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

Polymer xxx (2014) 1e15

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Past and present developments in polymer bead foams and bead foaming technology €dt a, * Daniel Raps a, 1, Nemat Hossieny b, 1, Chul B. Park b, *, Volker Altsta a b

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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 July 2014 Received in revised form 27 October 2014 Accepted 31 October 2014 Available online xxx

Polymer bead foaming technology has expanded the market for plastic foams by broadening their applications because of the breakthrough in the production of low-density foamed components with complex geometrical structure. This review presents the recent advances in the processing, sintering behaviour and properties of bead foam products, which possess unique advantages such as excellent impact resistance, energy absorption, insulation, heat resistance, and flotation. The key features such as the mechanical properties of the commercially available bead foams, namely expanded polystyrene (EPS) and expanded polypropylene (EPP), are presented. Furthermore, recent developments of new types of bead foams based on biopolymer such as expanded polylactide acid (EPLA) and engineering thermoplastics such as expanded thermoplastic polyurethane (ETPU) and expanded poly(butylene terephthalate) (EPBT) are discussed. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Bead foam Expanded polypropylene Expandable polystyrene

1. Introduction The parts manufactured from polymer bead foams consist of numerous foamed particles, which are welded with each other into three dimensionally shaped products with densities in the range of 15e120 g/l. Generally, they show similar properties as extruded foams of the same density range with regards to the mechanical properties (as high energy adsorption at impact [1]), low thermal conductivity and acoustical insulation [2]. Compared to extruded foams, their main advantage is that extremely lightweight parts with complex geometries and a high dimensional accuracy can be produced [3]. In fact, bead foams are the only foams that combine a relatively free choice of shape with a very low density below 2% of the unfoamed polymer. Therefore, they are not only used in packaging i.e. for electronic devices but also for insulation and furniture [4]. Recently, bead foams have received a lot of attention in the automotive industry due to their unique combination of low density and free shapeability making them interesting candidates for parts, which have to withstand comparatively small stresses only,

* Corresponding authors. E-mail addresses: [email protected] (D. Raps), [email protected] (N. Hossieny), [email protected] (C.B. Park), [email protected] (V. Altst€ adt). 1 N. Hossieny and D. Raps contributed to this work equally.

e.g. in the interior of the automobile like sun visors [5]. Furthermore, bead foams are also gaining popularity for structural parts in automotive industry, such as crash absorbers in bumpers, due to their high specific energy absorption at impact [6]. The driving force for the growing use of bead foams is weight-reduction, which correlates directly to saving fuel and material. Currently, bead foams made of three base-polymers are established in the market as expanded/expandable beads, namely expandable polystyrene (EPS), expanded polyethylene (EPE) and expanded polypropylene (EPP) as well as blend-systems. EPS is the oldest bead foam product, which was invented 1949. In 2011, the worldwide demand of EPS has grown to 5.8 million tons a year [7]. In the 1970s, EPE was released into the market, followed by EPP in the early 1980s [8]. Since not every polymer can fulfil the requirements for bead foaming like the ability for welding (also referred to as sintering), the focus of scientific interest went in the direction of modification of these less appropriate materials with the aim to make more and more polymers suitable for bead foaming and thereby widening the area of possible applications. For example, the development of bead foams made of bio-based polymers like PLA is a major field of development. Also an improvement in mechanical properties and especially an enhanced thermal stability of beads foams is desired, which is a critical requirement for the application in the motor compartment of cars.

http://dx.doi.org/10.1016/j.polymer.2014.10.078 0032-3861/© 2014 Elsevier Ltd. All rights reserved.

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To achieve the goal of using bead foams for the motor compartment, the development of high-temperature steam-chest moulding for bead foams made of technical thermoplastics is crucial. Another trend goes towards more energy-efficient processing of the beads by optimisation of the steam-chest moulding process. This article will focus mainly on the unique properties and processing of EPS and EPP, but also present the efforts made with different, alternative materials and give an overview of future prospects of bead foaming technology. Besides the abovementioned commodity thermoplastic polymers (EPS and EPP), there is a huge interest on the processing of bead foams and products based on advanced polymers and biopolymers, which is also presented in this review literature. 2. Basics of foaming In order to understand the morphological development of bead foams and their processing techniques, knowledge of the general background of foaming is necessary. This is explained in many publications [9,10], so the subject will be explained only briefly. The foaming process can be divided into four steps: 1. 2. 3. 4.

