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PDF Handbook of plastic foams : types, properties, manufacture, and applications

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Even within a certain type of rigidity it is possible to vary the physical properties. Thus, one can make flexible foams that are resilient and also flexible foams which have high energy absorbing properties. In addition, the density of each type can be varied from above 60 lb. Considering a "six pound" density foam, it can vary in resilience from a firm foam to a very soft one. The firm foam will support the weight of a man without crushing, whereas the soft one can be balled up by handling and will slowly return to shape in a period of minutes.

It is seen, therefore, that polyurethane foam can not be regarded as a single entity, but rather as a broad class of materials from which it is possible to select individual foams to fit a desired application. Foam, in its different forms, has many properties which make it desirable for use in artificial limbs.

It is easily fabricated; it has a high strength-to-weight ratio; it resists wear; its shrinkage and resilience can be controlled; it resists bacteria and fungi: it can be made fire-retardant or self-extinguishing; it is easily cleaned and is not affected by soap, detergents and most solvents. Flexible foam finds most applications in cushioning, as for instance in the Total-Contact Socket Fig.

Here a shaped pad is foamed inside the socket, so that close contact is provided between the stump-end region and the soft socket bottom. During the weight-bearing phases of prosthetic use, the forces acting against the stump-end are compensated by the rather free compression of the foam material, providing an even distribution of contact. During the swing phase the intimate contact pressure is lessened but maintained, and by this alternating action a genuine massaging of the stump-end takes place, which works for improvement of the circulation in this stump region.

Another use is in the soft-wall socket. Here, the complete inner socket liner is made of flexible material, a construction which first of all allows the stump muscles to expand, aiding thereby the action of the muscle pump through the elastic counter-pressure of the socket walls; secondly, the pressure-sensitive bone contours are softly bedded in.


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This liner is part of a socket techniaue which often makes is possible to do away with the conventional thigh-lacer and mechanical joints for the below-knee prosthesis. The ideal socket-liner should have varing degrees of flexibility: stiffer ;at weight-bearing areas, more flexible where only padding is desired.

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Also needed is a suitable coating that will do two things: provide a perfect cosmetic finish, and seal the cells. If the cells are open, the foam ads like a cheese grater on the stump. It is hoped that these problems will be solved before long. As another example, in the polyester functional orthesis to aid persons with paralyzed hands and wrists, the layer of material next to the patient's skin is one-eighth inch foam. The foam helps eliminate the problem of discomfort from pressure over the bony prominences in the extremely atrophied hand where little muscular padding is present.

The ability of flexible foam to absorb energy or shock is put to good use in the SACH foot. Besides absorbing impact in the heel and simulating plantar flexion, the flexibility in the rest of the foot is designed so that it simulates normal toe break Fig. When properly constructed, the foot will show little toe curl after millions of flexion cycles.

It is readily molded to any shoe last desired; its fleshy feel is an advantage cosmetically. Lately, shank covers for hydraulic legs are being made out of foam. These serve not only as a cosmetic cover, but also as a cushion to protect the mechanism. They can be pigmented to any flesh color desired, from fair caucasion to dark negroid. Flexible foam is also used in padding and fillers in other prosthetic and orthopedic appliances.

Rigid foam has found different but equally useful applications. Its most prominent use is in the substitution for wood. Wood was, and still is, used extensively in the production of sockets and shanks, feet, and as a filler material. Balsa wood is used as a filler between the ankle block and the below-knee socket because of its lightness. At the present time, wood is still preferred over foam because the wood carving technique has been mastered by limb-fitters, whereas the foam molding technique is still new. Also, wood is finished more readily than is foam. The strength of the foam in comparison with wood is still largely an unknown quantity.

As is common to the general field of plastics, there is a paucity of physical data relative to the rigid foams discussed here.


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Most of the data have been obtained from core samples and, hence, may not be a realistic measure of the physical data desired. An approximate comparison is given in the following tabulation properties of basswood, willow wood and poplar are somewhat similar :. Table 1.

Handbook of Plastic Foams - 1st Edition

These figures are taken from Wood Handbook 72, U. Department of Agriculture, and Modern Plastics Encyclopedia. In general it can be said that, on an equal density basis, rigid foam is somewhat stronger in tension and considerably weaker in compression than wood. It therefore becomes necessary to reinforce the foam in areas under compression. The advantage of foam over wood is that foam lends itself readily for mass-production, in contrast to the slow process of carving each individual wood shank or knee block.

With a mold and the liquid resin material, the model can be readily reproduced in the factory. The mixing is done in batches as needed, and according to the density desired. In newer methods, the "one-shot" polyether foaming system has contributed to production economies. Most low density polyurethane and soy FPF used in furniture have a density between Soy foam density is known to increase with increased biomass [13].

