Plastic Product Material and Process Selection Handbook Part 11 pps

In addition to the basic plastics in liquid and bead forms with foaming agents, fillers, additives that include cell controllers and fire-retardants, catalysts, surfactants, styrene mono

Table 8.I Examples o f r'gid Flas:'c fcarr prcpe'ties

Polyvinyl Chloride Pheaytene Phenolic

Rigid Oxide

ASTM Foamed in Syntactic Closed Foamable

Polyethylene Polystyrene Polyurethane

Polycarbonate Density Foam Molded Extruded Cell

8 9 Foaming 3 3 5

their uses and applications continue to advance at a rapid pace The major plastics used as foams are the polyurethanes and polystyrenes (Chapter 2)

In addition to the basic plastics in liquid and bead forms with foaming agents, fillers, additives that include cell controllers and fire-retardants, catalysts, surfactants, styrene monomer, systems that vary viscosity from liquid to paste form, and other additives are used The gas can be put directly in to the plastic before the plastic solidifies Reactant chemicals can be put in the plastic formulation that during polymerization will release a gas and produce the foam

Very popular are extruding expanded polyethylene, polypropylene, polyvinyl chloride, and polystyrene (Chapter 5) Specially designed extruders can handle a mixture of plastic and a gas foaming agent such

as nitrogen The material expands as it leaves the die Foaming will take place with a mixture of plastic and blowing agents when put under pressure Blowing agents used include methyl chloride, propylene, or butylene A wide range of properties can be obtained in foamed vinyls

by just using carbon dioxide These types of foam materials find applications in the liquid and food serving container consumer markets Foam sheet made from expandable polystyrene beads containing pentane is extruded

The technology of polyurethane (PUR) foams has been developing since its inception during the early 1940's in Germany, followed by USA and the rest of the world This foam packaging material provides specific advantages It insures firm support and restraint for the product's interior by adapting itself to a product's complex contours Parts can perform multifunctional use: insulation and load carrying, insulation and ease of application, or buoyancy and structural rigidity For example, urethane foamed-in-place in a boat hull or hydrofoils makes the vessel virtually unsinkable, reduces noise level, and reduces structural vibration

Foamed plastics, like their solid counterparts, can be used for almost an unlimited range of products As an example there are different approaches

to spray-foamed homes Since about the 1950s foamed building structures where fabricated using polystyrene foamed plastics The initial development was by the US Army Since then many other foamed structures have been built worldwide using different plastics

An interesting approach was designed and built during 1966 Dome shaped buildings were being built using polystyrene (PS) boards by the Dow's spiral generation technique Craftsmen heat bond the boards in

a continuous pattern to produce the dome shaped medical clinical

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structures located in Lafayette, Indiana Boards were heated and bonded at the softening point of the PS in order to form a continuous pattern that produces the dome shape Sections cut from the dome were made into doors, connecting halls going from dome to dome These domes are structurally self supporting, requiring no internal or external support during or after manufacture It also provides its own insulation and other advantages

Similar to other materials, foams have limitations No foam is fireproof but many of them can be made flame-resistant Phenolics and silicones have excellent heat resistance but could crumble when subjected to vibrational stress if not modified There are foams that can be affected

by solvents, but fluorinated types resist them However these plastics with modifiers provide acceptable performances

There are various combinations of plastics and blowing agents to fabricate different products Basically during the process a blowing agent expands the plastic initiating cells that grow to produce the final foam As gas is produced equilibrium is established between material in the gas phase and the material dissolved in the solid state The gas dissolved in the solid state migrates from the solution into the gas phase The cells formed are initially under higher than ambient pressure because they must counteract the effects of the plastic's surface tension The pressure due to surface tension depends on the reciprocal of the cell radius so the pressure within the cell is reduced as the cell grows Different techniques are used to control this foaming action

