Foam plastic is a type of polymer material formed by a large number of gas micropores dispersed in solid plastic. It has the characteristics of light weight, heat insulation, sound absorption, shock absorption, etc., and its dielectric properties are better than the matrix resin, and it has a wide range of uses. Almost all kinds of plastics can be made into foam plastics, and foam molding has become an important field in plastic processing.

Definition

Also known as microporous plastic. The whole plastic is covered with countless interconnected or non-connected micropores, which significantly reduces the apparent density. It has the advantages of light weight, heat insulation, sound absorption, shock resistance, moisture resistance and corrosion resistance. According to its cell structure, it can be divided into closed cell, open cell and reticulated foam plastics. Almost all cells of closed-cell foam are not connected. Almost all cells of open-cell foam are connected. Foams with almost no cell walls are called reticulated foams. According to the density of foam plastics, it can be divided into low foaming, medium foaming and high foaming foams. Those with a density greater than 0.4g/cm³ are low-foaming foams, those with a density of 0.1~0.4g/cm³ are medium-foaming foams, and those with a density less than 0.1g/cm3 are high-foaming foams.

Foam plastic is a polymer material with plastic as its basic component and containing a large number of bubbles. Therefore, it can also be said to be a 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, and good heat and sound insulation properties. Therefore, it has been widely used in industry, agriculture, construction, transportation and other fields. Since the advent of foam plastics, its uses have become increasingly widespread and its varieties have been continuously enriched. Among them, the more common traditional foam plastics mainly include polyurethane (PUR), polystyrene (PS), polyvinyl chloride (PVC), polyethylene (PE), phenolic Resin (PF) and other varieties.

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

Material characteristics

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

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

Plastic with many tiny pores inside. It is produced by mechanical method (injecting air or carbon dioxide while mechanically stirring to make it foam ) or chemical method (adding foaming agent). It is divided into two categories: 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 connected to each other and have no floating properties. It can be made of polystyrene, polyvinyl chloride, polyurethane and other resins. It can be used as thermal insulation and sound insulation materials and has many uses.

1. The bulk density is very low, which can reduce the packaging weight and transportation costs;
2. It has excellent impact and vibration energy absorption, and can greatly reduce product damage when used in cushioning and shock-proof packaging;
3. It has strong adaptability to changes in temperature and humidity and can meet the requirements of general packaging conditions;
4. Low water absorption, low hygroscopicity, good chemical stability, will not cause corrosion to the contents, and has strong tolerance to chemicals such as acids and alkalis;
5. Thermal conductivity is low and can be used for thermal insulation packaging, such as ice cream cups, fast food containers, and insulated fish boxes;
6. The molding process is convenient, and various foam liners, foam blocks, sheets, etc. can be made by molding, extrusion, injection and other molding methods. It is easy to carry out secondary molding processing. For example, foam sheets can be made into various fast food containers by thermoforming. In addition, foam plastic blocks can also be bonded to themselves or with other materials using adhesives to make various cushioning pads, etc.

Use categories

The micropores that are connected to each other are called open-cell foams, and the ones that are closed to each other are called closed-cell foams. There are two types of foam plastics: hard and soft. According to the standards of the American Society for Testing and Materials, at a temperature of 18 to 29°C, within 5 seconds, the test sample is wrapped around a round rod with a diameter of 2.5cm. If it does not break, the test sample is a soft foam plastic; otherwise, it is a rigid foam plastic.. Foam plastics can also be divided into low foaming and high foaming categories. 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 foaming ratio greater than 5 is called high foaming.

1. Rigid foam plastics means that at room temperature, the polymers that make up foam plastics are crystalline or amorphous, and their glass transition temperatures are higher than normal temperature. Therefore, the texture of foam plastics is relatively hard at room temperature.

2. Soft foam plastic means that the melting point of the polymer that constitutes the foam plastic is lower than normal temperature or the glass transition temperature of the amorphous polymer is lower than normal temperature, and the material is soft at normal temperature.

3. Semi-rigid (or semi-soft ) foam is a foam between the above two categories.

In addition, there are many classifications of polyurethane foam. 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 method, it can be divided into high, medium, low and ultra-low density foams, as well as self-skinned foams with very different densities or called integral 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 relatively normal foam inside the foam, while no bubbles or very few bubbles are produced on the surface. The surface forms a dense skin layer with a much higher density than the internal foam, and there is a very obvious density drop gradient from the skin to the center of the core. Among polyurethane flexible 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 advancement of raw materials and processing technology of polyurethane flexible foam plastics and the need for commercial specialization of products, classification based on product functions has emerged, such as soft foam, high resilience soft foam, thermo-molded soft foam, Cold molded soft foam, ultra-soft foam, antistatic soft foam, lipophilic soft foam, hydrophilic soft foam, energy-absorbing soft foam, etc., are all new foam varieties that have been developed in response to market refinement and specialization needs., is developing rapidly, and the classification is also very complicated. The classification of polyurethane flexible foams is still based on the traditional classification of processing methods, namely prepolymer method, semi-prepolymer method and one-step method. Polyurethane flexible foams are classified by processing methods, namely prepolymer method, semi-prepolymer method and one-step method, which is another daily classification.

