Foam Latex
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Foam latex or latex foam rubber is a lightweight form of latex containing bubbles known as cells, created from liquid
latex Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latices are found in nature, but synthetic latices are common as well. In nature, latex is found as a wikt:milky, milky fluid, which is present in 10% of all floweri ...
. The foam is generally created though the Dunlop or Talalay process in which a liquid latex is foamed and then cured in a mold to extract the foam. Structural enhancements are applied to a foam by making different choices of polymers used for the foam or through the use of fillers in the foam. Historically, natural rubber latex is used for the foam, but a similar commercial contender is styrene-butadiene latex, which is especially designed for use in latex foams. Mineral fillers may also be used for the enhancement of properties like stability, load bearing, or flame resistance, but these fillers often come at the cost of lowered tensile strength and extension at break, which are generally desirable properties in the product. Latex foam has properties of energy absorption, thermal conductivity, and compression that make them suitable for many commercial applications like upholstery, soundproofing,
thermal insulation Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with s ...
(especially in construction), and transportation of goods. Foam latex is also used in
masks A mask is an object normally worn on the face, typically for protection, disguise, performance, or entertainment, and often employed for rituals and rites. Masks have been used since antiquity for both ceremonial and practical purposes, ...
and facial prosthetics to change a person's outward appearance. ''
The Wizard of Oz ''The Wizard of Oz'' is a 1939 American Musical film, musical fantasy film produced by Metro-Goldwyn-Mayer (MGM). Based on the 1900 novel ''The Wonderful Wizard of Oz'' by L. Frank Baum, it was primarily directed by Victor Fleming, who left pro ...
'' was one of the first films to make extensive use of foam latex prosthetics in the 1930s. Since then, it has been a staple of film, television, and stage productions, in addition to use in a number of other fields. Single use plastics and polymer foams are often disposed of in landfills, and there is a growing concern about the amount of space this waste takes up. In an effort to make the foams more environmentally friendly, research is being done into fillers than can achieve the same enhancements as mineral while also increasing biodegradability of the product. Examples of such fillers include eggshell powders and rice husk powders.


Structure

Latex foam is a form of latex that is lightweight and expanded. Cellular air bubbles are created inside liquid latex, and they can be shaped into different shapes and sizes. The extension of the foam is defined by the amount of air inside of these cells. Lower density and more extended foams tend to have cells which are more
polyhedral In geometry, a polyhedron (: polyhedra or polyhedrons; ) is a three-dimensional figure with flat polygonal faces, straight edges and sharp corners or vertices. The term "polyhedron" may refer either to a solid figure or to its boundary surfa ...
, while less extended foams tend to have more spherical cells. While the density of the foam (\rho_f) can be measured, a more important property is relative density of the foam to the density of the original latex base (\rho_s). This is expressed as \phi=\dfrac. Polymer foams will also have some ratio of closed cells to open cells (air bubbles which have been burst open), which can be measured through the water permeability of the foam.


Creation

To create foam latex, a liquid latex base is mixed with various additives and whipped into a
foam Foams are two-phase materials science, material systems where a gas is dispersed in a second, non-gaseous material, specifically, in which gas cells are enclosed by a distinct liquid or solid material. Note, this source focuses only on liquid ...
, then poured or injected into a mold and baked in an oven to cure. The main components of foam latex are the latex base, a
foaming agent A foaming agent is a material such as a surfactant or a blowing agent that facilitates the formation of foam. A surfactant, when present in small amounts, reduces surface tension of a liquid (reduces the work needed to create the foam) or increas ...
(to help it whip into a froth), a
gelling agent In polymer chemistry, gelation (gel transition) is the formation of a gel from a system with polymers. Branched polymers can form Cross-link, links between the chains, which lead to progressively larger polymers. As the linking continues, larger ...
(to convert the liquid foam into a gel), and a curing agent (to turn the gelled foam latex into a solid when baked). A number of additional additives can also be added depending on the required use of the foam.