Creation of a homogeneous polymer/gas mixture Nucleation of cells Cell growth Cell stabilisation

The first step, the homogenisation of the polymer with the blowing agent, is mainly determined by mass-transfer process. During this step, the blowing agent has to diffuse into the melt or solid bead and remain in the polymeregas solution. The wellknown second Fick's law can describe the temporal and spacial dependency of this transport. Besides temperature, diffusion also depends on pressure and gas-concentration in the polymer [11]. So, it is time and space dependent. For diffusion, the free volume of the polymer is important [12] as with increasing free volume, diffusivity is increased. In contrast to diffusion, the solubility of the gas in the polymer is highly dependent on pressure. Henry's law describes the pressure dependent concentration of a dissolved blowing agent in a polymeric melt. The temperature dependency of solubility is exponential in a negative manner: with higher temperature solubility is reduced. Furthermore, high shear-rates reduce the solubility of the blowing agent in the polymer melt due the decrease in the free volume caused by the aligning polymer chains [13]. The second step of the foaming process, nucleation of cells in the polymer, is the creation of nuclei, which act as centre for cell growth after a pressure drop for example in an under-water pelletizing system or autoclave. A sudden pressure drop causes a reduction of solubility (the melt becomes super-saturated) creating a driving force to reduce the gas-content of the polymeregasmixture. Alternatively, a sudden drop in solubility can be achieved by a temperature jump. Nucleation can be either homogeneous or heterogeneous. The latter mechanism dominates the nucleation process, if a solid second phase exists (e.g. a particle or surface of the processing equipment), gas will diffuse to micro-voids on this surface and form a bubble. According to nucleation theory, nucleation starts at clusters of gas-molecules inside the melt [14]. Those voids act as nucleating sites. The homogeneous nucleation rate is heavily dependent on the pressure drop. A high pressure drop leads to a high nucleation rate. From the material's side it is dependent on the surface tension between polymer-melt and gas. Another nucleation process is

stress, both in elongation and shear [15,16]. Extensional stress around growing cells is responsible for pressure fluctuations, which reduces solubility and thereby increases super-saturation [17]. Shear introduces micro-voids and causes an elongation of already existing bubbles. Those mechanisms lead to an increased nucleation rate and higher cell densities. After nucleation, cell growth takes place. During cell growth the stored gas diffuses out of the melt to the nucleation sites. The driving force behind this process is super-saturation caused by the pressure drop or temperature increase. So, typical quantities determining this process are temperature, which is influencing diffusion, pressure drop rate and the actual pressure [17]. Another important factor for foaming is the visco-elastic properties of the melt, since the melt is subjected to elongational deformation during bubble growth [18]. To obtain foams with a favourable cell size, cell size distribution and thereby good properties (e.g. mechanical behaviour or thermal transport properties), the morphology must be stabilised and cell growth has to cease, otherwise cell coalescence or coarsening (large cells grow at expense of small ones) takes place and deteriorates the final foam morphology. The main factor for stabilisation is a reduction of the polymer's temperature due to which the melt's viscosity increases. As the blowing agent is diffusing out of the polymer, the viscosity increases even further, because dissolved gas in a polymer acts as a plasticising agent [19]. At large elongations strain-hardening is important. It raises elongational viscosity above the linear value due to the stretching of chains [20]. Due to strain hardening thin sections of a cell wall are more difficult to extend than thicker ones (thinner sections are subjected to higher strains and thereby a higher degree of stretching of chains). So the thick sections 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]. Effects countering cell stabilisation are the creation of crevices by large and fast deformations and rupture of cell walls [10]. 3. Production of foamed beads In principle, two approaches for the production of foamed bead exist: a) the creation of expandable beads, which must be preexpanded and b) the production of already expanded beads. The first approach can be applied for amorphous thermoplastic resins only, like polystyrene, since only they retain a blowing agent in solid state (temperature below TG). Expandable beads are polymer granules in which a blowing agent (e.g. pentane) is trapped and the impregnated polymer granules are expanded in a separate step (i.e. pre-expansion step) before the actual welding-process (i.e. sintering). Efficient transportation of the unfoamed blowing agent impregnated polymer material and control of density by the partmanufacturer are main advantages compared to expanded beads [3]. Expanded beads are produced from semi-crystalline thermoplastics, since the presence of crystalline domains prevents the storage of a blowing agent inside the solid bead [31]. An overview of the possible methods for producing polymer bead foams is given in Fig. 1. The most often used method to produce high quantities of expandable beads of polystyrene is the suspension-polymerisation with a blowing agent [32]. This process consists of two steps, namely the polymerisation where the granules are formed and the addition of pentane and/or other blowing agents, which diffuse into the granules [33]. After this process the beads are sieved to get several fractions with a narrow size distribution and coated with antistatic agents to prevent agglomeration [32]. Problems arise

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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 for expandable 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.)