Preparation method

Foaming method

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

1. Introducing gas into liquid or molten plastic to generate micropores;
2. Make the micropores grow to a certain volume;
3. Fix the microporous structure through physical or chemical methods.

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

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

Physical method: Low-boiling hydrocarbons or halogenated hydrocarbons are often dissolved into plastics. When heated, the plastic softens, and the dissolved liquid evaporates, expands, and foams. Such as polystyrene foam, during the suspension polymerization of styrene, pentane can be dissolved into the monomer first, or the polystyrene resin that has been polymerized into beads can be treated with pentane under heat and pressure to produce the so-called Expandable polystyrene beads. The beads are pre-expanded in hot water or steam, and then placed in a mold and passed through steam to cause the pre-expanded particles to expand twice and sinter with each other. After cooling, a product with the same shape as the mold cavity is obtained (see picture). They are widely used as insulation and shock-proof materials in packaging. Extrusion molding can also be used. At this time, you can use expandable beads, which are foamed and extruded into sheets at one time; you can also use ordinary polystyrene pellets and add halogenated polymers to the appropriate parts of the extruder. Hydrocarbon is mixed evenly with the plastic melt, and the material expands and foams when it leaves the machine head. The extrusion method is often used for sheets or plates, which can be made into food packaging boxes and trays after vacuum molding. Polyethylene can also be made into extruded foam products in a similar way. Physical methods for introducing gas include dissolution method, hollow microsphere method, etc. The dissolution method is to mix soluble substances such as salt, starch, etc. with resin, shape them into products, and then repeatedly process the products in water to dissolve the soluble substances to obtain open-cell foam products, which are often used as filter materials. The hollow microsphere method is to mix hollow glass microspheres with high melting temperature with plastic melt. Under the molding conditions that the glass microspheres will not break, special closed-cell foam plastics can be produced.

Chemical method: can be divided into two categories: ① Use chemical foaming agents, which decompose and release gas when heated. Commonly used chemical foaming agents, such as azodicarbonamide, azobisisobutyronitrile, N, N′ -dinitrosopentamethylenetetramine, sodium bicarbonate, etc. Many thermoplastics can be made into foams using this method. For example, polyvinyl chloride foam shoes are made by putting a batch of resin, plasticizer, foaming agent and other additives into an injection molding machine. The foaming agent decomposes in the barrel and the material is foamed in the mold.. Foam artificial leather is made by mixing the foaming agent into the polyvinyl chloride paste, scraping or calendering it on the fabric, and continuously passing through the tunnel heating furnace. The material is plasticized and melted, the foaming agent decomposes and foams, and is cooled and surface -finished, that is, Get foam leatherette. Rigid polyvinyl chloride low-foaming sheets, pipes or special-shaped materials 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 gas dissolved in it expands and foams. If If the foaming process and the cooling and shaping process are properly coordinated, structural foam products can be obtained. ② Utilize the by-product gas during the polymerization process. A typical example is polyurethane foam. When isocyanate and polyesterWhen polyether or polyether undergoes polycondensation reaction, part of the isocyanate will react with water, hydroxyl or carboxyl groups to generate carbon dioxide. As long as the gas release rate and polycondensation reaction rate are properly adjusted, highly foamed products with very uniform cells can be produced. There are two types of polyurethane foam. The soft open-cell type is shaped like a sponge and is widely used as cushions for various seats and sofas as well as sound-absorbing and filtering materials. The hard closed-cell type is ideal for heat preservation, insulation and shock absorption. and floating materials.

Modified

1. Fiber reinforced foam plastics

Generally, short fiber reinforced foam plastics are used. The specific method is to evenly disperse 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, stiffening 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 fibers most commonly used to reinforce foam are short glass fibers (SGF). In order to obtain good reinforcement effects, SGF must first be surface treated. When SGF is added to the resin, there will be interlaced resin molecular chains connecting the SGF, which is equivalent to cross-linking the resin. When subjected to bending, tension, compression and other loads, the resin transfers stress between the SGF, allowing the SGF and the resin to bear the same load, improving the strength of the foam plastic.

