Plastic Closed cell

The Dynamit-Nobel extmsion process (252) utilizes a volatile plasticizer such as acetone which is injected into the decompression section of a two-stage screw and is uniformly dispersed in the vinyl resin containing a stabilizer. The resulting PVC foam has low density and closed cells. 

Closed-cell foam Cellular plastic that is composed predominantly of noninterconnecting cells. 

One example is the use of rigid selfexpanding closed cell polyurethane foams as a method to inhibit corrosion of the interior surfaces of metal (steel, etc.) structural cavities exposed to seawater and moisture is one of many example of plastic providing corrosion protection. Unfilled metal cavities are a general feature of various structures or products used in the marine, building, electronics,... 

Most rigid polyurethane foams have a closed cell structure. Closed cells form when the plastic cell walls remain intact during the expansion process and are not ruptured by the increasing cell pressure. Depending on the blowing process a small fraction (5-10%) of the cells remain open. Closed cell structures provide rigidity and obstruct gaseous or fluid diffusional processes. 

A study was made of the impact and recovery behaviour of three HDPE closed-cell foams with varying densities. Impact stress-strain curves were measured using a falling striker impact rig and the recovery monitored from 10s after the impact. Cell deformation was observed during compression and recovery using SEM. Recovery was found to occur by the viscoelastic straightening of the buckled faces and to be incomplete due to plastic deformation in the structure. 6 refs. 

Dow Plastics is to unveil a new resilient polyolefin foam, Strandfoam, which offers packagers of lightweight electronic equipment and components reduced package size, and savings in materials and shipping costs. Strandfoam is made of a special blend of PE and is produced by a proprietary extrusion process that yields fused strands of closed-cell foam with a network of air channels parallel to the foam strands. Strandfoam provides the required levels of protection with substantially smaller volumes of foam compared to urethane. 

BXL Plastics ERP Division is to introduce new grades of antistatic and conductive closed-cell crosslinked PE and ethylene copolymer foam at the Internepcon Exhibition to meet a growing demand from the electronics industry, for ways of minimising the effect of static electricity on circuits embodying static-sensitive devices, particularly using metal oxide/silicone technology. Very brief details are noted of Evazote C conductive closedcell, crosslinked ethylene-vinyl acetate copolymer foam. 

BXL Plastics ERP division is introducing two new grades of antistatic and conductive foams at this years Intemepcon exhibition at the Birmingham NEC in the UK in March 1988. Brief details are given on conductive closed-cell crosslinked EVA copolymer foam, Evazote C, and antistatic PE foam Plastazote AS, both used for electronics packaging. 

Until 1920, the only flexible foam available was the natural sponge, but chemically foamed rubber and mechanically foamed rubber latex were introduced before World War II. These foams may consist of discrete unit cells (unicellular, closed cell), or they may be composed of interconnecting cells (multicellular, open cells) depending on the viscosity of the system at the time the blowing agent is introduced. Over 1.5 million tons of foamed plastic is produced annually in the United States. 

A low (<0.4 W / (m-K)) thermal conductivity polymer, fabricated into alow density foam consisting of a multitude of tiny closed cells, provides good thermal performance. Cellular plastic thermal insulation can be used in the 4—350 K temperature range. Cellular plastic materials have been developed in... 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the structural and thermal properties of insulation materials. Most closed-cell plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and construction practices require adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibility of moisture absorption and retention is always present. The freezing of moisture and rupturing of cells result in permanent reduction of thermal and structural performance. 

Foamed plastics can be classified in different ways, for instance by their nature (flexible vs. rigid), chemical composition of the matrix, density, cell size, cell structure (open-celled vs. closed-celled), processing method, and dimensions. It is the aimed combination of these properties that determines the final application of the cellular polymer. As an example, open-celled ultra-low density foams are highly desirable for acoustical insulation, while rigid foams with closed-cells and elevated densities are preferred as load-carrying core materials in composite materials. 

When plastics are foamed to low densities, containing more air than polymer, they acquire unique new properties and applications. Major uses are in crash padding and thermal insulation. Closed-cell foams are outstanding for flotation, rigidity, and insulation while open-cell foams are outstanding for softness, resilience, and comfort. 

Foam structures consist of at least two phases, a plastic matrix and gaseous voids or bubbles. A closed-cell or open-cell structure is formed, with cellular walls enclosing the gaseous voids. In closed cell foams, the gas cells are completely enclosed by cell walls, while in open-cell foams, the dispersed gas cells are unconfined and are connected by open passages. Plastic can be stabilized against cell rupture by crosslinking (Chapters 1 and 2). 

