Foam plastic are porous polymer materials made of resin with countless tiny cellular voids inside. They are also called microporous plastic or porous plastic and are one of the common porous materials. Foam plastic have the characteristics of low density, high specific strength, ability to absorb impact loads, good heat insulation and sound insulation performance, etc. They have been widely used in industry, agriculture, construction, transportation and other fields. They can be used as packaging materials, sound absorbing materials, thermal insulation materials, building materials, medical supplies, etc.

Foam plastic commonly used include polystyrene, polyvinyl chloride, polyurethane, adenaldehyde and polyethylene. During processing and molding, chemical methods (foaming with a foaming agent during molding) or mechanical methods (air or carbon dioxide is injected while stirring during molding) are used to produce micropores inside. According to the manufacturing conditions, it has open-cell type (pores are interconnected and do not float) and closed-cell type (pores are isolated from each other and can float). According to mechanical strength, it can be divided into three types: hard, semi-hard and soft. By controlling the size and number of pores, products of different densities can be obtained.

The foaming principle of foam plastic is that most polymer foam materials are produced by adding chemical foaming agents or physical foaming agents to resins and raw rubber. Under production process conditions, the foaming agent is decomposed or gasified to produce gas and form pores in the polymer material. Various foaming agents need certain conditions to produce gas and follow their own laws.

Definition

Also known as microporous plastic. Plastic that are covered with countless interconnected or unconnected micropores that significantly reduce the apparent density. It has the advantages of light weight, heat insulation, sound absorption, shock resistance, moisture resistance, and corrosion resistance. According to its pore structure, it can be divided into closed-cell, open-cell and reticulated foam plastic. All the pores of closed-cell foam plastic are almost unconnected. All the pores of open-cell foam plastic are almost connected. Foam plastic with almost no pore walls are called reticulated foam plastic. According to the density of foam plastic, it can be divided into low-foaming, medium-foaming and high-foaming foam plastic. The density is greater than 0.4g/cm3 for low foaming plastic, the density is 0.1~0.4g/cm3 for medium foaming plastic, and the density is less than 0.1g/cm3 for high foaming plastic.

Foamed plastic are polymer materials with plastic as the basic component and containing a large number of bubbles, so they can also be said to be composite plastic with gas as filler. Compared with pure plastic, it has many excellent properties, such as light weight, high specific strength, ability to absorb impact loads, good heat insulation and sound insulation, etc. Therefore, it has been widely used in industry, agriculture, construction, transportation and other fields. Since its advent, foamed plastic have been increasingly used and their varieties have been continuously enriched. Among them, the more common traditional foamed plastic are mainly polyurethane (PUR), polystyrene (PS), polyvinyl chloride (PVC), polyethylene (PE), phenolic resin (PF) and other varieties.

Foamed plastic can be divided into soft, hard and semi-hard foamed plastic in between according to their flexibility. Rigid foam plastic can be used as thermal insulation materials and sound insulation materials, thermal insulation materials for pipes and containers, floating materials and shock-absorbing packaging materials, etc.; soft foam plastic are mainly used as cushioning materials, foam leather, etc. Commonly used foam plastic include polyurethane, polystyrene, polyvinyl chloride, polyethylene, phenolic foam plastic, etc.

Material characteristics

Foam plastic are also called porous plastic. Plastic with countless micropores inside made of resin as the main raw material. Lightweight, heat-insulating, sound-absorbing, shock-proof, and corrosion-resistant. There are soft and hard types. Widely used as insulation, sound insulation, packaging materials and car and ship hulls.

Compared with pure plastic, foam plastic have low density, light weight, high specific strength, and their strength increases with increasing density. They have the ability to absorb impact loads, excellent buffering and shock-absorbing performance, sound insulation and sound absorption performance, low thermal conductivity, good thermal insulation performance, excellent electrical insulation performance, and corrosion resistance and mold resistance. Soft foam plastic have excellent elasticity and other properties.

