past and present developments in polymer bead foams and bead foaming technology

lable at ScienceDirect

Polymer xxx (2014) 1e15

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Polymer

journal homepage: www.elsevier .com/locate/polymer

Past and present developments in polymer bead foams and beadfoaming technology

Daniel Raps a, 1, Nemat Hossieny b, 1, Chul B. Park b, *, Volker Altst€adt a, *

a Department of Polymer Engineering, University of Bayreuth, Germanyb Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada

a r t i c l e i n f o

Article history:Received 30 July 2014Received in revised form27 October 2014Accepted 31 October 2014Available online xxx

Keywords:Bead foamExpanded polypropyleneExpandable polystyrene

* Corresponding authors.E-mail addresses: [email protected] (D. R

(N. Hossieny), [email protected] (C.B. Park)(V. Altst€adt).

1 N. Hossieny and D. Raps contributed to this work

http://dx.doi.org/10.1016/j.polymer.2014.10.0780032-3861/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Raps D, et(2014), http://dx.doi.org/10.1016/j.polymer.2

a b s t r a c t

Polymer bead foaming technology has expanded the market for plastic foams by broadening their ap-plications because of the breakthrough in the production of low-density foamed components withcomplex geometrical structure. This review presents the recent advances in the processing, sinteringbehaviour and properties of bead foam products, which possess unique advantages such as excellentimpact resistance, energy absorption, insulation, heat resistance, and flotation. The key features such asthe mechanical properties of the commercially available bead foams, namely expanded polystyrene (EPS)and expanded polypropylene (EPP), are presented. Furthermore, recent developments of new types ofbead foams based on biopolymer such as expanded polylactide acid (EPLA) and engineering thermo-plastics such as expanded thermoplastic polyurethane (ETPU) and expanded poly(butylene tere-phthalate) (EPBT) are discussed.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The parts manufactured from polymer bead foams consist ofnumerous foamed particles, which are welded with each other intothree dimensionally shaped products with densities in the range of15e120 g/l. Generally, they show similar properties as extrudedfoams of the same density range with regards to the mechanicalproperties (as high energy adsorption at impact [1]), low thermalconductivity and acoustical insulation [2]. Compared to extrudedfoams, their main advantage is that extremely lightweight partswith complex geometries and a high dimensional accuracy can beproduced [3]. In fact, bead foams are the only foams that combine arelatively free choice of shape with a very low density below 2% ofthe unfoamed polymer. Therefore, they are not only used in pack-aging i.e. for electronic devices but also for insulation and furniture[4]. Recently, bead foams have received a lot of attention in theautomotive industry due to their unique combination of low den-sity and free shapeability making them interesting candidates forparts, which have to withstand comparatively small stresses only,

aps), [email protected], [email protected]

equally.

al., Past and present developm014.10.078

e.g. in the interior of the automobile like sun visors [5]. Further-more, bead foams are also gaining popularity for structural parts inautomotive industry, such as crash absorbers in bumpers, due totheir high specific energy absorption at impact [6]. The drivingforce for the growing use of bead foams is weight-reduction, whichcorrelates directly to saving fuel and material.

Currently, bead foams made of three base-polymers are estab-lished in the market as expanded/expandable beads, namelyexpandable polystyrene (EPS), expanded polyethylene (EPE) andexpanded polypropylene (EPP) as well as blend-systems.

EPS is the oldest bead foam product, which was invented 1949.In 2011, theworldwide demand of EPS has grown to 5.8million tonsa year [7]. In the 1970s, EPE was released into the market, followedby EPP in the early 1980s [8].

Since not every polymer can fulfil the requirements for beadfoaming like the ability for welding (also referred to as sintering),the focus of scientific interest went in the direction of modificationof these less appropriate materials with the aim to make more andmore polymers suitable for bead foaming and therebywidening thearea of possible applications. For example, the development of beadfoams made of bio-based polymers like PLA is a major field ofdevelopment. Also an improvement in mechanical properties andespecially an enhanced thermal stability of beads foams is desired,which is a critical requirement for the application in the motorcompartment of cars.

ents in polymer bead foams and bead foaming technology, Polymer

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D. Raps et al. / Polymer xxx (2014) 1e152

To achieve the goal of using bead foams for the motorcompartment, the development of high-temperature steam-chestmoulding for bead foams made of technical thermoplastics iscrucial. Another trend goes towards more energy-efficient pro-cessing of the beads by optimisation of the steam-chest mouldingprocess.

This article will focus mainly on the unique properties andprocessing of EPS and EPP, but also present the efforts made withdifferent, alternative materials and give an overview of futureprospects of bead foaming technology. Besides the above-mentioned commodity thermoplastic polymers (EPS and EPP),there is a huge interest on the processing of bead foams andproducts based on advanced polymers and biopolymers, which isalso presented in this review literature.

2. Basics of foaming

In order to understand the morphological development of beadfoams and their processing techniques, knowledge of the generalbackground of foaming is necessary. This is explained in manypublications [9,10], so the subject will be explained only briefly. Thefoaming process can be divided into four steps:

1. Creation of a homogeneous polymer/gas mixture2. Nucleation of cells3. Cell growth4. Cell stabilisation

The first step, the homogenisation of the polymer with theblowing agent, is mainly determined by mass-transfer process.During this step, the blowing agent has to diffuse into the melt orsolid bead and remain in the polymeregas solution. The well-known second Fick's law can describe the temporal and spacialdependency of this transport. Besides temperature, diffusion alsodepends on pressure and gas-concentration in the polymer [11]. So,it is time and space dependent. For diffusion, the free volume of thepolymer is important [12] as with increasing free volume, diffu-sivity is increased. In contrast to diffusion, the solubility of the gasin the polymer is highly dependent on pressure. Henry's law de-scribes the pressure dependent concentration of a dissolvedblowing agent in a polymeric melt. The temperature dependency ofsolubility is exponential in a negative manner: with higher tem-perature solubility is reduced. Furthermore, high shear-ratesreduce the solubility of the blowing agent in the polymer meltdue the decrease in the free volume caused by the aligning polymerchains [13].

The second step of the foaming process, nucleation of cells in thepolymer, is the creation of nuclei, which act as centre for cellgrowth after a pressure drop for example in an under-waterpelletizing system or autoclave. A sudden pressure drop causes areduction of solubility (the melt becomes super-saturated) creatinga driving force to reduce the gas-content of the polymeregas-mixture. Alternatively, a sudden drop in solubility can be achievedby a temperature jump.

Nucleation can be either homogeneous or heterogeneous. Thelatter mechanism dominates the nucleation process, if a solid sec-ond phase exists (e.g. a particle or surface of the processingequipment), gas will diffuse tomicro-voids on this surface and forma bubble.