with additivation, since the additives have to meet high requirements. They are not allowed to change the polymerisation process and the interfacial tension between water and styrene [34]. Another drawback is that not all polymers can be synthesised via suspension-polymerisation. Another method to produce expanded beads is the impregnation (loading with blowing agent) of micro-granules, which contain all required additives, with the blowing agent in an autoclave. This is the main production process for EPP [35]. In a first impregnation vessel the solid PP-beads are saturated with gas close to the melting point of PP (i.e. 150  C). After the saturation step, the materials with blowing agent are released to an expansion vessel [35]. Afterwards the beads are washed to remove any residual suspension stabilizer, which would inhibit proper welding of the beads [3]. For amorphous polymers, expandable beads can be produced as well, if the saturation step takes place at temperature below the glasstransition of the polymer-blowing agent-solution. Alternatively, foam extrusion with under-water pelletizing allows the production of expandable beads or already expanded beads [36]. It is schematically shown on Fig. 2. In this method, gas-loaded polymer melt is extruded through a hole-plate into a water-stream and cut by rotating knifes. If the water-pressure is above the vapour pressure of the blowing agent (for example 10.1 bar for pentane at 125  C), the blowing agent is trapped within the solidifying polymer during cooling and expandable beads are produced. At low pressure, the dissolved gas evaporates and forms bubbles resulting in expanded beads. Advantages of this method are the exact dosing of the blowing agent(s) into the melt, a continuous and flexible process and the applicability of additive that cannot be used in suspension polymerisation [32,35], which theoretically allows the processing of

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

any thermoplastic polymer with additives. Furthermore, the bead size is rather uniform [32]. A challenge is the keeping of the required temperature, since a deviation of only a few degrees might lead to unusable products [35]. Variable process parameters are temperature and pressure of the water, the rotational speed of the knives and the temperature of the perforated plate. 4. Principles of the moulding-process and machine design Parts from bead foams are made in a complex, yet efficient process, which allows the production of parts with a high geometrical degree of freedom at very low density. For the production of parts, foamed beads are welded together in a steam-chest moulding machine. The surface of the beads is molten or softened [35], using high pressure (i.e. high temperature) steam, which leads to an inter-diffusion of polymer chains between different beads resulting in a cohesion of the beads [37]. Good cohesion between the beads and a low content of macro-voids (marked with arrow in Fig. 3) are necessary to ensure favourable mechanical properties [38,39]. 4.1. Description of the steam-chest moulding process The processing of foamed beads to a finished part is done in a steam-chest-moulding machine in five steps. The steps are shown on Fig. 4 and will be explained below.

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

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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 mould The first step in the steam-chest moulding cycle is the closing of the mould. 4.1.2. Filling of the mould Foamed beads are drawn by air pressure out of a container and blown into the mould by an injector, which usually functions according to the venturi-principle. This step is critical to achieve a homogenous distribution of the beads inside the mould and even considered to be the most important step [40]. 4.1.3. Welding of the beads After the filling process, the beads are fused together by hot steam flowing through the mould. During steaming, the beads form physical links due to inter-diffusion of chains of neighbouring beads. To ensure a high quality of welding between the beads, elevated temperatures and a sufficient steaming time are necessary

[38]. Furthermore, a high contact area and force between the beads is also important to achieve good bonding between the beads. With a low contact area, force is transferred only at a few points, which leads to bad mechanical properties. On the other hand, if the contact force is low, the beads might not touch sufficiently thus also leading 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 pressures between 2.5 and 4 bar are common [41]. Thus, a steamtemperature up to 150  C is achieved corresponding to steam pressure of 4 bar. The steaming process consists of three steps, which are shown on Fig. 5. At first, the air between the beads is purged out and the mould is pre-heated. During this step, steam is flowing parallel to the 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 called cross steaming. During this step, the steam supply and exit valves

Fig. 5. Steps for steaming bead foams: 1: purging, 2: cross-steam, 3: autoclave steaming.

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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 distribution as homogeneous as possible and to ensure uniform quality of welding throughout the entire part, the mould is steamed from both sides. Finally, steam is guided into the steam chamber while the exit valves are closed to improve surface quality by the creation of a skin (autoclave steaming, Fig. 5 e 3) [42]. During steaming, the characteristic imprints of the steam nozzles on the part's surface are formed. The nozzles and exemplary imprints are depicted in Fig. 6. 5. Cooling and stabilisation For dimensional stability of the part, cooling of the mould is a crucial step. If the part is ejected without cooling, further expansion of the beads is possible, which leads to a deviation of the original size. For cooling, the mould is sprayed with water until a temperature of around 80  C is reached [40]. 6. Ejection of the moulded part After moulding and cooling, the part is finally ejected. Pressurized 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. For example 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 beads that leads to a lower pressure compared to the outer atmosphere. For components requiring high dimensional accuracy, a tempering step of the parts is necessary, e.g. at a temperature of 80  C [44]. In this step, the original shape is restored, since air diffuses quickly into the part at this elevated temperature thus reducing the vacuum inside the beads and leading to a volume expansion. Furthermore, condensed water from the steaming step is removed as well [3].

Fig. 7. Concept of the crack filling method.

