylene technology is also in the grip of a few large companies in a few countries, the main reason is: polypropylene is a crystalline polymer, the crystalline melting point below little flow, but the crystallization melting point above the melt viscosity decreases sharply, so in the process of pp foam produced by gas is difficult to melt cover; In addition, polypropylene will release a lot of heat when it changes from molten state to crystalline state. In addition, due to the high permeability of polypropylene, foaming gas is easy to escape, so the temperature range suitable for polypropylene foaming is narrow, and the foaming process is difficult to control. Therefore, PP resin should be cross-linked before foaming in order to get uniform and fine foaming polypropylene products with high foaming ratio, so as to slow down the rate of its melting viscosity, so as to have proper foaming viscosity in a wide range of temperature. But owing to the high crystallinity of polypropylene, methyl beside the main chain, it is difficult to produce crosslinking reaction, and PP macromolecular degradation caused by free radicals is easy to break in the chain, so the crosslinked PP extrusion foaming technology need to be very strict process control, has the high difficulty, but also because of the polypropylene after crosslinking can reduce the degradation of material properties, so from the perspective of the trend of technological development, is the hope that through conventional polypropylene modified to get the crosslinked PP foam products.

2. Domestic and foreign research background and development trend

There are many kinds of polypropylene foaming products, but the largest amount is polypropylene foaming sheet and foam bead molding products. Therefore, this project will focus on the discussion of these two product forms.

Polypropylene foam sheet:

In Japan, toray and guhe electric company first launched foaming PP products in the late 1970s, and in the early 1980s, its PP foam output reached 50 ~ 60t/a. At present, about 4-5 enterprises in Japan can produce PP foam products. Europe began to produce foaming PE in the 1970s, while the crosslinked foaming PP was not industrialized until the 1980s.

In recent years, other countries have developed PP foam products for different USES. Germany Reifenhauser adopts co-extrusion method to produce PP foam sheet with density of 0.1-0.5g /cm3, which can be used for food or meat packaging, thin shell products, dishes, bowls, boxes and other utensils, as well as internal plug-ins for car muffler and insulation materials. Battenfield produces chemically foamed PP sheet with t-head, which is 50% lighter than non-foamed PP sheet. British Packaging Trays moulded foamed PP sheets on rotary thermal molding machines. The company USES chemical crosslinkers to produce foamed PP sheets with a density of 0.5g/cm3, which is mainly used in the automotive industry to make carpet backing materials, shading board, door lining and luggage rack, etc. PP foaming plates with Strandfoam, manufactured by Dow chemical in the United States, are first extruded into layers of foaming sheets and then quickly cooled into hard, highly oriented sheets that can be used as surfboards. The foamed PP, developed by the company, is also used in thermal insulation and cars. Swiss company Alveo has been producing radiation crosslinked PP/PE foam, mainly for the automotive industry. Compared with uncrosslinked foamed products, crosslinked foamed plastics are more likely to retain their shape after hot forming. Oman company adopts Compact EPX T joint extrusion line and adopts heat absorbing foaming agent to produce PP foam sheet with density less than 0.5g/cm3. They cooperate with Reedy-International to extrude 80% of the non-foaming skin and foamed core layer. Canadian Endex Polymer Additives company production of endothermic foaming agent (release CO2) has been used in the production of PP foam sheet for several years. The heat absorbing foaming agent produced by the company has been successfully used in the production of PP, HDPE and PET foaming sheets. With the high melt strength PP resin, the foaming agent produced by the company can be used to shape the foaming film with a thickness of 40 mm.

Polypropylene bubble beads (EPP) :

At present, the world has Japan JSP, Kaneka, Germany BASF, Berstorff with EPP production technology, the products are mainly for molding foam products, package and auto parts (as), the use of pp foam bead forming by the molding process of automobile vibration energy absorption components and impact resistant packaging materials, because of the production process is simple, easy to control, and forming of complex shape parts to occupy a larger market share. CSI has built a production line for producing foamed PP products from the hair PP pellets used to produce food packaging boxes, microwave oven trays and containers.

Pretest probability in China there is no bubble polypropylene (EPP) of industrial production line, the foamed polypropylene products manufacturers, such as the Shanghai tong auto parts co., LTD., Shanghai Iraq than the heat insulation products co., LTD., foshan nanhai sea packaging materials plant and tianjin embellish raw packaging materials co., LTD. Adopt imported pretest probability of polypropylene foam particles, through the secondary foaming and molding process in the production of auto parts and industrial products packaging. For example, some brands of foamed PP bead (EPP) of JSP company sell for up to 100,000 yuan/ton. In addition, EPP particle itself is already a foamed particle with low density (20 ~ 150kg/m2), and the corresponding volume is very large, and the transportation cost in the import price is very high.

