Hard foam plastics refer to foam plastics that have no flexibility, high compression hardness, can produce deformation when stress reaches a certain value, and cannot return to its original shape after stress is released. ISO also stipulates that foam plastics that are compressed to 50% and then reduce their thickness by more than 10% after the pressure is released are hard foam plastics. They are often used in related aspects due to their thermal insulation and mechanical properties. Representative products are polystyrene foam plastics, hard polyurethane foam plastics, and foam plastics such as phenolic, amino, epoxy, and thermosetting acrylic resins, as well as hard polyvinyl chloride foam plastics. They can be used as thermal insulation materials, interlayer materials, packaging materials, sound insulation and shockproof materials, building materials, etc.

Definition and explanation

Rigid foam plastic is a type of foam plastic, which refers to foam plastic with an elastic modulus greater than 700MPa.

Characteristics of foam plastic

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

(2) Excellent absorption of impact and vibration energy, which can greatly reduce product damage when used for cushioning and 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-insulating 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 processing, such as foam sheets can be made into various fast food containers through thermoforming. In addition, foam plastic blocks can also be bonded to themselves or to other materials with adhesives to make various cushioning pads.

Foaming methods

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, dinitrosopentamethylenetetramine, sodium bicarbonate, etc. Many thermoplastics can be made into foam plastics 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 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 sheets, 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 the by-product gas in the polymerization process. A typical example is polyurethane foam. When isocyanate and polyester or polyether undergo polycondensation 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 polycondensation reaction rate are properly adjusted, a highly foamed product with very uniform pores can be made. There are two types of polyurethane foam plastics. The soft open-cell type is similar to sponge and is widely used as seat cushions of various seats and sofas, as well as sound absorption and filtering materials; the hard closed-cell type is an ideal thermal insulation, insulation, shock absorption and floating material.

Classification

There are many classification methods for foam plastics, and there are three common ones.

Classification by hardness

It is divided into three categories: soft foam plastics, hard foam plastics and semi-hard foam plastics.

The elastic modulus of soft foam plastics is less than 70MPa

The elastic modulus of hard foam plastics is greater than 700MPa

The elastic modulus of semi-hard foam plastics is 70~700MPa

Classification by density

It is divided into three categories: low foaming, medium foaming and high foaming.

The density of low foaming foam plastics is greater than 400kg/m3

The density of medium foaming foam plastics is 100~400kg/m3

The density of high foaming foam plastics is less than 100kg/m3

Structural classification

The micropores that are interconnected are called open-cell foam plastics, and the micropores that are closed to each other are called closed-cell foam plastics.

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