Small cells tend to disappear and large cells tend to get larger This is because the gas migrates through the matrix or substrate (plastic) or the cell walls break After forming cells, the foam has to be stable; the gas must not diffuse out of the cell too quicldy, thereby causing collapse or excessive shrinkage The stability of the foam depends on the solubility and diffusivity of the gas in the matrix The many processes make for many methods of cell initiation, cell growth, and cell stabilization 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 arc connected by open passages Plastic can be stabilized against cell rupture by crosslinking (Chapters 1 and 2)

A basic distinction is made between closed-cell systems, where spherical

or roughly spherical voids (cells) are fully separated by matrix material, and open cell systems where there are interconnections between voids

8 9 Foaming 3 3 7

The degree of interconnection can be assessed if a sample is subjected

to a moderate vacuum; a liquid is then allowed to fill the inter- connected spaces and the weight gain is measured The cell size or average cell size can be an important factor A distinction is sometimes made between microcellular foams 0.1 to 10 micron diameters They correspond roughly to cells indistinguishable with the naked eye and macrocellular foams (at least 250 micron) With microcellular foaming products can be produced that are lightweight, high strength, and are thin walled (such as 0.5 m m thick)

The cell density (number of cells per unit cross-section area or volume)

is also used to characterize the coarseness or fineness of foam Foamed products can feature a deliberately created inhomogeneous (nonuniform) morphology An example is when a foamed core is sandwiched between solid skins as in so-called structural foams, or in elastomeric products with so-called integral skins With cells elongated in the direction of foam rise or melt flow, the process will give an anisotropic structure and properties (Chapter 15)

Blowing agent

Different foaming agents (also called blowing agents) are used to produce gas and thus to generate cells or gas pockets in the plastics The type of blowing agents used influences all kinds of physical, mechanical, electrical, thermal conductivity, and other properties The amount of blowing agent used affects the properties of the foamed plastic, and different amounts are required for particular applications.: about 0.1wt% for elimination of sink marks in injection molded parts, 0.2 to 0.8% for production of injection molded structural forms, 0.3% for extruded foamed profiles, 1 to 15% for formation of vinyl foams, and 5 to 15% for compression-molded foam products Nucleating and cell-sizing agents can be added to produce cells of a more uniform size and to enhance the symmetrical expansion of cells during the foaming process

Foaming methods vary widely One is to whip air into suspension or a solution of the plastic, which is then hardened by heat curing A second

is to dissolve a gas in a mix, then expand it when the pressure is reduced Another is to heat a mixture until one of its liquid com- ponents volatilizes Similarly, water produced in an exothermic chemical reaction can be volatilized within the mass by the heat of reaction A different technique uses a chemical reaction to produce carbon dioxide gas within solid mass A related way is for a gas such as nitrogen to be

3 3 8 Plastic Product Material and Process Selection Handbook

liberated within a mass by thermal decomposition of a chemical blowing agent Other techniques disperse small solid particles, tiny beads of plastic, or even glass microballoons within a plastic mix or syntactic foam

The most common method disperses a gaseous phase throughout a fluid plastic phase then preserves the resulting combination, this is called the dispersion process The expansion process consists of the following actions:

1 creation of small discontinuities or cells in a plastic fluid phase,

3 stabilization of the resultant cellular structure by physical or

chemical means The gas phase is usually distributed in voids or pockets called cells They can be foamed open-cell but usually they are foamed closed-cell

The most popular blowing agents arc classified as physical or chemical, depending on how the gas is generated Physical blowing agents (PBAs) undergo a change of state during processing, while chemical blowing agents (CBAs) usually solids, undergo a decomposition reaction during processing that results in formation of a gas PBAs are compressed gases

or volatile liquids Compressed gases, usually nitrogen, are injected under high pressure such as 2,000 psi, into the plastic melt during processing As the pressure is relieved, the gas becomes less soluble in the plastic melt and expands to form cells Nitrogen is inert, non- flammable, and can be used at any processing temperature No residue

is left in the foamed plastic, so that recycling of the plastic part is easy When using compressed nitrogen, however, generally the result is to produce foams with a coarser cell structure and poorer surface appearance than nitrogen produced with CBAs, although nucleating agents can be added for a finer cell structure