7.1. Moulding technology In contrast to EPS, which still contains a certain amount of blowing agent, EPP-beads do not expand any further inside the mould without special treatment [35]. Therefore, this matter must be dealt with special processing techniques. In principle, the EPP beads 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 is shown on Fig. 7. With this method the beads are filled into a compression-mould at ambient pressure. Before the steamingprocess, the mould is closed to its final dimensions, so that the beads are compressed. With this technique very thin parts with a thickness even below the bead thickness can be achieved. The drawback of this method is an inhomogeneous density distribution in the final part [40] and if the wall thickness is not constant, the part shape is not optimal. Alternatively, counter pressure filling method can be used, which is depicted in Fig. 8. In this method, the beads are subjected to an elevated air-pressure from a compressor during the filling process, which leads to a compression of the beads [35]. After filling, the pressure is released and the beads re-expand thus reducing macro-porosity. According to the level of filling-pressure, the compression of the beads and thereby the final density of the part can be controlled. For EPP usually back-pressures between 1.5 and 3.5 bar are applied [44]. With the above-mentioned processing method only moderate densities can be achieved. To lower the density the moulding methods discussed earlier must be combined with pressure preloading [45]. During the pressure pre-loading step, the beads are subjected to pressurized hot air for several hours until the inside pressure of the beads reaches equilibrium with the outside prior to the actual moulding process. The trapped air leads to additional

Fig. 8. Concept of the pressure filling method.

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expansion of the beads during steaming thus allowing lower densities in the moulded parts. Furthermore, pressure pre-loading reduces macro-voids between the beads, which leads to better mechanical properties. 7.2. Aspects of energy saving processing and environmental friendly processing One fundamental topic in bead foaming is energy saving during the steam-chest moulding process, since the prices for energy are rising steadily (e.g. electricity in the industrial sector in Germany: 2010: 0.1207 V/kWh, 2013: 0.1487 V/kWh [46]). Also environmental protection is one of the most pressing questions amongst the population in the present [47]. Those two factors drive the trend of a more economical and ecological production of bead foam parts. Energy can be saved in every processing step from the manufacture of the expanded beads to moulding. For example, the production of beads can be optimised by more efficient extrusion equipment with better drives, since they consume most of the energy during extrusion [48]. Steam-chest moulding has a huge potential to reduce energy consumption as well. Most of the energy of the steam is lost for heating up the mould and steam chamber [49]. To overcome this issue, the walls of the steam chamber can be coated with an insulating material [50]. For EPS, moulding systems with steaming and cooling taking place in separate moulds are available, thus eliminating the need for heating and cooling the mould every cycle. Another measure is the construction of a mould without a steam chamber, but with a system distributing the steam directly within the mould [51]. Steaming without controlling steam-pressure for a short duration reduces energy consumption significantly [49]. Hossieny et al. studied the usefulness of hot air as a second energy transport medium [6]. It was found that an addition of hot air to the steam reduces the moulding time and energy consumption. Furthermore, mechanical properties (tensile strength) of the part were improved at the same time. In the cooling step there is also space for optimisation. Impulse cooling is such a measure. Instead of cooling the mould with constant spraying of water, the spraying is done in intervals. Those intervals are repeated until the mould reaches its target temperature. Thereby, cycle time can be reduced slightly and water consumption significantly. By applying vacuum to the mould during cooling the amount of residual water can be removed, which shortens the cooling time. A more extreme approach would be the complete change of the welding machinery and mechanism. Instead of steam, microwaves can be used to heat up the expandable beads thus expanding and welding them to a foamed part [52] with a reduced energy consumption [53]. Since the polymers commonly used for bead foaming are transparent for microwaves, the beads must be coated with a microwave active substance or a microwave active blowing agent must be used for expansion. With this approach the volatile organic blowing agent n-pentane could be substituted with 2propanol for EPS [54]. Starch-based bead foams were also prepared in the past using this method [55e58]. Especially for polymers requiring elevated temperatures for sintering this method is a promising approach to achieve good welding without the investment in high-temperature steam-chest moulding technology, which is not a state-of-the-art technology.

is that the former uses high temperature steam as an effective heating/cooling medium [37,59], while the latter normally uses hot air [60e62]. During the steam-chest moulding process [63,64], high temperature steam is injected into the mould in the three cycles explained in the previous section to soften and fuse the beads. In the case of EPS, the steam vaporises the volatile gas present in the beads and hence causes an expansion in volume, as well as reblowing of the beads. In case of EPP, the beads need to be compressed as discussed earlier. Through this process of steam-chest moulding, the empty space between the beads is filled and the inter-bead fusion is created. To improve bead foaming technologies and bead-moulded products, many researchers performed mechanical property tests of bead-moulded products based on the commercially available beads such as EPS and EPP. The formation of inter-bead bonding in EPS beads involves the diffusion of polymer chains across the inter-bead regions during the heating process of steam-chest moulding process. Whereas, the cooling cycle freezes the physical entanglement of the polymer chains at the inter-bead boundaries and results in the bonding of the EPS bead foams. The steam temperature and moulding time are two critical parameters affecting the extent of bead fusion and significantly affects the overall mechanical properties of the moulded EPS bead foam samples [37,39,65,66]. The physical effects during the bonding process between the beads are explained below. After the establishment of an intimate contact of the beads (Fig. 9), the actual “healing” of the interface takes place. The temporal evolution of intimate contact is dependent on pressure, temperature, time and the surface topography [67,68]. Either the establishment of the intimate contact of the surfaces or the healing can be the limiting mechanism [69]. The correct choice of processing settings cannot only reduce the macroporosity in the final part but also leads to good bonding strength between the beads. During steaming, the beads surface softens or melts. At first only wetting and van-der-Waals-forces dominate the welding process, which would result only in weak bonding. These are followed by the actual healing by inter-diffusion of polymer chains across the interfaces between the beads [68], as depicted in Fig. 10. The diffusion step is described well by several authors in the context of welding, healing of interfaces/cracks or novel measurement techniques to quantify the inter-diffusion across a surface [67,68,70e75]. A brief summary of the underlying processes, their time-scales and ultimately their relevance for bead foaming is given here. The most well known type of diffusion is Fickian diffusion, which applies to small molecules (like a blowing agent) diffusing in a homogeneous matrix without entanglements. Small molecules diffusing and dissolving in a network of entangled chains causes swelling, which leads to an increased entanglement density and changes the relaxation mechanism of the polymer resulting in non-