Plastic with many tiny pores inside. Made by mechanical method (air or carbon dioxide is introduced to make it foam while mechanically stirring) or chemical method (adding foaming agent). It is divided into two types: closed-cell type and open-cell type. The pores in the closed-cell type are isolated from each other and have floating properties; the pores in the open-cell type are interconnected and have no floating properties. It can be made of resins such as polystyrene, polyvinyl chloride, and polyurethane. It can be used as heat insulation and sound insulation materials and has a wide range of uses.

(1) Low bulk density, which can reduce packaging weight and transportation costs;

(2) Excellent impact and vibration energy absorption, which can greatly reduce product damage when used for shockproof packaging;

(3) Strong adaptability to changes in temperature and humidity, which can meet the requirements of general packaging conditions;

(4) Low water absorption, low hygroscopicity, good chemical stability, will not corrode the contents, and has strong tolerance to chemicals such as acids and alkalis;

(5) Low thermal conductivity, which can be used for heat insulation packaging, such as ice cream cups, fast food containers and insulated fish boxes;

(6) Convenient molding and processing, which can be made into various foam pads, foam blocks, sheets, etc. by molding methods such as molding, extrusion, and injection. It is easy to carry out secondary molding, such as foam sheets can be made into various fast food containers by thermoforming. In addition, foam plastic blocks can also be bonded to themselves or to other materials with adhesives to make various cushioning pads, etc.

Classification of use

Open-cell foam plastic are those whose micropores are interconnected, and closed-cell foam plastic are those whose micropores are closed. There are two types of foam plastic: hard and soft. According to the standard of the American Society for Testing and Materials, if the sample does not break after being wrapped around a round rod with a diameter of 2.5 cm within 5 seconds at a temperature of 18 to 29¡ãC, the test sample is a soft foam plastic; otherwise, it is a hard foam plastic. Foam plastic can also be divided into two categories: low foaming and high foaming. Usually, the foaming ratio (the multiple of the volume increase after foaming compared to before foaming) less than 5 is called low foaming, and the one greater than 5 is called high foaming.

1. Hard foam plastic: At room temperature, the polymers that constitute the foam plastic are in a crystalline or amorphous state, and their glass transition temperature is higher than room temperature. Therefore, the texture of the foam plastic is relatively hard at room temperature.

Second, soft foam plastic, that is, the melting point of the polymer constituting the foam plastic is lower than room temperature or the glass transition temperature of the amorphous polymer is lower than room temperature, and the material is soft at room temperature.

Third, semi-rigid (or semi-soft) foam plastic are foams between the above two categories.

In addition, there are many classifications of polyurethane foams. For example, according to the processing method, it can be roughly divided into two-step method (or prepolymer method), semi-prepolymer method, one-step method and spraying method. According to the foam density, it can be divided into high, medium, low and ultra-low density foams and self-skin foams with a large density difference or whole skin foams. The latter is a special form of foam. It does not have a consistent density like the previous foams. It only generates a more normal foam inside the foam, and does not generate bubbles or rarely generates bubbles on the surface. A dense skin layer with a much higher density than the internal foam is formed on the outer surface of the foam. There is a very obvious change in the density drop gradient from the skin to the core center. Among polyurethane soft foams, it is a semi-rigid molded foam product developed to adapt to the application of automobile steering wheels and armrests as well as bicycle and motorcycle saddles.

With the progress of raw materials and processing technology of polyurethane soft foam plastic and the need for commercial specialization of products, classifications based on product functions have emerged, such as soft foam, high-resilience soft foam, hot molded soft foam, cold molded soft foam, super soft foam, antistatic soft foam, lipophilic soft foam, hydrophilic soft foam, energy-absorbing soft foam, etc. These are new types of foam developed with the needs of market refinement and specialization. They have developed rapidly and their classification is also very complicated. The classification of polyurethane soft foams is still based on the traditional classification of processing methods, namely prepolymer method, semi-prepolymer method and one-step method. The classification of polyurethane soft foams based on processing methods, namely prepolymer method, semi-prepolymer method and one-step method, is also another daily classification.

Preparation method

Foaming method

No matter what foaming method is used, the basic process is:

¢ÙIntroduce gas into liquid or molten plastic to produce micropores;

¢ÚMake the micropores grow to a certain volume;

¢ÛFix the micropore structure by physical or chemical methods.