There are liquid PBAs that are volatile and change from a liquid to a gaseous state when heated to the plastic processing temperatures They are short-chain chlorinated and fluorinated aliphatic hydrocarbons (CFCs) Although they can be used over a wide temperature range and

at low (atmospheric) pressures, they have been gradually discontinued due to their role in the reduction of stratospheric o z o n e 249 Other PBAs are reviewed in Table 8.2

Chemical blowing agents (CBAs) decompose at various processing temperatures to form a gas (Table 8.3) The most important criterion for selection of a chemical blowing agent is that the decomposition temperature matches the processing temperature of the plastic Little or

Sodium Bicarbonate (NaHCO 3) 100-140 135

Alkali Carbonate (Hydrocerol)

Alkali Carbonate (Activex)

Alkali Carbonate (Safoam~

160+ 100- 160

120 140 170-210 130

Exo EVA, HDPE, LLDPE, LDPE, PE TPE, FPVC Exo HDPE, FPVC

Exo EVA, HDPE, LLDPE, LDPE, PE TPE, FPVC Exo PP, PC

Endo LDPE, EVA, FPVC, TPE Endo LDPE EVA, LLDPE, FPVC Endo LDPE, EVA, FPVC Endo EVA HDPE LLDPE

no foaming will occur at processing temperatures below the decomposition temperature With processing temperatures too high the result can be in overblown or ruptured cells and poor surface quality

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~ ~ ~ ~ ~ - ~ ~ : : : ~ ~ - : : : ; c ~

Activators such as alcohols, glycols, antioxidants, and metal salts, can be added to lower the decomposition temperature Other selection considerations include the type and amount of gas liberated and its effect on the final product

CBAs can be classified as inorganic or organic Their decomposition can

be endothermic or exothermic Endothermic blowing agents, usually inorganic, require the input of energy for the decomposition reaction to take place, while exothermic blowing agents, usually organic, release energy during decomposition Exothermic CBAs commonly have a higher gas yield than endothermic CBAs The lower gas yield and pressure associated with endothermic CBAs produce foams with a smaller cell structure, resulting in improved appearance and physical property performance Endothermic and exothermic CBAs have been combined in a single product, in which the exothermic CBA provides the gas volume and pressure necessary for lower densities, and the endothermic CBA produces a fine, uniform cell structure

CBAs are available as dry powders, liquid dispersions, and pellet concentrates They can be incorporated by dry-blending with the resin powder, tumble-blended with resin pellets, blended using a hopper blender, metered in at the feed throat, or pumped into the barrel Typical inorganic blowing agents are sodium bicarbonate, sodium borohydride, polycarbonic acid, and citric acid, which primarily evolve into carbon dioxide gas upon decomposition Sodium bicarbonate is the most common inorganic blowing agent It is inexpensive, and it decomposes endothermically at a low temperature, over a broad temperature range (100 to 140C (212 to 284F) At temperatures _142C (287F), decomposition becomes more rapid, facilitating its use

in polyolefins Its decomposition is less controllable than organic blowing agents, however, and it can form an open-celled foam structure Its gas yield is 267 cc/g Polycarbonic acid decomposes at about 160C (287F), with a gas yield of about 100 cc/g It is also used as a nucleating agent in physical blowing agents

Organic CBAs evolve gas over a specific, narrow temperature range and are selected according to the processing temperature of the plastic The most common low temperature blowing agent is 4,4"oxybis benzenesulfonyl hydrazide) (OBSH), with a decomposition temp- erature of 157 to 160C (315 to 320F) and gas yield of 125 cc/g High temperature blowing agents, with decomposition temperatures of greater than about 230C (450F), include 5-phenyltetrazole, with a decomposition temperature of 240 to 250C (460 to 480F) and trihydrazine triazine (THT)