1.2 Nylon fiber reinforcement

Nylon fiber can be used to reinforce PUR foam. This is because the nylon molecular chain is highly polar and has good intermolecular force with PUR. In addition, -NH- on the nylon main chain can form hydrogen bonds with -C=O on PUR, further increasing the intermolecular force. force. Therefore, nylon fiber modified PUR foam can achieve good results.

2. Inorganic particle reinforced foam plastics

The main purpose of adding inorganic particles to foam plastics is to change its properties and reduce costs. Due to the interface between inorganic particles and resin, when the interface adhesion force is large enough, the inorganic particles can enhance the foam; in addition, because the inorganic particles often act as nucleating agents in the foam It can make the cells of rigid foam plastics more fine and uniform, thereby improving the performance of foam plastics.

2.1 CaCO3 enhancement

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 plastics can improve their strength and heat resistance, and reduce their 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, compressive elastic modulus, tensile strength, tensile elastic modulus, flexural strength, flexural elastic modulus and heat resistance of rigid foam plastics, while also improving It can improve the dimensional stability and friction performance of rigid foam plastics and reduce shrinkage. Compared with other particles used for foam reinforcement, hollow glass microspheres are easy to produce low-density reinforced foam due to their low density (only about 0.3 g/cm3).

2.3 Nanoparticle enhancement

Extensive research has been carried out on the application of nanotechnology to modify polymers, many technological breakthroughs have been achieved, and various polymer/nanoparticle composite materials have been successfully prepared, such as polymer/nano CaCO3, polymer/nano SiO2, polymer/nano TiO2 and polymer/nano clay and other nanocomposites. Compared with the original polymer, its performance has been greatly improved, and its processing performance has also been improved to a certain extent. Although there are few china’s studies on the use of nanoparticles to enhance foam plastics, the small size of nanoparticles relative to the cell walls and their strong surface activity make it easy to form a good interface and have a greater number of particles per unit volume. The role of the nucleating agent makes the cell density larger and the cells smaller, which has great potential for the reinforcement of foam plastics. It is believed that nanoparticle-enhanced foam plastics will definitely become a new hot spot in the research on high-performance foam plastics in the future.

3.Polymer alloy foam

Polymer alloys are produced by blending and copolymerizing two or more polymers at the same time through physical or chemical methods. The networks formed by these two or more polymers interpenetrate and entangle each other, and different polymerizations Covalent bonds may exist between substances. 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 plastics, and some people in China are also conducting research on polymer alloy foam plastics.

4. Microcellular foam plastic

MCF based on thermoplastic plastic is called thermoplastic microcellular foam, and MCF based on thermosetting plastic is called thermosetting microcellular foam. There are many studies on thermoplastic MCF, which has excellent impact toughness (up to more than 5 times that of solid plastics), high specific stiffness (up to 3 to 5 times that of solid plastics), and high fatigue life (up to more than 5 times that of solid plastics). ), high thermal stability, low dielectric constant and thermal conductivity. Compared with unfoamed solid plastics, MCF has low density, low cost, can absorb energy, can blunt cracks, and has high impact strength. In addition, its cells are extremely small, allowing foam plastic parts to be very thin (such as 0.1~1.0 mm). MCF is therefore ideally suited for the manufacture of thin-walled covers, packaging, and electrical and thermal insulation parts. MCF has a unique micropore morphology, uniform bubble core distribution 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 plastics with high cost performance, no pollution to the environment and easy recycling have been widely used in construction, transportation, aerospace, packaging, bioengineering and other fields, the research and development of microcellular foam plastics has become a hot topic.

Main purpose

Structural foam plastics developed in the 1960s are characterized by foaming in the core layer and non-foaming in the cortex. They are hard on the outside and tough on the inside. They have high specific strength (strength per unit mass) and consume less material. They are increasingly widely used to replace wood. Used in the construction and furniture industries. The success of chemical or radiation cross-linking foaming technology of polyolefins has greatly increased the production of foam plastics. Foam plastics made by blending, filling, reinforcing and other modified plastics have better comprehensive properties and can meet the needs of various special purposes. For example, glass fiber-reinforced polyurethane foam made by reaction injection molding has been used as structural parts for aircraft, automobiles, computers, etc.; and foam made by filling polybenzimidazole with hollow glass beads is lightweight and resistant to high temperatures., has been used in spacecraft.

As the performance requirements for foam plastics in special fields such as aviation and aerospace continue to increase, traditional foam plastics can no longer meet the special requirements for material strength, stiffness and heat resistance in these fields. Therefore, high performance has become a new direction and hot spot in foam plastics research. High-performance foam plastics have been used abroad as load-bearing structural materials in aviation, aerospace, transportation and other fields, such as the skeleton of satellite solar cells, the fairing at the front of a rocket, the vertical tail of an unmanned aircraft and the body and wing of a cruise missile., large radar domes for ships, etc.