According to the way of introducing gas, the foaming methods include mechanical method, physical method and chemical method.

Mechanical method: With the help of strong stirring, a large amount of air or other gas is introduced into the liquid plastic. This method is mainly used in industry to produce urea-formaldehyde foam plastic, which can be used as thermal insulation materials or film and television scenery materials (such as artificial snowflakes).

Physical method: Low-boiling hydrocarbons or halogenated hydrocarbons are often dissolved in plastic. When heated, the plastic softens, and the dissolved liquid evaporates, expands and foams. For example, polystyrene foam can be made by dissolving pentane into monomers during the suspension polymerization of styrene, or by treating the polystyrene resin that has been polymerized into beads with pentane under heating and pressure to obtain so-called expandable polystyrene beads. The beads are pre-foamed in hot water or steam, and then placed in a mold and steam is passed through to make the pre-foamed particles expand for a second time and fuse with each other. After cooling, a product with the same shape as the mold cavity is obtained (see figure). They are widely used as shockproof materials in insulation and packaging. Extrusion molding can also be used. In this case, either expandable beads can be used and extruded into sheets after foaming once; or ordinary polystyrene pellets can be used, and halogenated hydrocarbons can be added to the appropriate part of the extruder to mix them evenly with the plastic melt. When the material leaves the head, it expands and foams. Extrusion is often used to make sheets or plates. The sheets can be made into food packaging boxes and trays by vacuum forming. Polyethylene can also be made into extruded foam products by similar methods. There are also physical methods for introducing gas, such as dissolution method and hollow microsphere method. The dissolution method is to mix soluble substances such as salt, starch, etc. with resin, and then put the products in water for repeated treatment to dissolve the soluble substances, so as to obtain open-cell foam products, which are mostly used as filter materials. The hollow microsphere method is to mix hollow glass microspheres with a very high melting temperature with plastic melts. Under the molding conditions where the glass microspheres will not break, special closed-cell foam plastic can be obtained.

Chemical method: It can be divided into two categories: ¢Ù Using chemical foaming agents, which decompose and release gas when heated. Commonly used chemical foaming agents include azodicarbonamide, azobisisobutyronitrile, N, N'-dinitrosopentamethylenetetramine, sodium bicarbonate, etc. Many thermoplastic can be made into foam plastic by this method. For example, polyvinyl chloride foam shoes are made by putting the mixture made of resin, plasticizer, foaming agent and other additives into an injection molding machine, the foaming agent decomposes in the barrel, and the material foams in the mold. Foamed artificial leather is made by mixing the foaming agent into the polyvinyl chloride paste, applying or calendering it on the fabric, and continuously passing through a tunnel-type heating furnace. The material is plasticized and melted, the foaming agent decomposes and foams, and after cooling and surface finishing, the foamed artificial leather is obtained. Rigid polyvinyl chloride low-foaming plates, pipes or profiles are formed by extrusion. The foaming agent decomposes in the barrel. When the material leaves the machine head, the pressure drops to normal pressure, and the dissolved gas expands and foams. If the foaming process is properly coordinated with the cooling and shaping process, structural foam products can be obtained. ¢Ú Utilize by-product gases in the polymerization process. A typical example is polyurethane foam plastic. When isocyanate and polyester or polyether undergo condensation reaction, part of the isocyanate will react with water, hydroxyl or carboxyl to generate carbon dioxide. As long as the gas release rate and the condensation reaction rate are properly adjusted, a highly foamed product with very uniform pores can be produced. There are two types of polyurethane foam plastic. The soft open-cell type is similar to sponge and is widely used as cushions for various chairs and sofas, as well as sound-absorbing and filtering materials; the hard closed-cell type is an ideal material for heat preservation, insulation, shock absorption and floating.

Modification

1. Fiber-reinforced foam plastic

Short fibers are generally used to reinforce foam plastic. The specific method is to evenly disperse the short fibers in the polymer system or reaction system to be foamed. After foaming, the fibers are evenly distributed on the foam wall, which plays a role in strengthening, increasing rigidity and improving heat resistance. It is generally believed that the better the interface and the longer the fiber, the better the performance and the more ideal the reinforcement effect.