7.3. Physical processes during steam-chest moulding 7.3.1. Processes for inter-bead bonding The working mechanism of inter-bead bonding is similar to a sintering process. The major difference between the two processes

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

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Fig. 10. The molecular arrangement before and after steam-chest moulding. At the initial 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 interdiffusion thus forming a solid bond.

Fickian behaviour (case II diffusion) [70]. The inter-diffusion of polymers across an interface is a complicated topic depending on molecular weight, distribution of chain ends and temperature history [67]. It can be modelled from the reptation theory taking into account the so-called minor chain segments, which are the non-constrained segments outside of their original confining tube. At the initial stage, all chains are constrained to a tube and can just move along this tube due to Brownian motion. After some time, the ends escape the tube, which are now called minor chains. This process is illustrated in Fig. 11. The minor chains grow in length with time t since more segments of the chain are moving out of the constraining tube. The minor chains can move across the bead boundary, since this is a thermodynamically more favourable state, thus leading to a polymer network across the interface. For interfaces without an excess of chain ends as expected in bead foaming, the time scale for the bond strength varies with (t/tRep)½, which is also depicted in Fig. 12. In contrast, fractured surfaces with an excess of chain ends vary with (t/tRep)¼, where tRep (reptation time) is the time necessary for the chain to move completely out of its original tube. It gives the temperature dependency of the process according to the Arrhenius-equation or other equations. Bousmina et al. [70] observed a transition of the time scaling laws: at small times the t½-law holds, at longer the t¼-law. Healing is also dependent on molecular weight (Mw) of the polymer. For a linear and monodisperse polymer, the reptation time scales to tRep ~ M3w, the diffusion coefficient to D ~ Mw2 [69]. Therefore, the melts with lower molecular weight exhibit faster healing. Those dependencies emphasize that both steaming time and temperature must be sufficiently long and high for a given material to achieve good bonding.

Fig. 12. The strain energy release rate GIc as an indicator of bond strength. GIc is plotted vs. t1/2*M 1/2 for the welding of polystyrene with the molecular weights 152,000 and 400,000 (Pressure Chemical) at 115  C [reprint from Ref. [68] with permission from John Wiley and Sons].

Stupak et al. [38] conducted fracture toughness experiments on EPS, which was moulded at different temperatures and for differently long time intervals. He observed, that fracture toughness varied with moulding time by t1.25 and moulding pressure (equivalent to steam temperature) by p6.7. The time-scales differ from the ones shown above, which can be attributed to several reasons. Firstly, the contact area between the beads is highly timedependent, which changes the flow resistance of the beads bead for the steam and makes the degree of healing inhomogeneous. Secondly, the polymer chains are neither mono-disperse nor in thermo-dynamic equilibrium at initial condition. Rossacci et al. [37] investigated the effects of varying moulding pressure and moulding time on the tensile property of EPS part with densities ranging from 19 kg/m3 to 34 kg/m3. The EPS beads were sintered in a mould with a thickness of 60 mm. They reported that the steam temperature and moulding time are two critical parameters affecting the extent of bead fusion, which significantly affected the overall mechanical properties of the EPS moulded bead foam samples [47]. Zhai et al. [39,65] showed that at high steam pressure, the tensile properties of EPP moulded part with a thickness of 10 cm was much higher than the samples moulded at lower pressure as a result of improved bead-to-bead bonding. However, experience and literature shows that if the foamed beads are steamed for too long, their cell structure might collapse and deteriorate the surface property of the moulded product [38].

Fig. 11. Formation of minor chains by diffusion of chain ends out of their former constraining tube into a new one.