All three foam alternatives pass the regulated smouldering cigarette test, during which the upholstered furniture must not ignite in the presence of a smouldering cigarette [2]. The cellular structure of the FPF alternatives makes it quite flammable even after the addition of phosphorus containing flame retardants. However, flame retardants help these materials meet all fire related regulations [12]. Natural latex foam, on the other hand, surpasses all fire safety and flammability requirements without needing fire-repelling additives [15].

Solid state FPF regular and soy based is non-toxic, and completely inert when in contact with the human body [16].

Similarly, latex foam is naturally hypoallergenic and non-toxic. The open cell structure of latex foam, combined with a careful washing process, ensures removal of most or all of the protein culprits in latex allergic reactions [17]. Latex foam is also dust mite resistant, which is beneficial for some allergy sufferers those allergic to dust mite excrement [18]. If an ultraviolet UV process is used for treating the soy polyol, no odour is created, whereas using a heat treatment creates an odour [19]. The Streamlined Lifecycle Assessment SLCA technique is a method of evaluating the environmental impact of a product or process over its entire lifecycle.

For products, five life stages pre-manufacturing, manufacturing, delivery, use and end of life are evaluated based on five categories of environmental impacts materials choice, energy use, solid residues, liquid residues and gaseous residues. A set of guidelines and scoring criteria are used to aid the evaluation, these can be found in various reference books such as "Streamlined Life Cycle Assessment" by T. The production of natural latex does not leave any solid residues, resulting in a perfect score for the process.

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Polyurethane foam premanufacturing produces more solid residues than the latex foam due to the large sulphur releases associated with mining of the coal used in FPF production. This results in the acidification of the soil and surrounding bodies of water [10]. Conversely, the primary residue of soy farming is the plant stover, which does not have any substantial impact, and is used primarily as animal feed [21].

These possible spills are far more hazardous than spilling natural latex since latex is a natural substance. A major liquid residue, and the largest environment impact of soy farming, is the nitrogen-rich agricultural runoff which is responsible for large dead zones in the aquatic bodies it enters [23] [24] [25] [26]. Emissions from transporting raw materials are the only gaseous residues associated with latex foam. However, since these emissions are also present for the other alternatives, they were disregarded when formulating the scores.

Finally, the emissions from the machinery used in the production of soybeans are also considered adverse gaseous residues. Chemicals of relatively low toxicity are added to the primary raw materials to increase the stability of all three foam alternatives. Likewise, both processes use readily available, natural blowing agents see SLCA Manufacturing - Basics to produce the characteristic bubbles of the foam.

FPF requires the addition of flame retardants, whereas latex foam is able to complete the task naturally. However, since the flame retardants have little if any environmental effects [27] , both processes merit a score of 2, due to the relatively low toxicity of the materials used. The production of latex foam is more energy intensive than that of FPF since latex foam must pass through steam curing and washing chambers and finally a drying oven, each of which consume a lot of energy due to their high operating temperatures [8].

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The production of molded FPF does not require any curing since the chemical reaction which produces the CO 2 is exothermic and produces all the energy needed to cure the foam [16]. Additionally, the latex foam production process does not attempt to utilize a single heating system for the drying oven and the steam curing chamber; a design that would have increased the overall energy efficiency. The only solid residues produced while manufacturing the foam alternatives are small scrap pieces.

Latex foam manufacturing produces substantially more liquid residues than the FPF manufacturing process due to the need to wash the molds after the production of each latex foam cushion, thereby diluting the mold release agents into the cleaning water. This water is neither treated nor reused. In comparison, the FPF manufacturing process produces no liquid residues directly related to the manufacturing line [8].

For both processes, the main contributor to VOC emissions are the naphtha based mold release agents [2]. Additional gaseous residues for FPF foam include carbon dioxide produced in the chemical reaction between isocyanates and water, and the emissions of isocyanates themselves.

Though these residues have low environmental impacts due to their low relative concentrations and the treatment of the emissions [28] , they do not exist in latex foam production, resulting in the latter being given a slightly higher score. The delivery of the final molded foam part to the client is generally the same for polyurethane, soy and latex based foams.

Corrugated fiberboard cardboard boxes are the preferred packaging material in the foam padding industry due to their low cost and ability to contain multiple parts [29]. Since corrugated fiberboard is paper-based, a functioning recycling infrastructure also exists [30]. Depending on the location of the customer, transportation of the product may require air, land or sea vehicles. Since transportation is a major component of the delivery phase, numerous gaseous emissions i. CO 2 are expelled from the various vehicles. The size of an order may also influence the mode of transportation and in turn the amount of emissions due to weight considerations [29].

Once manufactured and assembled, the three alternatives do not require any form of maintenance or care over the course of their useful life. Therefore no material or energy inputs are required, and no liquid, gaseous or solid residues are created with the exception of polyurethane [8]. Generally, the various foams are encased with fabric in office chair use. Therefore, chunking or tearing of the foam is usually not a concern [4]. In the case of polyurethane foam, it has been found that trace amounts of VOCs may be released into the atmosphere over the course of the products life.