1.1 Glass fiber reinforcement

The short fiber most commonly used to reinforce foam plastic is short glass fiber (SGF). In order to obtain a good reinforcement effect, the SGF must first be surface treated. When SGF is added to the resin, there will be staggered resin molecular chains connecting the SGF, which is equivalent to cross-linking the resin. When subjected to loads such as bending, stretching, and compression, the resin transfers stress between SGF, allowing SGF and resin to carry the load together, thereby increasing the strength of the foam.

1.2 Nylon fiber reinforcement

Nylon fiber can be used to reinforce PUR foam. This is because the polarity of the nylon molecular chain is strong, and there is a good intermolecular force between it and PUR. In addition, the -NH- on the nylon main chain can form a hydrogen bond with the -C=O on PUR, further increasing the intermolecular force. Therefore, nylon fiber-modified PUR foam can achieve good results.

2. Inorganic particle reinforcement of foam

The addition of inorganic particles to foam is mainly to change its performance and reduce costs. Since there is an interface between inorganic particles and resin, when the interfacial adhesion is large enough, inorganic particles can enhance the foam; in addition, since inorganic particles can often act as nucleating agents in foam, the pores of rigid foam are more fine and uniform, thereby improving the performance of foam.

2.1 CaCO3 reinforcement

CaCO3 used as foam filler mainly has two types: solid phase crushing type and precipitation type. The particle size of the former is about 20 um, and the particle size of the latter is 0. 05~ 10 um. Adding CaCO3 to foam plastic can improve its strength and heat resistance, reduce linear expansion coefficient and shrinkage rate.

2.2 Hollow glass microsphere reinforcement

The diameter of hollow glass microspheres is 10 ~ 100 um, and its spherical surface can reduce stress concentration inside the resin. Under good interface conditions, hollow glass microspheres can improve the compressive strength and compression elastic modulus, tensile strength and tensile elastic modulus, bending strength and bending elastic modulus and heat resistance of rigid foam plastic, and at the same time improve the dimensional stability and friction performance of rigid foam plastic and reduce shrinkage rate. Compared with other particles used for foam plastic reinforcement, hollow glass microspheres are easy to make low-density reinforced foam plastic due to their low density (only about 0.3 g/cm3).

2.3 Nanoparticle reinforcement

China and other countries have conducted extensive research on the application of nanotechnology to modify polymers, and have achieved many technological breakthroughs. They have also successfully prepared various polymer/nanoparticle composite materials, such as polymer/nano-CaCO3, polymer/nano-SiO2, polymer/nano-TiO2, and polymer/nano-clay and other nanocomposites. Compared with the original polymers, their performance has been greatly improved, and the processing performance has also been improved to a certain extent. Although there are few studies on the use of nanoparticles to enhance foam plastic in China, due to the small size of nanoparticles relative to the pore wall and their strong surface activity, it is easy to generate a good interface, and there are more particles per unit volume, which acts as a nucleating agent, making the pore density larger and the pores smaller, which has great potential for the enhancement of foam plastic. It is believed that nanoparticle-enhanced foam plastic will definitely become a new hot spot in the future research on the high performance of foam plastic.

3. Polymer alloy foam plastic

Polymer alloys are obtained by blending and copolymerizing two or more polymers by physical or chemical methods at the same time. The networks formed by these two or more polymers are interpenetrating and entangled with each other, and covalent bonds may exist between different polymers. Polymer alloys are developing rapidly. This method can achieve high performance, low cost, high efficiency and variety of polymer materials. These advantages of polymer alloys can also be reflected in foam plastic. Some people in China are also conducting research on polymer alloy foam plastic.