New uses for Styrofoam: Combining Styrofoam blocks with reinforced concrete provides positive buoyancy for a floating airport in the Port of Vancouver, an economical and durable solution. The total cost of the entire project is 1.6 million Canadian dollars, or about 570 Canadian dollars/m2. Compared with development on land, not only the cost is reduced, but it also has the following advantages:

1. Reduces initial investment;
2. The construction period is shortened and medium-term investment is minimized;
3. The use of floating airports saves commercial area land that can be publicly developed;
4. The floating airport 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 thermal damage in the event of fire;
8. The structure is aesthetically appealing and fits well into the shoreline landscape.

General varieties and applications

Molded polystyrene foam (EPS) board

(1) It is light in weight and has certain compressive and tensile strength. It can support the plaster protective layer by its own strength. It does not require pull-joints and can avoid the formation of thermal bridges.

(2) EPS board has the smallest thermal conductivity value in the density range of 30-50 kg/m; at an average temperature of 10°C and a density of 20kg/m, the thermal conductivity is 0.033-0.036W/(m·K); density When less than 15 kg/m, the thermal conductivity increases sharply as the density decreases; EPS boards with a density of 15 to 22 kg/m are suitable for external insulation.

(3) When used for exterior wall and roof insulation, there is generally no obvious moisture problem. 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 closed by a material with poor water vapor permeability; or when the roof waterproof layer fails, the EPS board may be seriously affected by moisture, thus affecting its thermal insulation performance. Severely reduced.

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

Extruded polystyrene foam (XPS) board

(1) It has a unique fine closed-cell honeycomb structure. Compared with EPS boards, it has the characteristics of high density, high compression performance, small thermal conductivity, low water absorption, and small water vapor permeability coefficient. XPS board can still maintain its excellent thermal insulation performance in long-term high humidity or water-immersed environments. Among various commonly used insulation materials, it is the only one that can maintain a thermal resistance retention rate of more than 80% after two years at 70% relative humidity. Insulation material.

(2) Due to the low long-term water absorption of XPS boards, they are especially suitable for inverted roofs and air conditioning ducts.

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

Rigid polyurethane foam (PUR)

(1) The use temperature is high, generally up to 100°C. After adding temperature-resistant accessories, the use temperature can reach 120°C.

(2) The foaming agent in polyurethane will continue to replace the air in the environment due to diffusion, causing the thermal conductivity to gradually increase over time. In order to overcome this shortcoming, air-impermeable materials such as profiled steel plates can be used as surface layers to seal them to limit or slow down this displacement effect.

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

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

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

(6) The smoke temperature is low and a large amount of thick smoke and toxic gases are produced in case of fire. It is not suitable to be used as internal insulation material.

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

Polyethylene foam (PE)

(1) It is almost non-absorbent and almost impermeable to water vapor. It will not get damp when used in a humid environment for a long time, so the thermal conductivity can remain unchanged (EPS, PUR, PF, etc. cannot be compared), and it is a soft foam plastic. Has very good flexibility.

(2) The compression performance is poor, and compression creep occurs when used under pressure.

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

Phenolic foam (PF)

(1) The performance and price are equivalent to those of polyurethane, except that the compression performance is lower; however, because its temperature resistance and fireproof performance are far better than polyurethane, it is especially suitable for high-temperature pipelines and occasions with strict fireproof requirements.

(2) The heat resistance and flame retardancy are far superior to polyurethane and other foam plastics. The long-term use temperature can be as high as 200℃, and the intermittent temperature is allowed to be as high as 250℃.

(3) The PF oxygen index is as high as 50%, the smoke density level (SDR) is 4, it does not burn in the air, and does not melt or drip. When the fire resistance test was conducted according to GB 9978-90, the specimen showed no obvious deformation and no flashover of fire.

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

(1) Resistant to aging and mold, and does not corrode metal after drying.

(2) Suitable for filling insulation of sandwich walls and hollow blocks.

(3) Moisture is released during the hardening process, so the surrounding materials should have good water vapor permeability to allow the hardened foam to fully dry; if the application space is in a humid state for a long time, or the material is not used for heat preservation but cold preservation, it should be damp Issues are given special consideration.

(4) If the shrinkage is large during the drying process (drying shrinkage is not greater than 4%), cracks may occur in the material, and looseness may easily occur at the contact surface between the material and the space. If this phenomenon is not allowed to occur, this should be reported to the material supplier in advance.

(5) There is a formaldehyde release problem.