Steam-expanded foam

Once the product is allowed to emerge from the extruder the water flashes into steam, expanding the product to a foam. Any protein and emulsifiers present help stabilise the bubbles, which set as the product cools. As the water flashed to steam the latent heat of the steam is lost to the product thus cooling it rapidly. 

Expandable polystyrene is the usual name for the beads used to make molded polystyrene foam parts. Parts made with these beads are usually made in a two-step process. The first step consists of pre-expansion of the beads by heat. After this step, the beads are stored for 6-12 h to allow them to reach equilibrium. The beads are then conveyed to the mold, where they expand to the final dimensions. Steam is the preferred mode of heating, either by introducing live steam through perforations in the mold or by the means of steam probes, which are withdrawn as the beads are expanding. Expandable polystyrene foam can be distinguished from expanded polystyrene foam by the round bead outlines on the surface of the part. The expanded foam has a smooth surface. 

By steaming with saturated water steam, the foam particles are heated to the fusing temperature. The plastic material becomes soft and formable. At the same time, the blow agent evaporates and the gas in the cells tries to expand. 

Figure 6 summarizes results of a series of experiments in which the bromine sorption and ammonia stages were carried out on unfoamed PS beads, containing 6% of absorbed gaseous pentane. The porofica-tion treatment did not affect the pentane and the smaples could be expanded by steam in the usual manner, to obtain foam densities of 0.0145 g/cm. It is noteworthy that the amounts of NH Br deposited are much smaller than in the case of the foamed samples 1 and 2. 

Pre-expansion a steam flow heats the beads of EPS, which are expanded to about 40 times their original size by the boiling of the pentane. Thus, a closed-cell foam is formed. 

Expandable PS beads are a material devised to accommodate the transportation drawbacks of foams. Foams take up a lot of room, but not much weight, so a truck or boxcar cannot be used very efficiently. Expandable PS beads can be readily turned into foam at their destination. The beads are impregnated with a volatile liquid like pentane as they are extruded, chopped, and cooled. Later, on site, the beads are heated in small batches with steam. The vaporization temperature of the pentane is just below the melting point of the PS beads. As the beads soften, the pentane flashes (volatilizes) and causes the PS to foam. The polymer is then ready for molding. Coffee cups, ice chests, life preservers, buoys, and floats are often fabricated this way. 

The article describes the process of producing expanded polypropylene foam parts with a solid integral skin on one side, citing a bicycle helmet moulded from BASF s Neopolen EPP bead, as an illustration. The self-skinning EPP process reqnires some modification of standard beadmoulding equipment, needing an extra steam chamber. Automotive interior trim is a potential application for the process. 

The Polymer. Amorphous Polystyrene. All of the expandable polystyrene referred to above is the amorphous type that is obtained by free radical initiation. This polymer is completely noncrystalline, and in the absence of impurities such as monomer and blowing agent it exhibits a glass-transition temperature of about 100°C. Both the rate of expansion and the extent of expansion are enhanced by reducing molecular weight, but the foam becomes less resistant to collapse on further steaming (63). Other polymeric modifications are discussed below. 

Isotactic Polystyrene. The familiar steam molding of pre-expanded particles has so far not been applied successfully to isotactic polystyrene. However, the polymer has been foamed, according to three disclosed methods. For example, finely divided acetone-insoluble polymer, with a melting point in excess of 200°C., is blended with a liquid selected from methylene chloride, aromatic hydrocarbons, or halogenated aromatic hydrocarbons. This blend is then heated (84). A mixture of molten polymer and methyl chloride, propane, or butane is suddenly depressurized (8). Foam may also be generated in a continuous manner directly from a butyllithium-initiated polymerization conducted in the presence of a 4/1 blend of benzene and petroleum ether (15). 

The production and application in 1967 of 193 million pounds of polystyrene foam reflects considerable scientific, engineering, and commercial activity. The continuing growth of expandable polystyrene in this field is attributed to its ability to be steam molded economically into a variety of useful items. 

The single most important factor responsible for the rapid commercial growth of expandable PS is its ability to be steam-moulded into lightweight, closed-cell, low-cost foams suitable for beverage cups, packages, ice buckets, picnic chests, insulation board, etc [11]. (Figure 1.10). 

Spherical beads that can be expanded into foam under the influence of heat or steam are produced directly by suspension polymerization in the presence of blowing agent. The term suspension polymerization describes a process in which water-insoluble monomers are dispersed as liquid droplets with suspension stabilizer and vigorous stirring to produce polymer particles as a dispersed solid phase. Initiators used in suspension polymerization are oil-soluble. The polymerization takes place within the monomer droplets. The kinetic mechanism of the suspension process is considered to be a free radical, water-cooled microbulk polymerization [1]. 

The collets from the expander are hot and moist and are often cooled and dried prior to entering the extraction process. The dryers, which may be equipped with steam heating, typically remove about 2% moisture (most of that added by live steam in the expander) and reduce temperamre by about 40°C. The dryers also allow time for the collets to approach a more uniform moisture and temperature. Some plants report an increased tendency for the misceUa in the second effect evaporator to foam or to coat or plug the evaporator tubes. It is thought that this may be related to a lack of drying and a nonuniformity of the collet such that the center is hot and moist, causing water-related problems in extraction. 

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