4. Microporous foam plastic

MCF with thermoplastic plastic as the matrix is called thermoplastic microporous foam plastic, and MCF with thermosetting plastic as the matrix is called thermosetting microporous foam plastic. Thermoplastic MCF has been studied more and has excellent impact toughness (up to 5 times that of solid plastic), high specific stiffness (up to 3 to 5 times that of solid plastic), high fatigue life (up to 5 times that of solid plastic), high thermal stability, low dielectric constant and thermal conductivity. Compared with unfoamed solid plastic, MCF has low density, low cost, can absorb energy, passivate cracks, and has high impact strength. In addition, its bubbles are extremely small, allowing foam plastic parts to be very thin (such as 0.1~1.0 mm). Therefore, MCF is very suitable for manufacturing thin-walled covers, packaging parts, electrical and thermal insulation parts. MCF has a unique microporous morphology, uniform distribution of bubble nuclei and excellent mechanical properties resulting from it. It is very suitable for extremely small-sized foam plastic parts and has attracted much attention in theoretical research and industrial applications. In recent years, as engineering plastic with high performance-price ratio, no pollution to the environment and easy recycling are widely used in construction, transportation, aerospace, packaging, bioengineering and other fields, the research and development of microporous foam plastic has become a hot topic.

Main uses

Structural foam plastic developed in the 1960s are characterized by core foaming and non-foaming skin. They are hard on the outside and tough on the inside, with high specific strength (strength per unit mass) and low material consumption. They are increasingly widely used to replace wood in the construction and furniture industries. The success of chemical or radiation cross-linking foaming technology of polyolefins has greatly increased the output of foam plastic. Foam plastic made by modified plastic such as blending, filling and reinforcement have better comprehensive properties and can meet the needs of various special uses. For example, glass fiber reinforced polyurethane foam made by reaction injection molding has been used as structural components of aircraft, automobiles, computers, etc.; and foam made by filling polybenzimidazole with hollow glass microspheres is light and resistant to high temperatures, and has been used in spacecraft.

With the continuous improvement of the performance requirements of foam plastic in special fields such as aviation and aerospace, traditional foam plastic can no longer meet the special requirements of these fields for material strength, stiffness and heat resistance. Therefore, high performance has become a new direction and hot spot in the research of foam plastic. Foreign countries have used high-performance foam plastic as load-bearing structural materials in the fields of aviation, aerospace, transportation, etc., such as the skeleton of satellite solar cells, the fairing at the front end of rockets, the vertical tail of unmanned aircraft, the body and wings of cruise missiles, and the large radar cover of ships.

New use of polystyrene foam plastic: Combining polystyrene foam blocks with reinforced concrete provides positive buoyancy for the floating airport in Vancouver Port, which is an economical and durable solution. The total cost of the project is 1.6 million Canadian dollars, or about 570 Canadian dollars per square meter. Compared with land development, not only is the cost lower, but it also has the following advantages:

(1) Reduced initial investment;

(2) Shortened construction period, minimizing mid-term investment;

(3) The use of floating airports saves commercial land that can be publicly developed;

(4) Floating airports can be moved at any time;

(5) The maintenance cost of the structure is low;

(6) The friction of the deck is large;

(7) The material is fire-resistant and has excellent tolerance to heat damage in the event of a fire;

(8) The structure is aesthetically attractive and fits well into the coastline landscape.

Production Statistics

Foamed plastic have been increasingly widely used to replace wood in industries such as construction and furniture. Since the fourth quarter, although the investment growth rate of downstream industries such as construction and furniture manufacturing has gradually recovered, the inventory of foam plastic has increased, resulting in a wide range of fluctuations in the output of foam plastic. In 2012, China's foam plastic output reached 1.7206 million tons, a year-on-year increase of 23.13%, an increase of 1.07 percentage points over the first three quarters and a decrease of 2.25 percentage points over the same period last year. Among them, the monthly output increased from 155,700 tons in October to 168,100 tons in November, and then fell to 153,200 tons in December.

Chemical materials

1 Molded polystyrene foam (EPS) board

(1) It has a light weight and a certain compressive and tensile strength. It can support the plastering protective layer by its own strength. It does not require pull-joint parts and can avoid the formation of thermal bridges.

(2) The thermal conductivity of EPS board is the smallest in the range of density of 30-50 kg/m; when the average temperature is 10¡æ and the density is 20kg/m, the thermal conductivity is 0.033-0.036W/(m¡¤K); when the density is less than 15 kg/m, the thermal conductivity increases sharply with the decrease of density; EPS board with density of 15-22 kg/m is suitable for external insulation.