According to nucleation theory, nucleation starts at clusters ofgas-molecules inside the melt [14]. Those voids act as nucleatingsites. The homogeneous nucleation rate is heavily dependent on thepressure drop. A high pressure drop leads to a high nucleation rate.From the material's side it is dependent on the surface tensionbetween polymer-melt and gas. Another nucleation process is

Please cite this article in press as: Raps D, et al., Past and present developm(2014), http://dx.doi.org/10.1016/j.polymer.2014.10.078

stress, both in elongation and shear [15,16]. Extensional stressaround growing cells is responsible for pressure fluctuations, whichreduces solubility and thereby increases super-saturation [17].Shear introduces micro-voids and causes an elongation of alreadyexisting bubbles. Those mechanisms lead to an increased nucle-ation rate and higher cell densities.

After nucleation, cell growth takes place. During cell growth thestored gas diffuses out of the melt to the nucleation sites. Thedriving force behind this process is super-saturation caused by thepressure drop or temperature increase. So, typical quantitiesdetermining this process are temperature, which is influencingdiffusion, pressure drop rate and the actual pressure [17]. Anotherimportant factor for foaming is the visco-elastic properties of themelt, since the melt is subjected to elongational deformation dur-ing bubble growth [18].

To obtain foams with a favourable cell size, cell size distributionand thereby good properties (e.g. mechanical behaviour or thermaltransport properties), the morphology must be stabilised and cellgrowth has to cease, otherwise cell coalescence or coarsening (largecells grow at expense of small ones) takes place and deterioratesthe final foam morphology. The main factor for stabilisation is areduction of the polymer's temperature due to which the melt'sviscosity increases. As the blowing agent is diffusing out of thepolymer, the viscosity increases even further, because dissolved gasin a polymer acts as a plasticising agent [19]. At large elongationsstrain-hardening is important. It raises elongational viscosity abovethe linear value due to the stretching of chains [20]. Due to strainhardening thin sections of a cell wall are more difficult to extendthan thicker ones (thinner sections are subjected to higher strainsand thereby a higher degree of stretching of chains). So the thicksections are extended preferentially, the so called self-healing effect[21]. Strain-hardening can be induced by long chain branching[22e25], the introduction of high-aspect-ratio nano-additives[26e29] or by blends having a fibril morphology [30]. Effectscountering cell stabilisation are the creation of crevices by large andfast deformations and rupture of cell walls [10].

3. Production of foamed beads

In principle, two approaches for the production of foamed beadexist: a) the creation of expandable beads, which must be pre-expanded and b) the production of already expanded beads. Thefirst approach can be applied for amorphous thermoplastic resinsonly, like polystyrene, since only they retain a blowing agent insolid state (temperature below TG). Expandable beads are polymergranules inwhich a blowing agent (e.g. pentane) is trapped and theimpregnated polymer granules are expanded in a separate step (i.e.pre-expansion step) before the actual welding-process (i.e. sinter-ing). Efficient transportation of the unfoamed blowing agentimpregnated polymer material and control of density by the part-manufacturer are main advantages compared to expanded beads[3].

Expanded beads are produced from semi-crystalline thermo-plastics, since the presence of crystalline domains prevents thestorage of a blowing agent inside the solid bead [31]. An overviewof the possible methods for producing polymer bead foams is givenin Fig. 1.

The most often used method to produce high quantities ofexpandable beads of polystyrene is the suspension-polymerisationwith a blowing agent [32]. This process consists of two steps,namely the polymerisation where the granules are formed and theaddition of pentane and/or other blowing agents, which diffuse intothe granules [33]. After this process the beads are sieved to getseveral fractions with a narrow size distribution and coated withantistatic agents to prevent agglomeration [32]. Problems arise


Fig. 1. Methods for the production of expandable and expanded bead. The base-material is highlighted in green, the processing steps in blue and the final bead either in black forexpandable beads or red for already expanded products. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

D. Raps et al. / Polymer xxx (2014) 1e15 3

with additivation, since the additives have to meet high re-quirements. They are not allowed to change the polymerisationprocess and the interfacial tension betweenwater and styrene [34].Another drawback is that not all polymers can be synthesised viasuspension-polymerisation.

Another method to produce expanded beads is the impregna-tion (loading with blowing agent) of micro-granules, which containall required additives, with the blowing agent in an autoclave. Thisis the main production process for EPP [35]. In a first impregnationvessel the solid PP-beads are saturatedwith gas close to themeltingpoint of PP (i.e. 150 �C). After the saturation step, the materials withblowing agent are released to an expansion vessel [35]. Afterwardsthe beads are washed to remove any residual suspension stabilizer,which would inhibit proper welding of the beads [3]. For amor-phous polymers, expandable beads can be produced as well, if thesaturation step takes place at temperature below the glass-transition of the polymer-blowing agent-solution.

Alternatively, foam extrusion with under-water pelletizing al-lows the production of expandable beads or already expanded beads[36]. It is schematically shown on Fig. 2. In this method, gas-loadedpolymer melt is extruded through a hole-plate into a water-streamand cut by rotating knifes. If the water-pressure is above the vapourpressure of the blowing agent (for example 10.1 bar for pentane at125 �C), the blowing agent is trappedwithin the solidifying polymerduring cooling and expandable beads are produced. At low pressure,the dissolved gas evaporates and forms bubbles resulting inexpanded beads. Advantages of this method are the exact dosing ofthe blowing agent(s) into themelt, a continuous and flexible processand the applicability of additive that cannot be used in suspensionpolymerisation [32,35], which theoretically allows the processing of

Fig. 2. Schematic of under-water pelletisation as a following unit of a foam extrusionsystem. For expandable beads the water pressure is set above the vapour pressure ofthe blowing agent, for expanded it lies at atmospheric pressure.


any thermoplastic polymer with additives. Furthermore, the beadsize is rather uniform [32]. A challenge is the keeping of the requiredtemperature, since a deviation of only a few degrees might lead tounusable products [35]. Variable process parameters are tempera-ture and pressure of thewater, the rotational speed of the knives andthe temperature of the perforated plate.

4. Principles of the moulding-process and machine design

Parts from bead foams are made in a complex, yet efficientprocess, which allows the production of parts with a highgeometrical degree of freedom at very low density.

For the production of parts, foamed beads are welded togetherin a steam-chest moulding machine. The surface of the beads ismolten or softened [35], using high pressure (i.e. high temperature)steam, which leads to an inter-diffusion of polymer chains betweendifferent beads resulting in a cohesion of the beads [37]. Goodcohesion between the beads and a low content of macro-voids(marked with arrow in Fig. 3) are necessary to ensure favourablemechanical properties [38,39].

4.1. Description of the steam-chest moulding process

The processing of foamed beads to a finished part is done in asteam-chest-moulding machine in five steps. The steps are shownon Fig. 4 and will be explained below.

Fig. 3. Macro-voids in bead foams at the intersection of several beads.


Fig. 4. Bead foam processing in a steam-chest moulding machine: 1: closing and 2: filling the mould, 3: steaming, 4: cooling, 5: ejection of moulded part.


4.1.1. Closing of the mouldThe first step in the steam-chest moulding cycle is the closing of

the mould.