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7.3.2. Formation, function and evolution of the double meltingpoint of EPP In the case of moulding of EPP beads with a steam-chest moulding machine, good sintering requires a desirable double crystal melting peak structure as shown in Fig. 13. The hightemperature melting peak crystals (Tm-high) are formed during the isothermal annealing step in a batch-based EPP bead foaming process. The low-temperature (Tm-low) is formed during the cooling as foaming occurs. The hatched area in represents the desirable steam temperature range between the low and high melting peaks (Tm-low and Tm-high) of EPP beads within the steam-chest moulding machine [76e83]. When EPP beads are processed in the steamchest moulding machine, crystals associated with Tm-low melt and contribute to the fusing and sintering of individual beads. The unmolten Tm-high crystals help to preserve the overall cellular morphology and dimensional stability of the moulded EPP product. A very narrow processing window between the two melting peaks poses a significant challenge in setting the processing steam temperature during the moulding process in steam-chest moulding machine. A slight variation in steam temperature may cause the Tmhigh crystals to get affected and destroy the cellular morphology of the EPP beads and cause shrinkage of the moulded EPP product. The steam penetrates into the EPP beads during the steam-chest moulding process. During the cooling cycle, at the end of the moulding process, the high-temperature steam, which diffused into the beads, tends to condense in the cells and leads to a negative pressure. Due to the characteristic closed cell structure of EPP beads, air cannot penetrate into the foam within a short span of time, which results in a dramatic decrease in the internal pressure of the foams. Consequently moulded EPP parts tend to shrink after completion of the moulding process. An annealing process is generally used at 80  C for 4 h to enhance the diffusion rates of steam and air and thus prevent shrinkage [39].

shape of the mould, cavity the temperature distribution and thereby also density is not uniform. Hence the optimum processing condition required for the desired properties in the moulded bead foam 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 conduction, using numerical simulation techniques. They reported reduced heat conduction to the core area of the mould caused by a decrease in steam temperature as a result of drop in the steam pressure. Generally, higher operating steam pressure is implemented to improve the heat conduction to the core area of the mould. However, a higher operating steam pressure relates to higher operating cost and a higher temperature leading to an increase in localized temperature near the steam entry. Beads exposed to this high temperature may melt resulting in shrinkage at the surface of the product. This dramatically deteriorates the surface property of the moulded product. The introduction of dry hot air into the steam has shown an improvement of the heat transfer in the core of the moulded bead foam product (Fig. 14) and hence improved the overall mechanical property across the moulded bead foam products as compared to the sample moulded with pure steam [6]. Thermodynamically, the lower JouleeThomson coefficient (mJ) of hot air reduces the sensitivity of a decrease in the steam temperature with a drop in steam pressure [6,85,86]. However, steam cannot be eliminated completely due to its high thermal conductivity and heating capability due to condensation and high heat capacity compared to hot air. The introduction of hot air also reduces the local temperature at the steam entry port and hence the melting of beads on the surface is decreased resulting in better surface quality [6]. 8. Commercially available bead foams 8.1. EPS

7.3.3. Challenges and improvements of steam-chest moulding The processing steam in a steam-chest moulding machine is in the superheated state and its temperature is coupled with the processing pressure [84]. However, as the steam enters the mould cavity via small ports, the overall pressure starts decreasing due to condensation of the steam on the beads. Furthermore, the pressure of the steam decreases because of the resistance of the flow through the beads, which subsequently reduces the temperature and makes it difficult to determine the actual temperature of the mould. Moreover, considering the large volume and complicated

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

8.1.1. Advantages and disadvantages EPS is the most widely used bead foam material with a consumption of 4.7 Mt per year [87], because of its low price and high availability [88]. It is heavily used for packaging applications. This also causes major problems due to enormous amounts of EPSwaste. So, recyclability is very important [89]. EPS offers a less competitive compression set compared to EPP, which makes the latter material the favourite for applications with multiple impact deformation. With EPS, lower densities can be achieved compared to EPP but it has a less favourable chemical and temperature resistance. 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 having a bulk density of 0.64 g/cm3 [3], where EPP has to be transported in the form of foamed beads, which have a much lower density thus requiring much more space for a given mass. EPS offers good thermal insulation capabilities, which leadein combination with the low price e to the second highest market share of insulation materials after glass wool [90]. 8.1.2. Mechanical properties In principle, the mechanical behaviour of EPS is similar to EPP, since they possess the same basic structure. However, EPS has a higher 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 the maximum temperature of usage for EPS is lower than EPP, so EPP exhibits a size alteration after 4 days at 110  C of less than 2% [44], whereas EPS can only stand long-term service temperatures between 80 and 85  C [91]. Mechanical properties are

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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 the processing temperatures of T1 and T3 were measured.