(3) When used for external wall and roof insulation, it generally does not cause obvious moisture problems. However, when one side of the EPS board is in a high temperature and high humidity environment for a long time, and the other side is in a low temperature environment and is sealed by a material with poor water vapor permeability; or when the roof waterproof layer fails, the EPS board may be seriously damp, resulting in a serious reduction in its thermal insulation performance.

(4) When used for insulation of low-temperature pipes such as cold storage and air conditioning, a vapor barrier must be set on the outer surface of the EPS board.

2 Extruded polystyrene foam (XPS) board

(1) It has a unique micro-closed cellular structure. Compared with EPS board, it has the characteristics of high density, high compression performance, low thermal conductivity, low water absorption rate, and low water vapor permeability coefficient. In a long-term high humidity or water immersion environment, XPS board can still maintain its excellent thermal insulation performance. Among various commonly used thermal insulation materials, it is currently the only thermal insulation material that can retain a thermal resistance of more than 80% after two years at 70% relative humidity.

(2) Due to the low long-term water absorption rate of XPS board, it is particularly suitable for inverted roofs and air conditioning ducts.

(3) It also has good freeze-thaw resistance and good compression creep resistance.

3 Rigid polyurethane foam (PUR)

(1) The use temperature is high, generally up to 100¡ãC. After adding heat-resistant auxiliary materials, the use temperature can reach 120¡ãC.

(2) The foaming agent in polyurethane will continuously replace the air in the environment due to diffusion, causing the thermal conductivity to gradually increase over time. In order to overcome this disadvantage, airtight materials such as corrugated steel plates can be used as the surface layer to seal it to limit or slow down this replacement effect.

(3) On-site spray polyurethane foam has a high operating temperature, high compression performance, and simple construction. It is more suitable for roof insulation than EPS boards.

(4) When used for pipelines (especially underground direct buried pipelines) and roof insulation, reliable waterproof and moisture-proof measures should be taken. At the same time, it should be considered that the thermal conductivity will increase over time, and sealing materials should be used as a protective layer as much as possible.

(5) Due to the high operating temperature, it is mostly used for heating pipe insulation.

(6) The smoke temperature is low. When it encounters fire, it produces a lot of thick smoke and toxic gases. It is not suitable for use as an internal insulation material.

(7) Although the water absorption rate is low, as an insulation material, it must not be used as a waterproof material.

4 Polyethylene foam (PE)

(1) Almost no water absorption and almost no water vapor permeability. It will not be damp when used in a humid environment for a long time, so the thermal conductivity can remain unchanged (EPS, PUR, PF, etc. cannot compare with it). It is a soft foam plastic with good flexibility.

(2) Poor compression performance. There is compression creep when used under pressure.

(3) Suitable for low-temperature pipes and air-conditioning ducts.

5 Phenolic foam (PF)

(1) The performance and price are comparable to polyurethane, but the compression performance is lower; however, because its temperature resistance and fire resistance are far superior to polyurethane, it is particularly suitable for high-temperature pipes and occasions with strict fire protection requirements.

(2) Heat resistance and flame retardancy are far superior to polyurethane and other foam plastic. The long-term use temperature can be as high as 200¡ãC, and the intermittent temperature can be as high as 250¡ãC.

(3) PF oxygen index is as high as 50%, smoke density level (SDR) is 4, non-flammable in air, non-melting and dripping. When conducting fire resistance test according to GB 9978-90, the specimen has no obvious deformation and no fire phenomenon.

6 Urea formaldehyde cast-in-place foam plastic (UF)

(1) Aging and mildew resistance, no corrosion to metal after drying.

(2) Suitable for sandwich wall and hollow block filling insulation.

(3) Water is released during the hardening process, so its peripheral materials should have good water vapor permeability to allow the hardened foam to dry fully; if the application space is in a humid state for a long time, or the material is not used for heat preservation but for cold preservation, the humidity problem should be specially considered.

(4) The shrinkage is large during the drying process (the drying shrinkage rate is not more than 4%), cracks may occur in the material, and it is easy to loosen at the contact surface between the material and the space. If this phenomenon is not allowed to occur, it should be proposed to the material supplier in advance.

(5) There is a formaldehyde release problem.

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