4.1.2. Filling of the mouldFoamed beads are drawn by air pressure out of a container and

blown into the mould by an injector, which usually functions ac-cording to the venturi-principle. This step is critical to achieve ahomogenous distribution of the beads inside the mould and evenconsidered to be the most important step [40].

4.1.3. Welding of the beadsAfter the filling process, the beads are fused together by hot

steam flowing through themould. During steaming, the beads formphysical links due to inter-diffusion of chains of neighbouringbeads. To ensure a high quality of welding between the beads,elevated temperatures and a sufficient steaming time are necessary

Fig. 5. Steps for steaming bead foams: 1: purg


[38]. Furthermore, a high contact area and force between the beadsis also important to achieve good bonding between the beads. Witha low contact area, force is transferred only at a few points, whichleads to bad mechanical properties. On the other hand, if the con-tact force is low, the beads might not touch sufficiently thus alsoleading to bad welding.

For EPP, the steam has an inlet pressure between 7 and 8 bar[41]. However, the pressure inside the mould is lower e pressuresbetween 2.5 and 4 bar are common [41]. Thus, a steam-temperature up to 150 �C is achieved corresponding to steampressure of 4 bar.

The steaming process consists of three steps, which are shownon Fig. 5. At first, the air between the beads is purged out and themould is pre-heated. During this step, steam is flowing parallel tothe mould (Fig. 5 e 1) with all the valves open. In the second step,the steam flows through the mould (Fig. 5 e 2), which is calledcross steaming. During this step, the steam supply and exit valves

ing, 2: cross-steam, 3: autoclave steaming.


Fig. 6. a) Steam nozzles to allow the flow of steam from the steam chamber into the mould and b) their imprints on the final part (EPS).


opposite to each other are open. To ensure a temperature distri-bution as homogeneous as possible and to ensure uniform qualityof welding throughout the entire part, the mould is steamed fromboth sides. Finally, steam is guided into the steam chamber whilethe exit valves are closed to improve surface quality by the creationof a skin (autoclave steaming, Fig. 5 e 3) [42].

During steaming, the characteristic imprints of the steam noz-zles on the part's surface are formed. The nozzles and exemplaryimprints are depicted in Fig. 6.

5. Cooling and stabilisation

For dimensional stability of the part, cooling of the mould is acrucial step. If the part is ejected without cooling, further expansionof the beads is possible, which leads to a deviation of the originalsize. For cooling, the mould is sprayed with water until a temper-ature of around 80 �C is reached [40].

6. Ejection of the moulded part

After moulding and cooling, the part is finally ejected. Pressur-ized air and mechanical ejectors are used to eject the part.

7. Post-processing of the final part

At low part density, shrinkage can be a major challenge. Forexample EPP at a density of 22 g/l can have a shrinkage up to 2.8%[43], which comes from the condensation of steam inside the beadsthat leads to a lower pressure compared to the outer atmosphere.For components requiring high dimensional accuracy, a temperingstep of the parts is necessary, e.g. at a temperature of 80 �C [44]. Inthis step, the original shape is restored, since air diffuses quicklyinto the part at this elevated temperature thus reducing the vac-uum inside the beads and leading to a volume expansion.Furthermore, condensed water from the steaming step is removedas well [3].

Fig. 7. Concept of the crack filling method.


7.1. Moulding technology

In contrast to EPS, which still contains a certain amount ofblowing agent, EPP-beads do not expand any further inside themould without special treatment [35]. Therefore, this matter mustbe dealt with special processing techniques. In principle, the EPPbeads can be processed with two different moulding techniques,namely the crack filling process and the pressure filling process[41]. Both can be combined with the so-called pre-loading step[45].

At first, the crack-filling method will be explained. Its concept isshown on Fig. 7. With this method the beads are filled into acompression-mould at ambient pressure. Before the steaming-process, the mould is closed to its final dimensions, so that thebeads are compressed. With this technique very thin parts with athickness even below the bead thickness can be achieved. Thedrawback of this method is an inhomogeneous density distributionin the final part [40] and if the wall thickness is not constant, thepart shape is not optimal.

Alternatively, counter pressure filling method can be used,which is depicted in Fig. 8. In this method, the beads are subjectedto an elevated air-pressure from a compressor during the fillingprocess, which leads to a compression of the beads [35]. Afterfilling, the pressure is released and the beads re-expand thusreducing macro-porosity. According to the level of filling-pressure,the compression of the beads and thereby the final density of thepart can be controlled. For EPP usually back-pressures between 1.5and 3.5 bar are applied [44].

With the above-mentioned processing method only moderatedensities can be achieved. To lower the density the mouldingmethods discussed earlier must be combined with pressure pre-loading [45]. During the pressure pre-loading step, the beads aresubjected to pressurized hot air for several hours until the insidepressure of the beads reaches equilibriumwith the outside prior tothe actual moulding process. The trapped air leads to additional

Fig. 8. Concept of the pressure filling method.


Fig. 9. Schematic process of the formation of intimate contact between the beadsduring steam chest moulding. Initially the contact is limited to a few points, when thebeads expand further the contact area, where inter-diffusion can occur is increased.


expansion of the beads during steaming thus allowing lower den-sities in the moulded parts. Furthermore, pressure pre-loading re-duces macro-voids between the beads, which leads to bettermechanical properties.

7.2. Aspects of energy saving processing and environmental friendlyprocessing

One fundamental topic in bead foaming is energy saving duringthe steam-chest moulding process, since the prices for energy arerising steadily (e.g. electricity in the industrial sector in Germany:2010: 0.1207 V/kWh, 2013: 0.1487 V/kWh [46]). Also environ-mental protection is one of the most pressing questions amongstthe population in the present [47]. Those two factors drive thetrend of a more economical and ecological production of bead foamparts.

Energy can be saved in every processing step from the manu-facture of the expanded beads to moulding. For example, the pro-duction of beads can be optimised by more efficient extrusionequipment with better drives, since they consume most of theenergy during extrusion [48]. Steam-chest moulding has a hugepotential to reduce energy consumption as well. Most of the energyof the steam is lost for heating up the mould and steam chamber[49]. To overcome this issue, the walls of the steam chamber can becoated with an insulating material [50]. For EPS, moulding systemswith steaming and cooling taking place in separate moulds areavailable, thus eliminating the need for heating and cooling themould every cycle. Another measure is the construction of a mouldwithout a steam chamber, but with a system distributing the steamdirectly within the mould [51]. Steaming without controllingsteam-pressure for a short duration reduces energy consumptionsignificantly [49]. Hossieny et al. studied the usefulness of hot air asa second energy transport medium [6]. It was found that an addi-tion of hot air to the steam reduces the moulding time and energyconsumption. Furthermore, mechanical properties (tensilestrength) of the part were improved at the same time.