8 9 Foaming 341

As an example azodicarbonamide (ABFA), with a decomposition temperature of 204 to 213C (400 to 415F) is commonly used in PP (melting temperature is 168C; 334F) The use of activators can reduce the decomposition temperature to 150C (300F) ABFA is a yellow powder that decomposes exothermically, with a gas yield of about 220 cc/g, to produce a gas mixture containing 65% nitrogen ABFA produces a fine, uniform cell structure but can produce discoloration in the foamed part It is nontoxic and is FDA-approved for a wide variety

of applications, including those involving food contact The high gas yield, good performance, and low cost of ABFA make it a widely used foaming agent

Other agents used include p-toluenesulfonyl semicarbazidc (TSSC), although it decomposes at an intermediate-to-high temperature [228

to 236C (442 to 456F)] Activators can be used to decrease the decomposition temperature It has a gas yield of about 140 cc/g; the gas mixture consists of nitrogen, carbon monoxide, carbon dioxide, and trace amounts of ammonia Its white color and nonstaining residue are important in applications requiring color quality It is flammable and burns rapidly when ignited, producing a large amount of residue

The overwhelming majority of foams are TPs, but TSs are also foamed with CBAs, although some of them do create problems Popular TS foams are made from polyurethane, polyester, phenolic, epoxy, and rubber Thermal decomposition of the blowing agent with certain plastics such as TS polyesters cannot be applied in this system because the heat of polymerization is not high enough to induce decom- position But chemical reactions simultaneously produce gas and free radicals; they typically involve oxidation and reduction of a hydrazine derivative and peroxide The reactions are catalyzed by metals, which can be used repeatedly

Polyurethane foams (often referred to as urcthane foams) are prepared

by the reaction of a polyisocyanate with a polyol in the presence of a blowing agent, a surfactant, and a catalyst without external beating of the foaming system The principle of preparation of urcthanc foams is based on the simultaneous occurrence of two reactions, i.e., poly- urethane formation and gas generation in the presence of catalyst and surfactant In flexible urcthane foams, the major blowing agent is water and, at the same time, auxiliary blowing agents An example of a P U R foam mix is the polyol, polyisocyanate, chemical blowing agent, catalyst, and surfactant that generates gas and produces P U R foam

With the ban on the use of CFCs (chlorofluorocarbons) major changes

in foam formulations developed 249 A number of studies were carried

342 Plastic Product Material and Process Selection Handbook

out on the use of 100% water-blown foams for both rigid and flexible foams Other agents included pentane These studies required modifi- cations or improvements in raw materials (such as polyisocyanates, polyols, catalysts and surfactants) TM, 438,468

The polyisocyanates which can be used for preparing isocyanate-based foams are mainly aromatic compounds and some aliphatic or aralkyl polyisocyanates TDI (toluene diisocyanate) is widely used for flexible foams Pure MDI (diphenylmethane diisocyanate) is used for elastomers and coatings Modified TDI and modified M D I are used for high- resilience flexible foams Polymeric isocyanates (polymeric M D I or oligomeric MDI) are mostly used for preparing rigid urethane and isocyanurate foams, and in part, for preparing flexible and semi-flexible foams

Water may at first appear to be an unlikely blowing agent for plastic foams because of its low volatility and low solubility compared to CBAs 249 However, because manufacturers have started to realize the cost, storage, handling and environmental benefits of using water, its use as a blowing agent has increased They offer reduction of product weight and increased production rate opportunities In addition, the components are recyclable and exhibit excellent long-term physical properties for scaling and weathering Water foaming can be accomplished by modifying a standard single screw extruder Special requirements are focused, as with other blowing agents, on precise metering of water injection, temperature control, and mixing of the water with the plastic such as TPE Liquid temperature control of barrel zones and a minimum of a 30:1 L / D extruder are usually required Water injection takes place at the 18:1 L / D position with a pump capable of at least 10 MPa with an adjustable flow rate up to 10 ml/min

Chlorofluorocarbon and Alternate

CFCs are a family of inert, nontoxic, nonflammable, and easily produced liquefied chemicals that have principally been used in refrigeration, air conditioning, packaging, and insulation or as solvents and aerosol propellants (medical and other devices) The plastics industry, as well as other industries, has been phasing o u t C F C s , 252

which were once widely used in producing foam products 249 CFCs chlorine components reportedly destroy ozone in the upper atmosphere A targeted worldwide complete phase-out of CFCs was soon among the amendments to the Montreal protocol approved unanimously by 93 nations at a 1987 meeting in London Participating