Dunlop Process

The Dunlop process can be performed in batch form and in a continuous form. The following is a description for the batch process. # Different ingredients for the latex foam are prepared, including the choice of liquid latex, compounding agents, and stabilizers, are prepared for usage. # Deammoniated
liquid latex Liquid latex is a compound often used for special effects Cosmetics, makeup, body painting, mask making, and casting applications. Composition Liquid latex is usually made of 33% latex, 66% water, and less than 1% ammonia (to increase its she ...
is mixed with stabilizer and other ingredients, either as dispersions or
emulsion An emulsion is a mixture of two or more liquids that are normally Miscibility, immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloi ...
s depending on solubility in water. # The compound is gently stirred and allowed to mix. Fillers may be added at this point. The compound may be left to mature for 24 hours. # A Hobart mixer whips the compound to cause it to foam, incorporating differently sized bubbles into it and allowing it to expand to a desired size. # The whipping speed is reduced, and the bubbles assume a more regular size. A foam stabilizer can be added now. # A gelling agent can be added next, and then the compound is poured into a mold where it is allowed to gel and cure over time. Uniformity is a highly sought after property commercially, and performing the Dunlop process in a continuous manner rather than in batches helps increase the uniformity of the produced foams. Other advantages of the continuous process is the decreased labor cost and lowered waste product from the mold. The continuous process includes the use of a machine with different chambers for the creation and foaming of the mixture, addition of fillers, and molding and curing.


Talalay Process

# Different ingredients for the latex foam are prepared, including the choice of liquid latex, compounding agents, and stabilizers, are prepared for usage. # Deammoniated liquid latex is mixed with stabilizer and other ingredients, either as dispersions or emulsions depending on solubility in water. # The compound is gently stirred and allowed to mix. Fillers may be added at this point. The compound may be left to mature for 24 hours. # Through decomposition of hydrogen peroxide by yeast, bubbles are created which cause the foaming of the compound inside of the specialized mold. # A vacuum is applied to the mold to promote expansion. # The compound is then quickly frozen to create air bubbles. # Finally, the compound is allowed to cure and removed from the mold. The disuse of a gelling agent in preference for carbon dioxide makes the process more environmentally friendly, but the Talalay process is still not widely used for specialized latex foams industrially.


Properties


Expansion and density

In general, latex foams have lower density than the original polymer they are made of. This density can be measured regularly by taking a volume and mass measurement of the material. For a volume measurement of irregularly shaped foam, the foam pieces can be coated with wax and inserted in a known volume of water to measure volume change in the container. The purpose of the wax is to prevent water permeation into the foam, which may lead to a lower perceived volume (and higher perceived density as a result) if not accounted for. The density of a foam decreases as the expansion of the foam increases. Expansion, in turn, relates to the amount of air inside the cells of the foam. The more air inside the cells, the larger the expansion.


Compression

Latex foams demonstrate a stress-strain curve with three regions when compressed. This relates to the resistive force expressed by the foam when a load or force is applied to it. The shape of different regions of the curve will reflect some important quality of the foam relating to compression or relaxation stress and strain behaviour of the material. First, the foam will show a linear Hookian increase in stress. This happens because the gas contained in foam cells is compressed, and the walls of the cells maintain their structure. In the second region, the cell walls are being crushed, and no additional stress is experienced, and so the stress plateaus. In the third region, the foam increases in density as crushed cell wall material is compressed into itself. This leads to a steep increase in stress in the region of densification.


Resistance to dynamic fatigue

Relating to the longevity of the material, the resistance to dynamic fatigue is tested by recursively compressing a foam and allowing it to relax. The resistance of the foam to dynamic fatigue can then be measured either by visually observing the structure of the cells to note what proportion of cell walls has broken or ruptured, or by measuring the change in physical properties like the thickness of the material.


Thermal Conductivity

The low
thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...
of latex foams is affected by four factors: heat conduction of the polymer, heat conduction of the gas within the air bubbles,
convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...
of gas inside the cells (less important for small to medium size cells), and
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
through the foam. There are several ways conductivity can be affected through these factors: # lower temperature to lower heat radiation; # decrease cell size to decrease convection and radiation (due to more reflections within the cell walls); # decrease foam density to decrease conduction through the solid polymer; # replace air for a less conductive gas inside the cells.