In the cooling step there is also space for optimisation. Impulsecooling is such a measure. Instead of cooling the mould with con-stant spraying of water, the spraying is done in intervals. Thoseintervals are repeated until the mould reaches its target tempera-ture. Thereby, cycle time can be reduced slightly and water con-sumption significantly. By applying vacuum to the mould duringcooling the amount of residual water can be removed, whichshortens the cooling time.

A more extreme approach would be the complete change of thewelding machinery and mechanism. Instead of steam, microwavescan be used to heat up the expandable beads thus expanding andwelding them to a foamed part [52] with a reduced energy con-sumption [53]. Since the polymers commonly used for beadfoaming are transparent for microwaves, the beads must be coatedwith a microwave active substance or a microwave active blowingagent must be used for expansion. With this approach the volatileorganic blowing agent n-pentane could be substituted with 2-propanol for EPS [54]. Starch-based bead foams were also pre-pared in the past using this method [55e58]. Especially for poly-mers requiring elevated temperatures for sintering this method is apromising approach to achieve good welding without the invest-ment in high-temperature steam-chest moulding technology,which is not a state-of-the-art technology.

7.3. Physical processes during steam-chest moulding

7.3.1. Processes for inter-bead bondingThe working mechanism of inter-bead bonding is similar to a

sintering process. The major difference between the two processes


is that the former uses high temperature steam as an effectiveheating/cooling medium [37,59], while the latter normally uses hotair [60e62]. During the steam-chest moulding process [63,64], hightemperature steam is injected into the mould in the three cyclesexplained in the previous section to soften and fuse the beads. Inthe case of EPS, the steam vaporises the volatile gas present in thebeads and hence causes an expansion in volume, as well as re-blowing of the beads. In case of EPP, the beads need to be com-pressed as discussed earlier. Through this process of steam-chestmoulding, the empty space between the beads is filled and theinter-bead fusion is created. To improve bead foaming technologiesand bead-moulded products, many researchers performed me-chanical property tests of bead-moulded products based on thecommercially available beads such as EPS and EPP. The formation ofinter-bead bonding in EPS beads involves the diffusion of polymerchains across the inter-bead regions during the heating process ofsteam-chest moulding process. Whereas, the cooling cycle freezesthe physical entanglement of the polymer chains at the inter-beadboundaries and results in the bonding of the EPS bead foams. Thesteam temperature and moulding time are two critical parametersaffecting the extent of bead fusion and significantly affects theoverall mechanical properties of the moulded EPS bead foamsamples [37,39,65,66].

The physical effects during the bonding process between thebeads are explained below. After the establishment of an intimatecontact of the beads (Fig. 9), the actual “healing” of the interfacetakes place. The temporal evolution of intimate contact is depen-dent on pressure, temperature, time and the surface topography[67,68]. Either the establishment of the intimate contact of thesurfaces or the healing can be the limiting mechanism [69]. Thecorrect choice of processing settings cannot only reduce the macro-porosity in the final part but also leads to good bonding strengthbetween the beads.

During steaming, the beads surface softens or melts. At first onlywetting and van-der-Waals-forces dominate the welding process,which would result only in weak bonding. These are followed bythe actual healing by inter-diffusion of polymer chains across theinterfaces between the beads [68], as depicted in Fig. 10. Thediffusion step is described well by several authors in the context ofwelding, healing of interfaces/cracks or novel measurement tech-niques to quantify the inter-diffusion across a surface[67,68,70e75]. A brief summary of the underlying processes, theirtime-scales and ultimately their relevance for bead foaming isgiven here.

The most well known type of diffusion is Fickian diffusion,which applies to small molecules (like a blowing agent) diffusing ina homogeneous matrix without entanglements. Small moleculesdiffusing and dissolving in a network of entangled chains causesswelling, which leads to an increased entanglement density andchanges the relaxation mechanism of the polymer resulting in non-


Fig. 10. The molecular arrangement before and after steam-chest moulding. At theinitial state, a sharp interface between the beads exists, so the beads are just touching.After long enough steaming, the polymer chains crossed the interface due to inter-diffusion thus forming a solid bond.

Fig. 12. The strain energy release rate GIc as an indicator of bond strength. GIc is plottedvs. t1/2*M�1/2 for the welding of polystyrene with the molecular weights 152,000 and400,000 (Pressure Chemical) at 115 �C [reprint from Ref. [68] with permission fromJohn Wiley and Sons].


Fickian behaviour (case II diffusion) [70]. The inter-diffusion ofpolymers across an interface is a complicated topic depending onmolecular weight, distribution of chain ends and temperaturehistory [67]. It can be modelled from the reptation theory takinginto account the so-called minor chain segments, which are thenon-constrained segments outside of their original confining tube.At the initial stage, all chains are constrained to a tube and can justmove along this tube due to Brownian motion. After some time, theends escape the tube, which are now called minor chains. Thisprocess is illustrated in Fig. 11.

The minor chains grow in length with time t since more seg-ments of the chain are moving out of the constraining tube. Theminor chains can move across the bead boundary, since this is athermodynamically more favourable state, thus leading to a poly-mer network across the interface. For interfaces without an excessof chain ends as expected in bead foaming, the time scale for thebond strength varies with (t/tRep)½, which is also depicted in Fig. 12.In contrast, fractured surfaces with an excess of chain ends varywith (t/tRep)¼, where tRep (reptation time) is the time necessary forthe chain to move completely out of its original tube. It gives thetemperature dependency of the process according to theArrhenius-equation or other equations. Bousmina et al. [70]observed a transition of the time scaling laws: at small times thet½-law holds, at longer the t¼-law. Healing is also dependent onmolecular weight (Mw) of the polymer. For a linear and mono-disperse polymer, the reptation time scales to tRep ~ Mw

3 , thediffusion coefficient to D ~ Mw

�2 [69]. Therefore, the melts withlower molecular weight exhibit faster healing.

Those dependencies emphasize that both steaming time andtemperature must be sufficiently long and high for a given materialto achieve good bonding.

Fig. 11. Formation of minor chains by diffusion of chain end


Stupak et al. [38] conducted fracture toughness experiments onEPS, which was moulded at different temperatures and for differ-ently long time intervals. He observed, that fracture toughnessvaried with moulding time by t1.25 and moulding pressure (equiv-alent to steam temperature) by p6.7. The time-scales differ from theones shown above, which can be attributed to several reasons.Firstly, the contact area between the beads is highly time-dependent, which changes the flow resistance of the beads beadfor the steam and makes the degree of healing inhomogeneous.Secondly, the polymer chains are neither mono-disperse nor inthermo-dynamic equilibrium at initial condition.

Rossacci et al. [37] investigated the effects of varying mouldingpressure and moulding time on the tensile property of EPS partwith densities ranging from 19 kg/m3 to 34 kg/m3. The EPS beadswere sintered in a mould with a thickness of 60 mm. They reportedthat the steam temperature and moulding time are two criticalparameters affecting the extent of bead fusion, which significantlyaffected the overall mechanical properties of the EPSmoulded beadfoam samples [47]. Zhai et al. [39,65] showed that at high steampressure, the tensile properties of EPP moulded part with a thick-ness of 10 cmwas much higher than the samples moulded at lowerpressure as a result of improved bead-to-bead bonding. However,experience and literature shows that if the foamed beads aresteamed for too long, their cell structure might collapse and dete-riorate the surface property of the moulded product [38].

s out of their former constraining tube into a new one.