highly dependent on the quality of welding of the beads, which was studied in numerous publications [37,38,93,94]. For the application in the sectors of thermal insulation and packaging, the knowledge of creep behaviour of EPS is of utmost importance [95e97]. Protective systems often put EPS to use as a shock absorber, therefore the dynamic properties of this material were studied in many publications [89,98,99]. In contrast to EPP, EPS has lower elasticity imposing constrictions on the use of EPS for packaging of high-value goods, which might be subjected to multiple impacts. This lead to the development of bead foams from PS based blends, which offer higher elasticity, toughness at low temperature and better chemical resistance [100]. 8.1.3. Thermal properties In contrast to EPP, EPS is a commonly used material for thermal insulation, since it offers advantageous thermal insulation properties (EPS: 33 mW/mK [91]; EPP: 36 mW/mK [44]) and a more competitive price in this field of application. Thermal transport in foams comprises of conduction through the solid cell walls and struts as well as the cell gas, convection and radiation. Convection can be neglected for cell smaller than 3 mm [101]. Thermal conduction of foams consists of the conduction through the solid and the cell gas. The conduction of the polymeric matrix is affected by crystallinity and their orientation [102e104], which are heavily affected by the foaming process. Either conduction through the solid or through the cell gas can be dominant, depending on density. For low densities, the cell gas dominates the thermal transport over the solid polymer because of its high volume fraction. For very small cells with a cell diameter in the same order of magnitude as the free path length of an air molecule, the Knudsen-effect becomes important and the thermal conductivity of the cell gas is reduced drastically [105].

Besides conduction, radiation is a very important transport mechanism of thermal energy in foams. This effect is mainly dependent on cell morphology and temperature. Several workers try to separate the total heat conductivity into its parts [101,106e111]. However, those contributions cannot be separated in normal measurements without modelling [112], so the authors used more or less complex models for separation. One major drawback of the above-mentioned results of the theoretical models is an assumed independence of the radiative and conductive contributions. This matter was tackled by Ferkl et al. [101] in a (at the time being) spatially one-dimensional model. No assumption on the propagation of radiation and the individual geometry was made. In literature, EPS is often used to investigate the thermal properties of foams in general [2,90,109]. However, the special particle-structure of bead foams was never investigated in detail. The authors will consider this matter in an upcoming publication. To reduce thermal radiation EPS bead foams are filled with Graphite-particles, which act as reflectors for infrared radiation thus reducing the overall thermal conductivity [113]. 8.1.4. Applications EPS is well known for packaging applications. For example, electronic devices are kept safe from transport-damage using EPS crash absorbers or spacers. Also in areas, where rigorous safety restrictions exist, such as helmets for cyclists or bikers or car-seats for children, EPS is used often [114]. In the automotive industry it is used for crash-absorbers as well. Thanks to its advantageous thermal insulation capability, it is used for the insulation of houses in form of blocks. EPS is used for acoustic insulation for example against footfall sound. For the cooling of perishable goods as drugs, food or human blood, EPS

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contributes to keep energy cost low as insulation and makes the transportation of such goods affordable and practical. Besides the application as structural or insulation material, EPS is widely used for lost-foam casting, where it is mostly used as the core material [115]. The lost-foam casting technique allows the production of complex parts, mainly from aluminium and iron, for the automotive industry ranging from cylinder heads to motor blocks [116,117]. 8.2. EPP 8.2.1. Advantages and disadvantages Among bead foams, EPP has unique advantages, such as excellent impact resistance, energy absorption, insulation, heat resistance and flotation. In addition, it is lightweight and recyclable, and exhibits good surface protection as well as resistance against oil, chemicals and water. Thanks to these advantages, the use of EPP is gaining increased momentum in the automotive, packaging, and construction industries. The combination of its flexible applicability, reasonable tooling cost, high resilience, good sound dampening at high frequencies, and, especially, its low weight, has made EPP the material of choice for numerous applications. For instance, EPP foams are now utilized as bumper cores, providing significantly higher energy absorption upon impact as opposed to conventional systems. However, unlike expandable polystyrene EPS, which is supplied as expandable pellets, suppliers can only provide EPP beads in an expanded form. The beads are then shipped to the parts manufacturers for further moulding. Due to the presence of bubbles in the bead (i.e. the large volume of the bead), the cost of storing, packaging, and transporting EPP is very high, ultimately rendering it far more expensive than EPS or a normal PP resin. Moreover, very little research has been conducted on EPP manufacturing, sintering behaviour, and steam chest moulding process. Consequently, when an EPP concept product is targeted, the manufacturer can only depend on the EPP supplier to obtain a prototype, thus having little or no control over material selections and processing conditions. 8.2.2. EPP morphology and expansion ratio The EPP beads feature high closed-cell content, which is typically 95e98 % as shown in Fig. 15 and is measured using a pycnometer in accordance to ASTM D6226. The closed-cell structure provides high expansion force, while steam-chest moulding assists with the bonding of EPP beads. Depending on the bulk density, EPP beads have cell diameters from 200 to 500 mm and cell densities in the range of 105e106 cells/cm3. The cell density, expansion ratio and crystal characteristic of the individual EPP bead foams have a significant effect on the overall mechanical properties of the moulded EPP bead foam product [2,118]. Guo et al. [119] investigated the critical processing parameters to produce EPP beads in a lab-scale autoclave system. The pressure drop was systematically controlled using a modular die at the discharge port. The die geometry (L/D) was decided to maintain a high enough pressure inside the chamber to prevent pre-foaming of the gas-impregnated EPP beads. The cell density was not affected by die geometry. On the other hand, the volume expansion of the EPP beads slightly decreased as the die length increased. The saturation pressure of the blowing agent plays a crucial role in achieving high cell densities and expansion ratios in autoclave bead foaming. During foaming of EPP beads with CO2 in an autoclave, a higher saturation pressure of CO2 allows a higher gas content to be dissolved into the PP pellets [120,121]. This higher CO2 content helps to reduce the energy barrier for cell nucleation and increases the cell nucleation rate, which leads to a higher final cell density [122,123]. The volume expansion of EPP beads was observed to increase dramatically as the saturation pressure was