Energy Absorption

Energy absorption is a particularly important quality of latex foam. Most energy absorption occurs in the first and second regions of the strain-stress curve. In less elastomeric polymers, the cell walls are more brittle and therefore can get crushed more easily. In this case, most of the absorption occurs in the second region of the curve caused by the deformation and crushing of cell walls. This means that each cell can only contribute once to such absorption (that is, cells are getting crushed and therefore used up). For a more elastomeric polymer, the cell walls are more flexible and can take more impact. The cell wall in this case may bend and the cell becomes squeezed, but the cell will eventually return to its original shape. Most energy absorption therefore occurs in the first region of the stress-strain graph. The foam can also handle more instances of impact as the cells do not become depleted as easily. This is a significant environmental improvement.


Classification and Additives


Choice of Polymer


Traditional polymer choices

Historically, natural rubber latex was used, and foams were produced using the Dunlop processes.
Styrene-butadiene Styrene-butadiene or styrene-butadiene rubber (SBR) describe families of synthetic rubbers derived from styrene and butadiene (the version developed by Goodyear is called Neolite). These materials have good abrasion resistance and good aging ...
rubber latex rose to prominence once high-solids concentrates, which were designed specifically for foaming, began to be sold on the market. Properties of this polymer were fairly similar to natural rubber latex, so the competition between the two choices here is mostly economical.


Polymer choices for variation in properties

Other kinds of polymers were chosen for their properties and how they affect the properties of the foam in turn. For example,
polychloroprene Neoprene (also polychloroprene) is a family of synthetic rubbers that are produced by polymerization of chloroprene.Werner Obrecht, Jean-Pierre Lambert, Michael Happ, Christiane Oppenheimer-Stix, John Dunn and Ralf Krüger "Rubber, 4. Emulsion Rub ...
foam rubber is more difficult to burn and provides a less flammable alternative to traditional latex foam. Acrylonitrile-butadienelatex foam rubber is resistant to swelling in hydrocarbon oils.


Fillers


Structural fillers

These are fillers meant to increase the stability and load bearing capabilities of the foam latex while increasing expansion and therefore lowering the coast of materials. However, adding fillers also affects the desirable properties of the latex foam, such as by decreasing extension at break and resistance to repeated occurrences of stress and relaxation. Mineral fillers like kaolinite clays and
calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...
s can be added during the whipping phase (in the batch process) or mixing phase (in the continuous process) to the latex foam. Wet-ground
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...
s can similarly be added into the latex during foaming, and they tend to have a lower impact on
tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
and extension at break. However, micas tend to cause more shrinking to the product at the unmolding phase.


Flame Retardants

Since latex foams are a fire hazard, there are efforts to incorporate fillers into the foams to decrease their flammability. Such fillers include chlorinated paraffin hydrocarbons,
antimony trioxide Antimony(III) oxide is the inorganic compound with the formula Sb2O3. It is the most important commercial compound of antimony. It is found in nature as the minerals valentinite and senarmontite. Like most polymeric oxides, Sb2O3 dissolves in ...
, zinc borate, and hydrated
aluminum oxide Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...
.


Naturally Sourced Fillers

These are materials that improve the structural properties of latex foam while also making it more environmentally friendly through increased
biodegradability Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...
. A particular interest is using organic waste products to create these fillers. Eggshell powder is an example of such a filler which can be added into the latex foam to manipulate the properties of the product and increase its environmental friendliness. Similarly to mineral fillers, eggshell powder increases compression stress, compression set, hardness, and density of the foam while decreasing tensile strength and extension at break. This filler also decreases the
thermal stability In thermodynamics, thermal stability describes the stability of a water body and its resistance to mixing.Schmidt, W. 1928. Über Temperatur und Stabilitätsverhältnisse von Seen. Geogr. Ann 10: 145 - 177. It is the amount of work needed to tra ...
of the material produced, but adding
resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
, another possible organic filler, was found to increase the tensile strength of eggshell powder filled natural rubber polymer foam. Another proposed filler with similar properties was rice husk powder, which increases load bearing properties of the foam while decreasing tensile strength and extension at break. This was also found to increase the biodegradability of the foam for improved control over post-consumer waste of these products.