7.3.2. Formation, function and evolution of the double melting-point of EPP

In the case of moulding of EPP beads with a steam-chestmoulding machine, good sintering requires a desirable doublecrystal melting peak structure as shown in Fig. 13. The high-temperature melting peak crystals (Tm-high) are formed during theisothermal annealing step in a batch-based EPP bead foamingprocess. The low-temperature (Tm-low) is formed during the coolingas foaming occurs. The hatched area in represents the desirablesteam temperature range between the low and high melting peaks(Tm-low and Tm-high) of EPP beads within the steam-chest mouldingmachine [76e83]. When EPP beads are processed in the steam-chest moulding machine, crystals associated with Tm-low melt andcontribute to the fusing and sintering of individual beads. Theunmolten Tm-high crystals help to preserve the overall cellularmorphology and dimensional stability of the moulded EPP product.A very narrow processing window between the two melting peaksposes a significant challenge in setting the processing steam tem-perature during the moulding process in steam-chest mouldingmachine. A slight variation in steam temperaturemay cause the Tm-

high crystals to get affected and destroy the cellular morphology ofthe EPP beads and cause shrinkage of the moulded EPP product.The steam penetrates into the EPP beads during the steam-chestmoulding process. During the cooling cycle, at the end of themoulding process, the high-temperature steam, which diffusedinto the beads, tends to condense in the cells and leads to a negativepressure. Due to the characteristic closed cell structure of EPPbeads, air cannot penetrate into the foam within a short span oftime, which results in a dramatic decrease in the internal pressureof the foams. Consequently moulded EPP parts tend to shrink aftercompletion of the moulding process. An annealing process isgenerally used at 80 �C for 4 h to enhance the diffusion rates ofsteam and air and thus prevent shrinkage [39].

7.3.3. Challenges and improvements of steam-chest mouldingThe processing steam in a steam-chest moulding machine is in

the superheated state and its temperature is coupled with theprocessing pressure [84]. However, as the steam enters the mouldcavity via small ports, the overall pressure starts decreasing due tocondensation of the steam on the beads. Furthermore, the pressureof the steam decreases because of the resistance of the flowthrough the beads, which subsequently reduces the temperatureand makes it difficult to determine the actual temperature of themould. Moreover, considering the large volume and complicated

Fig. 13. A typical double-peak melting behaviour of foamed beads.


shape of the mould, cavity the temperature distribution andthereby also density is not uniform. Hence the optimum processingcondition required for the desired properties in the moulded beadfoam products must be achieved by trial and error. Nakai et al. [79]investigated some fundamental aspects of steam-chest moulding,such as the evaporation and condensation of steam and heat con-duction, using numerical simulation techniques. They reportedreduced heat conduction to the core area of the mould caused by adecrease in steam temperature as a result of drop in the steampressure. Generally, higher operating steam pressure is imple-mented to improve the heat conduction to the core area of themould. However, a higher operating steam pressure relates tohigher operating cost and a higher temperature leading to an in-crease in localized temperature near the steam entry. Beadsexposed to this high temperature may melt resulting in shrinkageat the surface of the product. This dramatically deteriorates thesurface property of the moulded product.

The introduction of dry hot air into the steam has shown animprovement of the heat transfer in the core of the moulded beadfoam product (Fig. 14) and hence improved the overall mechanicalproperty across the moulded bead foam products as compared tothe sample moulded with pure steam [6]. Thermodynamically, thelower JouleeThomson coefficient (mJ) of hot air reduces the sensi-tivity of a decrease in the steam temperature with a drop in steampressure [6,85,86]. However, steam cannot be eliminatedcompletely due to its high thermal conductivity and heatingcapability due to condensation and high heat capacity compared tohot air. The introduction of hot air also reduces the local temper-ature at the steam entry port and hence themelting of beads on thesurface is decreased resulting in better surface quality [6].

8. Commercially available bead foams

8.1. EPS

8.1.1. Advantages and disadvantagesEPS is the most widely used bead foam material with a con-

sumption of 4.7 Mt per year [87], because of its low price and highavailability [88]. It is heavily used for packaging applications. Thisalso causes major problems due to enormous amounts of EPS-waste. So, recyclability is very important [89]. EPS offers a lesscompetitive compression set compared to EPP, which makes thelatter material the favourite for applications with multiple impactdeformation. With EPS, lower densities can be achieved comparedto EPP but it has a less favourable chemical and temperatureresistance. However, transport and storage of EPS is much cheaper.In contrast to EPP, much higher masses of EPS can be transported.EPS is transported in the form of gas-loaded micro-granules havinga bulk density of 0.64 g/cm3 [3], where EPP has to be transported inthe form of foamed beads, which have a much lower density thusrequiring much more space for a given mass.

EPS offers good thermal insulation capabilities, which leadeincombination with the low price e to the second highest marketshare of insulation materials after glass wool [90].

8.1.2. Mechanical propertiesIn principle, the mechanical behaviour of EPS is similar to EPP,

since they possess the same basic structure. However, EPS has ahigher modulus and strength (compressive stress at 10% strain;EPS: 110 kPa [91]; EPP: 70 kPa [43]; 20 g/l) at the cost of elasticity(compression set; 50% strain, 24 h; EPS: 45% [35]; EPP: 28e33 kPa[92]). Also themaximum temperature of usage for EPS is lower thanEPP, so EPP exhibits a size alteration after 4 days at 110 �C of lessthan 2% [44], whereas EPS can only stand long-term service tem-peratures between 80 and 85 �C [91]. Mechanical properties are


Fig. 14. Effect of hot air and its flow rate on the processing temperature during (a)1st steaming cycle and (b) 2nd steaming cycle. (c) A schematic illustrating the locations where theprocessing temperatures of T1 and T3 were measured.


highly dependent on the quality of welding of the beads, which wasstudied in numerous publications [37,38,93,94]. For the applicationin the sectors of thermal insulation and packaging, the knowledgeof creep behaviour of EPS is of utmost importance [95e97]. Pro-tective systems often put EPS to use as a shock absorber, thereforethe dynamic properties of this material were studied in manypublications [89,98,99].

In contrast to EPP, EPShas lower elasticity imposing constrictionson the use of EPS for packaging of high-value goods, whichmight besubjected tomultiple impacts. This lead to the development of beadfoams fromPS based blends,which offer higher elasticity, toughnessat low temperature and better chemical resistance [100].

8.1.3. Thermal propertiesIn contrast to EPP, EPS is a commonly used material for thermal

insulation, since it offers advantageous thermal insulation proper-ties (EPS: 33 mW/mK [91]; EPP: 36 mW/mK [44]) and a morecompetitive price in this field of application.