8 9 Foaming 3 4 3

nations also agreed to use hydrochlorofluorocarbon (HCFC) only where other alternatives were not feasible The alternative H C F C (hydrochlorofluorocarbon) is 98% less ozone depleting than CFCs

Fully halogenated CFCs were eliminated in polystyrene foam food packaging and containers Substitute blowing agents used are either no threat to the ozone or are a 95% improvement over fully halogenated CFCs Action has been taken such as where PS foam cups now are 100% CFC-free, etc

Type of foam

Not all types of plastic foams possess all properties in the desirable ratio

As an example, those of most interest to hospitals are polyurethane PUR) and vinyl (PVC) foams The latter compete with foam rubber and P U R foam as cushioning and padding material Among the advantages of PVC foam over other types are good resiliency, chemical resistance, and nonflammability P U R foams have many things in their favor They do not have undesirable bounce-back, they do not mat, stiffen, or crumble after long use or aging, they are nonallergenic, they are odorless, and they are unaffected by dampness Moreover, since they are chemically inert, common cleaning chemicals, water, body acids, spilled foods, or liquids do not affect urethane foams

Polyethylene (PE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), and P U R are the most common foams; however, PP foams can provide favorable properties at a lower material cost PP is stiffer than

PE and can perform better in load bearing or structural applications The low glass transition temperature of PP compared to PS provides increased flexibility and impact strength Use of PP foams include packaging, automotive, insulation, and protection of underground pipe

Structural foam is a term originally used for cellular TP articles with integral solid skins and possessing high strength-to-weight ratios (Table 8.4) Eventually the term covered high-density rigid cellular plastics strong enough for structural applications As an example TS foams, such as polyurethane, are frequently referred to as structural foams In general structural foams can be made from virtually any high- molecular-weight TP organic polymer and will have a cellular core and

an integral skin on all sides The sldn is relatively non-porous in relation

to the cellular core

Table 8 4 Pro3~rtes o f ":-" thic< t h e r n ' o p l a s ( c struc:ural f o a m [20% w e i g h t reduction)

Proger~ t~n~

Modff~d High Potyphen- Me41ao~ of Densil!t Fler~

Te:s',.~m P ~ h # e ~ e A B S Oxide

High High Impact lml~ct Poi~'car- Thermoplastic Polypro- Polyst~ Polysty- bonate Polyester py~ene rene r~ne w/FR Sp¢¢iI~c ib:~./1; ' A S[M- D-7~ 60

8 9 Foaming 3 4 5

Structural-foam construction, when compared to an equivalent amount

of conventional foam plastics, results in a 3- to 4-fold increase in rigidity A broad and overlapping division of TPs exist between commodity and engineering groups of plastics used for structural foams The commodity group consist of the styrenics (PSs, styrene- acrylonitriles, etc.), olefins (PPs, PEs, etc.) and vinyl chlorides (PVCs), while the engineering group includes acetals, ABSs, nylons, PCs, polyester and polyetherimide, plus various glass- or carbon-reinforced plastics

The fairly dense varieties of TP and TS foams may be reinforced, usually with short glass fibers, but long fibers can also be used to provide increased performance (Chapter 15) The fibers are generally intro- duced into the basic ingredients and are blown along with them, to form part of, and to reinforce the walls of the cells These plastic foamed composites are lightweight with high strength

PC foam has outstanding impact strength, high heat resistance (deflection temperature of 280F (138C) at 66 psi (0.45 MPa), as well

as very good flexural characteristics, creep resistance, and processability

PC is a good choice for structural components where load-bearing capability at elevated temperatures is a key requirement It is an excellent alternative to metal for large components in the automotive, appliance, telecommunications, materials handling, and business machine industries Foamable PC combines an unusual blend of rigidity, impact strength, and toughness with UL 94 V-O and 5V flammability ratings