Applications


Transportation

Due to their energy absorption properties, latex foams are useful for transportation applications, such as in packaging to decrease impact on the shipped product or in vehicle
upholstery Upholstery is the work of providing furniture, especially seats, with padding, springs, webbing, and fabric or leather covers. The word also refers to the materials used to upholster something. ''Upholstery'' comes from the Middle English wor ...
. While packaging foams may be single-use with low resistance to dynamic fatigue, upholstery tends to benefit from being denser and more resistant to fatigue as it absorbs lower impacts but needs to do so more repeatedly.


Furniture

Latex foams can be used in items like bedding, upholstery, and pillows for cushioning purposes due to their expressed stress-strain curve when experiencing a load.


Soundproofing

As they contain air bubbles, latex foams carry some soundproofing properties. In particular, both natural rubber and styrene-butadiene latex foam are found to be good at soundproofing, but styrene-butadiene foams tend to be better for this purpose.


Separation of oil and water

Oil pollution An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially the marine ecosystem, due to human activity, and is a form of pollution. The term is usually given to marine oil spills, where oil is released into ...
in water bodies is a major environmental concern. Separating oil and water is helpful both to clean the water and recover the oil. Latex foams are
hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...
and absorbent, in addition to being resilient and recyclable, and can therefore be used to absorb the oil in water-oil mixtures to separate them.


Sports, arts, and recreation

Foam latex is used in masks and facial prosthetics to change a person's outward appearance. The Wizard of Oz was one of the first films to make extensive use of foam latex prosthetics in the 1930s. Theatrical latex foam is a specialized latex foam which is softer than commercial latex foam. It can be used in various arts and crafts including puppetry and costumes because of its ability to pick up small details of painting as well as its strength. Miss Piggy, Statler and Waldorf in
Jim Henson James Maury Henson (September 24, 1936 – May 16, 1990) was an American puppeteer, animator, actor, and filmmaker who achieved worldwide notability as the creator of the Muppets. Henson was also well known for creating ''Fraggle Rock'' ( ...
's
The Muppet Show ''The Muppet Show'' is a variety sketch comedy television series created by Jim Henson and starring the Muppets. It is presented as a variety show, featuring recurring sketches and musical numbers interspersed with ongoing plot-lines with ru ...
as well as characters in Henson's next production,
The Dark Crystal ''The Dark Crystal'' is a 1982 dark fantasy, dark fantasy film directed by Jim Henson and Frank Oz. It stars the voices of Stephen Garlick, Lisa Maxwell (actress), Lisa Maxwell, Billie Whitelaw, Percy Edwards, and Barry Dennen. The film was prod ...
, were some of the first puppets created from latex foams used on a large scale. Artists such as
Lordi Lordi () is a Finnish Rock music, rock band. Formed in 1992 by the band's lead singer, songwriter and costume maker Mr Lordi (Tomi Petteri Putaansuu), Lordi are known for wearing monster masks and using horror elements with pyrotechnics during c ...
and
GWAR Gwar, often stylized as GWAR, is an American heavy metal band formed in Richmond, Virginia, in 1984, composed of and operated by a frequently rotating line-up of musicians, artists, and filmmakers collectively known as Slave Pit Inc. Since ...
wear costumes that include this material. Latex foam is also widespread in the manufacture of modern soccer goalkeeper gloves. The material has proven to be the most effective way of allowing players to grip the football in wet and dry playing conditions, as well as providing damping properties which help in catching. A variety of treatments are applied to latex foam to produce different types of foam with varying properties to assist performance. Some, for example, are designed to offer a high level of grip; whereas others are designed to offer maximum durability.{{Cite web, title=Guide: All you need to know about latex for goalkeeper gloves {{!, url=https://www.unisportstore.com/blog/7163-guide-all-you-need-to-know-about-latex-for-goalkeeper-gloves/, access-date=2021-05-23, website=www.unisportstore.com, date=25 February 2017


References

Cosmetics Prosthetics Rubber Special effects Stagecraft