Thermal transport in foams comprises of conduction throughthe solid cell walls and struts as well as the cell gas, convection andradiation. Convection can be neglected for cell smaller than 3 mm[101]. Thermal conduction of foams consists of the conductionthrough the solid and the cell gas. The conduction of the polymericmatrix is affected by crystallinity and their orientation [102e104],which are heavily affected by the foaming process. Either conduc-tion through the solid or through the cell gas can be dominant,depending on density. For low densities, the cell gas dominates thethermal transport over the solid polymer because of its high vol-ume fraction. For very small cells with a cell diameter in the sameorder of magnitude as the free path length of an air molecule, theKnudsen-effect becomes important and the thermal conductivity ofthe cell gas is reduced drastically [105].


Besides conduction, radiation is a very important transportmechanism of thermal energy in foams. This effect is mainlydependent on cell morphology and temperature. Several workerstry to separate the total heat conductivity into its parts[101,106e111]. However, those contributions cannot be separatedin normal measurements without modelling [112], so the authorsused more or less complex models for separation.

One major drawback of the above-mentioned results of thetheoretical models is an assumed independence of the radiativeand conductive contributions. This matter was tackled by Ferklet al. [101] in a (at the time being) spatially one-dimensional model.No assumption on the propagation of radiation and the individualgeometry was made. In literature, EPS is often used to investigatethe thermal properties of foams in general [2,90,109]. However, thespecial particle-structure of bead foams was never investigated indetail. The authors will consider this matter in an upcomingpublication.

To reduce thermal radiation EPS bead foams are filled withGraphite-particles, which act as reflectors for infrared radiationthus reducing the overall thermal conductivity [113].

8.1.4. ApplicationsEPS is well known for packaging applications. For example,

electronic devices are kept safe from transport-damage using EPScrash absorbers or spacers. Also in areas, where rigorous safetyrestrictions exist, such as helmets for cyclists or bikers or car-seatsfor children, EPS is used often [114]. In the automotive industry it isused for crash-absorbers as well.

Thanks to its advantageous thermal insulation capability, it isused for the insulation of houses in form of blocks. EPS is used foracoustic insulation for example against footfall sound. For thecooling of perishable goods as drugs, food or human blood, EPS


Fig. 15. SEM micrograph of a cross-section of an EPP bead made with autoclavefoaming setup at 130 �C.[122].


contributes to keep energy cost low as insulation and makes thetransportation of such goods affordable and practical.

Besides the application as structural or insulation material, EPSis widely used for lost-foam casting, where it is mostly used as thecore material [115]. The lost-foam casting technique allows theproduction of complex parts, mainly from aluminium and iron, forthe automotive industry ranging from cylinder heads to motorblocks [116,117].

8.2. EPP

8.2.1. Advantages and disadvantagesAmong bead foams, EPP has unique advantages, such as excel-

lent impact resistance, energy absorption, insulation, heat resis-tance and flotation. In addition, it is lightweight and recyclable, andexhibits good surface protection as well as resistance against oil,chemicals and water. Thanks to these advantages, the use of EPP isgaining increased momentum in the automotive, packaging, andconstruction industries. The combination of its flexible applica-bility, reasonable tooling cost, high resilience, good sound damp-ening at high frequencies, and, especially, its low weight, has madeEPP the material of choice for numerous applications. For instance,EPP foams are now utilized as bumper cores, providing significantlyhigher energy absorption upon impact as opposed to conventionalsystems. However, unlike expandable polystyrene EPS, which issupplied as expandable pellets, suppliers can only provide EPPbeads in an expanded form. The beads are then shipped to the partsmanufacturers for furthermoulding. Due to the presence of bubblesin the bead (i.e. the large volume of the bead), the cost of storing,packaging, and transporting EPP is very high, ultimately renderingit far more expensive than EPS or a normal PP resin. Moreover, verylittle research has been conducted on EPP manufacturing, sinteringbehaviour, and steam chest moulding process. Consequently, whenan EPP concept product is targeted, the manufacturer can onlydepend on the EPP supplier to obtain a prototype, thus having littleor no control over material selections and processing conditions.

8.2.2. EPP morphology and expansion ratioThe EPP beads feature high closed-cell content, which is typi-

cally 95e98 % as shown in Fig. 15 and is measured using a pyc-nometer in accordance to ASTM D6226. The closed-cell structureprovides high expansion force, while steam-chest moulding assistswith the bonding of EPP beads. Depending on the bulk density, EPPbeads have cell diameters from 200 to 500 mm and cell densities inthe range of 105e106 cells/cm3.

The cell density, expansion ratio and crystal characteristic of theindividual EPP bead foams have a significant effect on the overallmechanical properties of the moulded EPP bead foam product[2,118]. Guo et al. [119] investigated the critical processing param-eters to produce EPP beads in a lab-scale autoclave system. Thepressure drop was systematically controlled using a modular die atthe discharge port. The die geometry (L/D) was decided to maintaina high enough pressure inside the chamber to prevent pre-foamingof the gas-impregnated EPP beads. The cell density was not affectedby die geometry. On the other hand, the volume expansion of theEPP beads slightly decreased as the die length increased.

The saturation pressure of the blowing agent plays a crucial rolein achieving high cell densities and expansion ratios in autoclavebead foaming. During foaming of EPP beads with CO2 in an auto-clave, a higher saturation pressure of CO2 allows a higher gascontent to be dissolved into the PP pellets [120,121]. This higherCO2 content helps to reduce the energy barrier for cell nucleationand increases the cell nucleation rate, which leads to a higher finalcell density [122,123]. The volume expansion of EPP beads wasobserved to increase dramatically as the saturation pressure was


increased. The higher cell density achieved at high saturationpressure decreases the amount of gas loss from the foamed EPPbeads and hence improves the expansion ratio.

8.2.3. Crystallization behaviour of EPP beadsThe production of EPP beads with double melting peak char-

acteristics has been well established [5,124e127]. The two-peakcrystal structure is generated by impregnating the PP micro-pellets with a physical blowing agent in an autoclave chamber atelevated pressures and temperatures around PP's melting pointover a certain period of time [125,127]. During the gas impregna-tion stage, a new crystal melting peak is created at a higher tem-perature, Tm-high. Since the saturation is done close to the meltingpoint of PP not all the crystals melt. The newly generated crystalpeak (Tm-high) during the isothermal gas-impregnation stage of theEPP beads stems from the perfection of the a crystal phase out ofthe unmolten crystals, which has a higher orientation and hence ahigher melting temperature than the original peak and is known asa2 [128,129]. The melting temperature of this peak is typicallyabove the annealing temperature. The Tm-low melting peak isgenerated during the rapid cooling process in autoclave foamingchamber and is known as a1. The a1 and a2 are a forms of crystalswith various degree of perfection [130e136]. Choi et al. [128] haveshown that by ramping the PP to the annealing temperature, theless perfect crystals melt and the more perfect crystals that existabove the annealing temperature remain unmolten. However, thework conducted by Choi et al. [128] was at ambient pressure. Theactual EPP bead manufacturing process is conducted at high pres-sure in the range of super-critical condition of the blowing agent(e.g. 74 bar for CO2), which leads to the dissolution of gas into the PPmatrix. The dissolved gas significantly affects the crystallizationbehaviour of PP [19].