At present, several factories of JSP, BASF, GEFINEX and other companies are producing PP foam molding products worldwide. According to incomplete statistics, the consumption of PP foam in the United States was 350kt in 1999, 170kt in Europe, 10kt in Japan and 7kt in other Asian countries. These PP foam products are mainly used to make auto parts and various packaging products.

The research on foaming PP in China started in the late 1970s. Sinopec Beijing chemical research institute undertook the national "ninth five-year" project to develop a new modified PP resin with high melt strength, but it has not yet been industrialized in China.

At present, the industrialization of PP foaming products is still blank in China, and many domestic manufacturers are trying to develop it. However, due to the weak accumulation of research on polypropylene raw materials and foaming process, industrial development is quite difficult, and now they mainly rely on imports to obtain special foaming polypropylene materials.

It can be predicted that the active development and application of polypropylene foaming materials will be of great significance to the development of domestic synthetic resin industry, the expansion of the application field of polypropylene foaming materials and the development of automobile, packaging, food and environmental protection.

3. Product application domain analysis

(1) packaging field

Compared with foamed PS and foamed PE sheet, foamed PP can be used in the microwave oven and is resistant to boiling water. The hot-formed plate has sufficient impact strength at low temperature and can be used in the cryogenic environment with a comfortable and soft surface. For example, the foamed PP sheet with a density of 0.1 ~ 0.5g/cm3 and a thickness of 1 ~ 3.5mm can be used for food or meat packaging. It can also be used for making thin shell products and various containers (plates, dishes, bowls, boxes, etc.). The foamed PP sheets with density of 0.5-0.7g /cm3 and thickness of 0.5-1.5mm are raw materials for the production of tableware (such as plates, bowls) and beverage cups with high rigidity and good heat insulation; The foamed PP board with density of 0.2-0.5g /cm3 and thickness of 1.0-3.5mm is used for the production of meat packaging materials, tableware, or processing into POTS and dishes, and for the packaging of low-acidity food such as apple sauce and lactic acid cheese in containers.

In order to reduce the use of PS resin which is harmful to human body, most developed countries use polyolefin resin to make various meal boxes. Previously used transparent PP sheet or extruded PP single - layer foaming sheet. Since May 1999, PP single-layer foamed sheet was officially adopted to make various tableware and containers that can be heated in microwave oven. Due to good safety and not easily affected by heat, it has been widely used in making food packaging containers since 1998. PP production lines have been built in the United States, Austria, Denmark and other places to produce food packaging boxes and microwave oven trays and containers.

Ofotec Folien, a German company that produces foamed PP sheets, believes that PP foam plates tend to replace EPS moulding plates, because users are interested in the fact that PP foam plates can be added directly from the roll to the molding -- canning -- sealing device, and the plates can be made steeper, so they occupy less shelves and are popular with merchants. In addition, the recyclable foamed PP moulded tray is very convenient for the processing and transportation of fruits and other agricultural products.

Due to the high heat resistance, PP is widely used in abroad instead of PS foam sheet heat resistance can not meet the requirements of the occasion, such as used in 90 ℃ ~ 120 ℃ heat carrier loop of insulation materials, engine room and battery workshop of insulation materials, heaters, and must be in the car and places, such as a visor, instrument on the insulation board and higher temperature easily between two doors. In summary, all the parts with heat resistance problems when using PE foaming materials can use foaming PP.

(2) automotive field:

PP foam material used in auto parts has become increasingly important in recent years. PP foam has high heat resistance, high impact energy absorption capacity, good resilience and thermal molding, recycling is also an important factor. This enables the implementation of a fully PP manufacturing component scheme, such as in the instrument panel, using GMT or talc to enhance PP manufacturing brackets, using PP foam as a flexible intermediate layer, and using soft sealing film as the outer layer. With this scheme, the plastic parts of the car can be disassembled, cleaned and remelted for use in lower grade products such as the inner lining of the wheel arch after being scrapped. However, this process is limited due to the high cost and complex process. All these problems will be solved if the foamed PP with a smooth surface is produced by thermal forming and bonded to the injection PP or ABS base. From this point of view, efforts continue to be made to use foamed PP for interior decoration materials such as car dashboards.

PP foam sheet can be used in the production of automotive doors waterproof or soundproof panels, luxury cars rear engine compartment. The rigid PP foam board reinforced with glass felt sheets can be used to produce automobile engine cover, load-bearing floor and spare tire cover. Currently, the foamed PP plates with a density of about 0.6g/cm3 and a thickness of 2 ~ 6mm can be processed into automobile LIDS and boxes, etc., and the pp-crosslinked foamed materials with a density of 0.035 ~ 0.085g/cm3 and a thickness of 1.5 ~ 6.0mm can be used as lining materials and damping materials in the automobile industry.