Two principal PS foams that are fabricated are extruded foam and expanded for molded foams PS foams are light, closed-cell foams with low thermal conductivities and excellent water resistance They provide for low-temperature insulation and buoyancy media The extruded PS foam is fabricated as billets and boards They are made by extruding molten PS containing a blowing agent, under elevated temperature and pressure, into the atmosphere, where the mass expands Billets and boards can be used directly or can be cut into many different f o r m s 254 The foam sheet is clcan, bright in appcarancc, has cxccllcnt cushioning properties, and is nonporous The foam is extruded as a sheet and is subsequently vacuum thermoformed into the desired shapes for packaging, etc (Chapters 5 and 7) Use includes low temperature insulation in freezers, coolers, and other types of refrigerated rooms; auto, truck bodies, and railroad cars; refrigerated pipelines; and low- temperature storage tanks for liquefied natural gas They find usage as a replacement for molded-paper-pulp board in meat and produce trays and egg cartons

3 4 6 Plastic Product Material and Process Selection Handbook

Popular is roof-deck PS foam insulation where the foam is placed in the last hot bitumen layer of the roof, which is then covered with gravel or stone to hold it in place Foam is used in the insulation of residential housing by using the foam in place of conventional sheathing This type

of foam when used in agriculture applications provides a means to insulate livestock buildings and low-temperature produce-storage buildings

In the low-density range, 0.5 to 1.0 lb/ft 3, EPS (expandable PS) is used

on boats as flotation, in packaging as an energy absorber, in building as insulation, and as a moisture barrier In the middle-density range, from 1.0 to 4.0 lb/ft 3 the foam is used in packaging as a structural support as well as an energy absorber Other applications include molding h o t / c o l d drinking cups, in the construction field for such applications as concrete forms, in the foundry industry as mold patterns, as insulated containers of all sizes and shapes, and in materials-handling pallets In the high-density range from 5.0 to 20.0 lb/ft 3 the foam exhibits almost wood-like properties Such products as thread spools, tape cores, and furniture parts have been made from these foams

Foams made from PVCs are of two types, open-cell and closed-cell The open-cell foams are soft and flexible, while the closed-cell foams are predominantly rigid Both types are made from plastisols, which are suspensions of finely divided plastics in a plasticizer (Chapter 16) The plastic does not dissolve appreciably in the plasticizer until elevated temperatures are used Vinyl foaming methods are by using a CBA type

or a mechanical frothing process in which a gas is also used as part of the blowing mechanism In the preparation of a soft open-cell foam using a CBA the plastisol is first chosen for the characteristics desired To the plastisol is added a paste made of powdered blowing agent dispersed in a plasticizer One class of materials used for the large majority of vinyl foams is the azocarbonamides and other azo CBA compounds They decompose at temperatures from about 250 to 425F (120 to 220C) Soft, very flexible vinyl foams used for garment insulation, upholstery and similar applications are made by this CBA process The more rigid foams used as underlays for rugs and flooring can also be made by this method, but require different plastics and lower plasticizer contents Open-cell chemically blown vinyl foams generally have densities in the range of 5 to 30 l b / f t 3

The open-cell vinyl foams produced by mechanical frothing, is used to produce sheets, such as flooring underlay, wall coverings, and other applications requiring relatively close thickness tolerances Plastisol is mixed with a given amount of air in a high-shear, temperature-

8 9 Foaming 3 4 7

resembling shaving cream, is cast onto a belt or fabric and knifed to a control thickness Passage through an oven or heating tunnel then causes fusion of the plastisol

Vinyl closed cell foams arc made by the process used to produce open- cell CBA foams except that much higher pressures are used and the process is accomplished in two steps (preparing a hardened mix and going through a reheating process) The vinyl plastisol containing the blowing agent is first placed in a mold in which very little space is left for expansion The mold is then heated, causing decomposition of the blowing agent and, at the same time, fusion of the foam This step raises the internal pressure in the mold to anywhere from 200 to as much as 1600 psi (13.8 to 110 MPa) At these high pressures the gas is dissolved in the plastic in the form of microscopically small bubbles It

is cooled to produce a harden product

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 l b / f t 3, although the usual range is 10 to 50 l b / f t 3 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