In the context of EPP bead foam manufacturing, the effect ofdissolved blowing agent on the generation of double crystalmelting peak structure can be systematically investigated using ahigh-pressure differential scanning calorimetry (HP-DSC). Theplasticising effect of dissolved blowing agent, decreases the satu-ration temperature required for the generation of the highermelting peak with perfected crystals in EPP bead foams [137].

8.2.4. Mechanical propertiesFor EPP bead foams, copolymers with polypropylene (PP) as

base monomer are preferred compared to homo-PP because thelatter has poor impact properties at low service temperatures[128,138e143]. The copolymers can be binary, such as a propyle-neeethylene copolymer or a propyleneebutene copolymer, or aternary copolymer, such as a propyleneeethyleneebutene copol-ymer [128,144]. By using branched high-melt-strength PP [77] and


Fig. 16. Failure mechanism: a) inter-bead, b) intra-bead.


metallocene-catalysed PP [5,145], the mechanical properties andcompressibility of EPP beads and their moulded foam products canbe improved. Other studies have shown that the mechanicalproperties of EPP beads can be improved significantly by choosingan appropriate PP copolymer that will lead to better control of thesecondary crystal form [146]. For instance, to improve EPP's in-mould foamability, researchers have employed a PP copolymerwith a lower melting temperature [147]; in another case, graphitewas introduced in order to increase heat resistance [138].Furthermore, it has been shown that the use of PP nano-compositescan also improve EPP bead properties [148]. Efforts have also beenmade to produce expandable PP beads. However, the use of eitheran encapsulated physical blowing agent [149] or a dispersedchemical blowing agent [150] in the beads has not become com-mon practice in the industry.

As mentioned earlier, for EPP foamed beads to have a goodsintering during the steam-chest moulding stage, they need topossess a double-peak (or at least broad)melting characteristic. Theratio between the Tm-low and Tm-high peaks is thus crucial indetermining the surface quality and mechanical properties of thesteam-chest moulded EPP product. If the Tm-low peak is dominant,then the moulded EPP product may shrink and the overall geom-etry would collapse. In contrast, if the Tm-high peak is dominant,then the sintering will be weak resulting in poor mechanicalproperties.

The failure mechanism in moulded EPP products has beenattributed to the bead boundaries and a potential fracture pathbetween the beads [151,152]. This is known as inter-bead bondingand it has been reported that they determine the mechanicalproperties of the bead products [151,152]. Inter-bead fracture arisesdue to weak sintering between the EPP beads. However, anotherfailure mechanism occurs within the EPP beads and is known asintra-bead fracture. This failure reflects that there is good sinteringbetween the EPP beads. The inter-bead and intra-bead failuremechanism can be investigated by observing the crack surfaceunder a scanning electron microscope as shown in Fig. 16.

The tensile strength of EPP samples has a strong dependency onthe processing steam pressure and corresponding temperatureused during the steam-chest moulding process. The tensile prop-erties of EPP samples increases at higher steam pressure. A similarphenomenon was observed in EPS bead processing, where a hightensile strength and a high degree of inter-bead fusion was ob-tained at high moulding pressure [152].

The tensile strength of EPP moulded samples also increasedsignificantly due to the development of crystals in the inter-beadareas during the cooling cycle of the steam-chest moulding pro-cess [119,153]. EPP bead size is another important parameter, whichaffects the inter-bead bonding and improves the mechanicalproperties of moulded products [119].

8.2.5. TrendsEPP is in a state of constant development and getting closer to

the customer. Previously EPP was mainly used in the automotiveindustry as structural material in the application as cores for crashbumpers or for tool boxes in the car boot. For those applications thespecific advantages of EPP as low density and good energy dissi-pation at impact are harvested.

Today's trends aim towards higher functionality. For examplehinges, snap fits or fasteners make the material fit for new appli-cations as furniture. Challenges are the steam nozzle imprints andits technical appearance (Fig. 6). Therefore, the development ofmulti-material systems is facilitated [154]. An EPP foam-core can becombinedwith a layer of TPE for decoration. The connection of bothcomponents can be achieved in an online process. However, if a


coating is desired, adhesion between the coating and EPP is stillchallenging making a surface treatment necessary [155].

Another approach to modify the properties of EPP is hybrid-isation [156]. So, the EPP beads are combined with metal beadfoams in order to create a hybrid material with highly elasticbehaviour at low stress (behaviour dominated by EPP) and highenergy dissipation at high stress (behaviour now dominated bymetal foam). The purpose is to produce better crash bumpers forcars to increase passenger safety while reducing weight.

The development of EPP is not at the end, but very dynamic andrapidly advancing, especially towards design and creativity.

9. Recent developments of new bead foams

9.1. Thermoplastic elastomers and thermoplastic polyurethane

Thermoplastic elastomers (TPEs) are unique materials with awide range of properties, filling the gap between thermoplasticsand elastomers. They combine the properties of elastomers witheasy processability of thermoplastics. TPEs differ according tostructure, rheological and thermal properties. There has been sig-nificant amount of work on the foaming of TPEs, however there isvery limited literature on the processing of TPE bead foams.

Thermoplastic polyurethanes (TPUs) consist of a phase-separated molecular structure of rigid hard segment (HS) do-mains dispersed in a soft segment (SS) matrix, which provides TPUswith a unique combination of strength, flexibility and process-ability. However, the application of TPUs is mostly limited to itshigh hardness and cost. Hence foaming could be a desirable way ofreducing density and thereby cost. TPUs do not still have a widelyused foaming method and is one group of TPEs under a lot ofresearch.



Expanded TPU bead foams (ETPU) and its product have beendeveloped recently [157]. One example is the shoe sole of theAdidas “Energy Boost” running shoe, where ETPU with brand name“Infinergy” from BASF SE with relatively high foam densities of 200… 300 g/l is used. This material provides good elastic propertiesand a high rebound effect. Generally softer grade TPUs with shorehardness between A 62 and A 80 are preferred to produce ETPUbeads for industrial applications. The advantage is that softer TPUshave a lower concentration of HSs and hence a lower melting pointwith much better flowability. Thus lower processing temperatureand pressure are required during the processing of ETPU beads.Furthermore, lower steam pressures are required during the sin-tering step of the ETPU beads, which reduces the sintering cost. Thesoftness of the beads also makes adhesive-bonding of the ETPUbeads more effective.