The final action required is reheating the molded part at which time the plastic softens and the gas expands to form a closed-cell foam. With this technique it is possible to produce foams with densities as low as 2 lb/ft3, although the usual range is 10 to 50 lb/ft3. Because of this two-step procedure the process is much slower than the foaming procedure for open-cell foams. Close cell use includes athletic mats and marine flotation products. 

The acoustic properties of polymers are altered in a cellular structure. Sound transmission changes only slightly, because it depends predominantly upon the barrier density, in this case the polymer phase. Therefore, closed-cell cellular polymers by themselves are poor materials for reducing sound transmission. They are, however, effective in absorbing sound waves of certain frequencies [64]. Materials with open cells on the surface are particularly effective in this respect. The combination of other advantageous physical properties with fair acoustic properties has led to the use of plastic foams in soundproofing [65,66]. The sound absorption of a number of cellular polymers has been reported [7,64,65,67]. 

Rodents chew through cellular polymers but do not ingest the foam as a foodstuff. The resistance to rot, mildew, and fungi is related to moisture absorption [64]. Therefore, open-cell foams support such growth better than closed-cell foams. High humidity and temperature are necessary for the growth of microbes on any plastic foam. 

In one of the few studies of plastic foam behavior at cryogenic temperatures Hingst discovered that upon cooling of closed-cell PUR foam in the 30-70 K range, an abrupt change in the linear temperature dependence of heat conduction occurs (Fig. 15). This phenomenon is closely related to the problem which we are... 

The chief advantage offered by this rule, proposed by Radushkevich is not only that it considerably reduces the number of systems being subject to statistical treatments but also simplifies the latter. For instance, by applying the reversal rule to closed-cell foamed plastics we convert the latter into their reciprocals in which the individual particles (formerly cells) are arranged in such a manner that they have very few contacts, if any. The latter systems formally all exhibit the characteristics of suspensions (or sols) which can at present readily be studied and described using the well-developed statistical apparatus, which is not possible for the original real system (foamed plastic), due to the insurmountable mathematical difficulties involved. 

In order to establish and evaluate quantitatively the relation between the morphology and the properties of foamed polymers the basic macrostructural parameters must be determined. These parameters include relative number of open and closed cells, volumetric weight or apparent density cell size, shape, wall thickness, cell distribution according to size and shape in a given volume and specific surface area of the foamed plastic material. 

These differences in physical structure are responsible for the different properties of foamed polymers containing varying proportions of open cells. In contrast to closed-cell foamed polymers, open-cell foamed plastics have a higher water and moisture absorptive capacity, a higher gas and vapor permeability, less pronounced electric and heat insulation characteristics but a stronger sound absorptive and damping power... 

According to Thomas elastic polymers often produce open-cell foamed plastics, whereas rigid polymers generally form closed-cell materials. However, there are many exceptions to this rule, owing to the variety of blowing techniques. Closed-cell structures are more likely to be produced from polyurethanes, epoxy resins, silicones, poly(vinyl chloride), polystyrene, etc., whereas open-cell materials mainly result from phenolic and carbamide foamed plastics. 

The relation between volumetric weight and the proportion in the polymer of opencell or closed-cell GSE has not yet been studied. It is only known that for any polymer composition the relative percentage of open GSE increases as the volumetric weight of the foamed plastic decreases. This is due to cell growth involving a decrease of the thickness of cell and struts. It adversely affects the aggregation stability and may ultimately cause fracture of the cell walls. 

The volumetric weight and the ratio between the number of open and closed cells are the fundamental morphological parameters of foamed plastics. Nevertheless, it has been reported that even for the same volumetric weight and number of open (or closed) cells the strength and thermophysical parameters may be widely different in plastic foams made of the same polymer grade. The differences in cell shapes and sizes are responsible for this fact. 

Foams made from vinyl resins are of two types, open-cell and closed-cell. The open-cell foams are soft and flexible, whUe the closed-cell foams are predominantly rigid. Both types are made from plastisols, which are suspensions of finely divided resins in a plasticizer. The polymer does not dissolve appreciably in the plasticizer until elevated temperatures are used. 

ASTM D 2856 is the method used to determine the open-cell content of rigid cellular plastics by use of the air-pycnometer. This method is used where porosity of the cellular plastic has a direct bearing on the end use involved. For example, in thermal insulation, a high percentage of closed cells is essential to prevent the escape of gases and thereby promote low thermal conductivity. Also, in flotation applications high closed-cell contents generally prevent water absorption. 

AMS 3635C-84 Plastic Sheet and Strip, Modified Vinyl, Foamed, Closed Cell, 7 pp (DOD Adopted) (FSC 9330) (AS)... 

---