Very popular for products such as metal and reinforced plastic laminates

is crosslinked rigid vinyl with exceptional strength It requires a combination of vinyl chloride polymer and monomer, plus maleic anhydride, isocyanate and catalyst The components are poured into a heated pressurized mold An exothermic reaction results in the maleic anhydride copolymerizing with the vinyl chloride monomer and grafting onto the PVC Following molding the TP is exposed to hot water or steam, thereby causing the isocyanate to liberate CO that acts

to expand the plastic mass After expansion is completed the water then reacts with the grafted maleic anhydride, and the resultant maleic acid reacts with the isocyanate and crosslinks it

PEs provide many unusual properties to the cellular plastics industry These foams arc tough, flexible and chemical and abrasion resistant They are known to have superior electrical and thermal insulation properties Their mechanical properties are intermediate between rigid and highly flexible foams Densities are 2 l b / f t 3 and higher, approaching that of the solid plastics The highly expanded polyolefin foams arc potentially the least expensive of the cellular plastics However, they require expensive processing techniques and for this

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reason their cost per unit volume is higher than that of low density polystyrene and polyurethane foams Low density ranges of 2 to 10 Ibflft 3 are used for producing extruded planks, rounds, tubes, and special purpose profiles Compression-molded items may also be produced from low density polyolcfins High density (10 to 40 lbflft 3) polyolefins were used initially for electrical cable coatings Low density polyolefin foams are being widely used in package cushioning Energy absorption under continued impact provides protection for delicate electronic parts as well as heavy metal assemblies

The production of cellular PE involves only one chemical reaction, the thermal decomposition of a blowing agent at a specific temperature, which action liberates an inert gas The choice of blowing agent for electrical service applications is critical because of several unusual requirements The gas from the blowing agent liberates gas This residual by-product must not absorb moisture, which would adversely affect the electrical properties of the product It is also important that the residue left by the blowing agent be nonpolar in order to avoid losses at high frequencies

PE crosslinkcd foams offer higher stability and mechanical strength, better insulation characteristics, and improved energy-absorption properties Most of these foams can be thermoformed, embossed, printed, laminated, or punched, using conventional equipment

PP foam sheeting is specified in Federal Specification PP-C-1797 There arc two types, Type I for general cushioning applications, and Type II for electrostatic protective cushioning applications These foams arc useful f r o m - 6 5 F to 160F (-54C to 71C) The foam sheeting

is intended for use as a protective cushioning wrap for low-density items For high-density items it can be used for protection of surfaces from abrasion PP foams in the structural foam field, supplanted H D P E foams Their use continues to increase because of the extreme range of grades and properties available, plus a favorable price advantage, compared with other TP foams 2~3 Glass-reinforced (30wt% chopped glass fiber) PP foams are commonly used Low-density flexible PP foam film can bc extruded in the 0.7 l b / f t 3 (11.2 k g / m 3) range

Film sheeting consists of a uniform matrix of small closed-cell gas-filled bubbles This film has outstanding toughness and strength over a wider range of temperatures and humidities Its major characteristics, compared with other packaging films, are its light weight, resistance to tearing, chemical resistance, and moisture-barrier properties Extrusion parameters arc similar to those used for LDPE A microccllular PP foam

of this type has been used as a furniture wrap for use in packaging

8 9 Foaming 3 4 9

furniture in interstate commerce The protective foam is wrapped around the item before insertion into a corrugated carton Even with movement in the carton the PP wrap will stay with the item it is intended to protect

The foam is non-dusting and non-linting Typical packaging appli- cations arc surface protection for optical lenses, equipment with critical surfaces, electrical and electronic equipment, glassware, ceramics, and magnetic-tape rolls There is microcellular PP foam sheeting that remains flexible and useful over the temperature range f r o m - 3 2 0 F (-196C) to 250F (121C) (DuPont's Microfoam| sheet)