The ETPU beads can be produced both in a batch setup using ahigh-pressure autoclave as well as a continuous setup using foamextrusion and an underwater pelletizer. During the processing ofETPU beads in a high-pressure autoclave setup, the TPU micro-pellets are fed into the high-pressure autoclave with suspensionmediumand saturatedwith theblowing agent. The autoclave is thenheated to the impregnation temperature, which is generally nearthe softening point of the TPU. The blowing agent impregnates intothe SS and results in the swelling of the TPU. The impregnation timeis generally from 0.5 to 10 h [157]. The impregnation with blowingagent near the softeningpoint of TPUresults in the rearrangementofthe existing HS crystalline domains. After completion of the satu-ration cycle, the autoclave chamber is depressurized, which resultsin production of ETPU beads. During the depressurization step,which results in cooling of the TPUmaterial new HS crystallites areformed in the microstructure. The HS crystallites in the TPUmicrostructure can be effectively utilized as heterogeneous bubblenucleating agents to produce microcellular ETPU bead foams [158].

9.2. Polyesters

Foaming of polyesters as for example polyethylene tere-phthalate (PET), polybutylene terephthalate (PBT) or polylactide-acid (PLA) is challenging, since they possess disadvantageousrheological properties for foaming like low melt elasticity and lowviscosity [159], which lead to an unfavourable cellular morphology.Below, the relevance of PET and PBT as a matrix will be described.PLA bead foams will be treated in the forthcoming chapter on bio-based bead foams.

Foams of PET are commercially available since the 1990s frommany companies, for example by Airex AG (AIREX T92), ArmacellBenelux S.A. (ArmaFORM PET) and BASF SE (Kerdyn). They aremainly used for foam-cores in sandwich applications. Extrusionfoaming is the mostly used production method and allows theproduction of foams with densities between 30 and 400 kg/m3

[160]. In contrast to extrusion foams, bead foams made of PET arenot available on the market yet. However, patents were filed since2011 [161e164].

In the past, studies on the foaming behaviour of PBT were doneby extrusion foaming by Jeong and Xanthos [165,166] and finallyachieved a density of 330 kg/m3.

Recently, the bead foaming capability of PBT was studied forexpanded PBT using extrusion with an under-water pelletizing sys-tem [167]. The lowest density achieved for the processed EPBT beadswas280kg/m3. Theeffectsofprocessingparameters likeknife-speed,water pressure and temperature as well as the viscosity of the PBT-grade were investigated. A variation of processing conditionsshowedmoderate influence on bead shape and cellular morphology.In contrast to that, the viscositywasmuchmore important. Increasedviscosity lead to a better bead shape and cellular structure.


9.3. Biopolymer-based

Among different biopolymers, polylactide acid (PLA) has gaineda lot of interest for bead foaming applications. The interest is due toits high potential to substitute EPS bead foam products used inpackaging and commodity products [168]. The production of EPLAbeads has been commercialised and the technique is similar to theprocessing of EPS beads. In this technique, the PLAmicro-pellets aresaturated with blowing agent below PLA's glass transition tem-perature and the pre-foaming is done in a pre-expander machineusing steam or hot air. The expanded EPLA beads are then sinteredtogether using a steam-chest moulding machine. Prior to sintering,the pre-foamed PLA beads are coated with a special coating toimprove the sintering of the EPLA beads during the steam-chestmoulding process. The thermal and mechanical properties ofEPLA beads processed with the technique described above aresimilar to the EPS bead foam product compared at the similardensity.

The technique of utilizing the double melting peak crystals(similar to the EPP beads) is believed to be a promising method toimprove the crystallization kinetics of PLA and also the sintering ofEPLA beads. It is believed that the presence of dissolved gas willsignificantly improve the crystallization kinetics of PLA and it willbe beneficial for the production and sintering of EPLA beads [169].The crystals formed during the processing of EPLA beads would alsoenhance the poor foaming behaviour of PLA. The generated crystalsduring the saturation process will promote the cell nucleation byacting as heterogeneous nucleating agents. Furthermore, theconnection of PLA molecular chains through the generated crystalswill improve the low melt strength of PLA and consequently in-crease the expandability of EPLA beads by minimizing the gas lossand cell coalescence.

10. Conclusion

This review-paper outlined the work done on bead foams bothfrom an industrial and scientific viewpoint and showed the trendsand perspectives of state-of the-art materials and machinery, aswells as new material developments.

Currently, EPS is the most widely used bead foam. It is used forcommodity applications requiring cost effective part production inhuge quantities as packaging and insulation. EPS has a high specificmodulus and strength at the drawback of low elasticity. In contrastto EPS, the other two commonly used bead foams EPE and EPP aremore expensive, but have a much higher elasticity making themsuitable for the packaging of more sensitive goods. Especially EPP isgaining more and more attention as new fields for its applicationbesides the traditional ones as structural material are found.Nowadays, EPP is used for furniture or multi-material combina-tions. An advancement of the recent past is ETPU, since thermo-plastic polyurethane is one of the most elastic thermoplasticallyprocessable materials. Bead foams made of this material shine outwith extraordinary elasticity making parts from ETPU an idealcandidate for damping high impact forces as found in shoe soles.ETPU bead foams will open new areas as the foaming reduces TPUhardness without the use of plasticizers. However, all state-of-the-art bead foams have a low usage temperature in common. There-fore, current developments aim to tackle this issue by makingtechnical thermoplastics like PBT ready for bead foaming. Beadfoams made of bio-based polymers like PLA are also a key-focus ofcurrent research.

Beside research on new materials or their optimisation, theother focus of investigation lies on the processing of bead foams.The trends in machinery show that steam-chest moulding ma-chines are made ready for new types of bead foams requiring high



steam temperature, which is critical since mould, steam chamberand injectors have to withstand much higher steam pressure.Optimisation towards lower energy consumption to facilitate anefficient production of huge quantities of bead foam parts is also achallenge to ensure efficient production. More efficient steamingand cooling cycles are key features for new steam-chest mouldingmachines.

Although bead foams are quite mature in terms of time on themarket, there are still many open scientific questions regarding theeffect of their unique morphology on properties, physical phe-nomena during steam chest moulding and how to achieve suitablewelding properties.

Acknowledgement

We thank our colleagues Kalaivani Subramaniam, TobiasStandau, Christian Trassl, Amir Fathi, Peter Schreier, Julia Genseland Janina Lauer for numerous fruitful discussions and for support.The supports of the German Research Foundation (DFG and SFB840) and the Consortium of Cellular and Micro-Cellular Plastics(CCMCP) are highly acknowledged.

List of abbreviations

(E)PBT (expanded) polybutylene terephthalate(E)PE (expanded) polyethylene(E)PET (expanded) polyethylene terephthalate(E)PLA (expanded) polylactid acid(E)PP (expanded) polypropylene(E)PS (expandable) polystyrene(E)TPU (expanded) polyurethane(HP-)DSC(high pressure) differential scanning calorimetryHS hard segmentSS soft segmentTG glass transition temperatureTm melting temperatureTPE thermoplastic elastomer

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