This foam tends to bc more difficult to foam due to weak melt strength and low melt elasticity Melt strength is the resistance of the melt to extension, while melt elasticity is a measure of elastic recovery Melt strength and melt elasticity are directly related; the higher the melt elasticity, the higher the melt strength With weak melt properties, cell walls separating gas bubbles in the foaming plastic are not strong enough to bear the extensional force as the gas expands, and they rupture As a result, PP foam has a high open cell foam content, which

is unsatisfactory for many applications Melt strength is commonly increased by plastic modification, such as crosslinking or use of high injection pressures

P U R continue to be important markctwisc such as in the furniture and mattress business They can be classified as flexible and rigid foams In some cases, flexible foams can be further subdivided into flexible and semi-flexible (or semi-rigid) foams Almost all mechanical and physical properties of rigid P U R foams depend on their foam densities Flexible urethane foams with its open-cells have the property of complete recovery immediately after compression They arc classified as polycther foams and polyester foams Polyethcr foams arc further classified as conventional flexible foams, high-resilience flexible foams, cold-molded foams, super-soft foams, and viscoelastic foams Microccllular flexible foams and integral-skin flexible foams arc classified as clastomcrs Different foams can bc prepared by the proper choice of polyols Polyisocyanates are used as joining agents for the polyols, and therefore, polyols arc considered to be the major components important to determining the physical and mechanical properties of the resulting foams.2Ss, 256

ABS foam provides properties that include impact, heat, and chemical resistance; low mold shrinkage rates; good long-term dimensional stability; and platability Improved flammability characteristics arc possible either by alloying (blending) with PVC or polycarbonatc, or by

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compounding with halogenated additives ABS compounds are slightly hygroscopic and should be dried prior to conventional injection molding to avoid splay marks (Chapter 2) High melt flow ABS grades display relatively stiff flow characteristics and, therefore, like all high- temperature TPs offer some resistance to foaming ABS is susceptible to degradation and discoloration upon exposure to ultraviolet (UV) radiation Modifying the flammability of ABS by means of halogen compounds significantly increases plastic cost and decreases color stability, especially in pastels, but to a lesser degree than with polystyrenes

ABS structural foam can be processed by injection molding, through conventional or low pressure injection machines (Chapter 4); by expansion casting in rotational molding machines (Chapter 13) or conveyorized oven systems; or it can be extruded into profiles through conventional extruders (Chapter 5)

Even though most plastics can be made into foamed products, from a practical and market oriented view only a few different types are used A few of these plastics will be reviewed As an example limited use has been made using cellular cellulose acetate (CCA) The CCA was one of the first rigid foams produced and was used rather extensively during the 1940s and 1950s mainly in aircraft sandwich constructions

Acetal translucent crystalline polymer is one of the stiffest TPs available

It provides excellent hardness and heat resistance, even in the presence

of solvents and alkalies Its low moisture sensitivity and good electrical properties permit direct competition with die-cast metal in a variety of applications In addition, acetal has extremely high creep resistance and low permeability Acetal is also available as a copolymer (Hoechst Celanese Corp.'s Celcon) for improved processability The homopolymer (DuPont's Delrin) has a very low coefficient of friction and its resistance to abrasion is second only to nylon 6 / 6 Acetals are frequently blended with fibers such as glass or fluorocarbon to enhance stiffness and friction properties Acetal is not particularly weather- resistant, but grades are available with UV stabilizers for improved outdoor performance Acetal, whether homopolymer or copolymer, is not used to any significant degree in forming structural foams

Ionomcr foams are produced by extrusion or injection molding Products are tough, closed-cell structures (6 to 9 lb/fff) The melt characteristics provide a tough skin in the low-density foam sheet and a better surface finish in the higher-density injection-molded products The higher tensile strength and low melt point characteristics of an ionomer provide strong heat-seal seams for fabricated sections used in packaging applications Products ranging from 3 to 30 l b / f t 3 are