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

increased. The higher cell density achieved at high saturation pressure decreases the amount of gas loss from the foamed EPP beads and hence improves the expansion ratio. 8.2.3. Crystallization behaviour of EPP beads The production of EPP beads with double melting peak characteristics has been well established [5,124e127]. The two-peak crystal structure is generated by impregnating the PP micropellets with a physical blowing agent in an autoclave chamber at elevated pressures and temperatures around PP's melting point over a certain period of time [125,127]. During the gas impregnation stage, a new crystal melting peak is created at a higher temperature, Tm-high. Since the saturation is done close to the melting point of PP not all the crystals melt. The newly generated crystal peak (Tm-high) during the isothermal gas-impregnation stage of the EPP beads stems from the perfection of the a crystal phase out of the unmolten crystals, which has a higher orientation and hence a higher melting temperature than the original peak and is known as a2 [128,129]. The melting temperature of this peak is typically above the annealing temperature. The Tm-low melting peak is generated during the rapid cooling process in autoclave foaming chamber and is known as a1. The a1 and a2 are a forms of crystals with various degree of perfection [130e136]. Choi et al. [128] have shown that by ramping the PP to the annealing temperature, the less perfect crystals melt and the more perfect crystals that exist above the annealing temperature remain unmolten. However, the work conducted by Choi et al. [128] was at ambient pressure. The actual EPP bead manufacturing process is conducted at high pressure 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 PP matrix. The dissolved gas significantly affects the crystallization behaviour of PP [19]. In the context of EPP bead foam manufacturing, the effect of dissolved blowing agent on the generation of double crystal melting peak structure can be systematically investigated using a high-pressure differential scanning calorimetry (HP-DSC). The plasticising effect of dissolved blowing agent, decreases the saturation temperature required for the generation of the higher melting peak with perfected crystals in EPP bead foams [137]. 8.2.4. Mechanical properties For EPP bead foams, copolymers with polypropylene (PP) as base monomer are preferred compared to homo-PP because the latter has poor impact properties at low service temperatures [128,138e143]. The copolymers can be binary, such as a propyleneeethylene copolymer or a propyleneebutene copolymer, or a ternary copolymer, such as a propyleneeethyleneebutene copolymer [128,144]. By using branched high-melt-strength PP [77] and

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metallocene-catalysed PP [5,145], the mechanical properties and compressibility of EPP beads and their moulded foam products can be improved. Other studies have shown that the mechanical properties of EPP beads can be improved significantly by choosing an appropriate PP copolymer that will lead to better control of the secondary crystal form [146]. For instance, to improve EPP's inmould foamability, researchers have employed a PP copolymer with a lower melting temperature [147]; in another case, graphite was introduced in order to increase heat resistance [138]. Furthermore, it has been shown that the use of PP nano-composites can also improve EPP bead properties [148]. Efforts have also been made to produce expandable PP beads. However, the use of either an encapsulated physical blowing agent [149] or a dispersed chemical blowing agent [150] in the beads has not become common practice in the industry. As mentioned earlier, for EPP foamed beads to have a good sintering during the steam-chest moulding stage, they need to possess a double-peak (or at least broad) melting characteristic. The ratio between the Tm-low and Tm-high peaks is thus crucial in determining the surface quality and mechanical properties of the steam-chest moulded EPP product. If the Tm-low peak is dominant, then the moulded EPP product may shrink and the overall geometry would collapse. In contrast, if the Tm-high peak is dominant, then the sintering will be weak resulting in poor mechanical properties. The failure mechanism in moulded EPP products has been attributed to the bead boundaries and a potential fracture path between the beads [151,152]. This is known as inter-bead bonding and it has been reported that they determine the mechanical properties of the bead products [151,152]. Inter-bead fracture arises due to weak sintering between the EPP beads. However, another failure mechanism occurs within the EPP beads and is known as intra-bead fracture. This failure reflects that there is good sintering between the EPP beads. The inter-bead and intra-bead failure mechanism can be investigated by observing the crack surface under a scanning electron microscope as shown in Fig. 16. The tensile strength of EPP samples has a strong dependency on the processing steam pressure and corresponding temperature used during the steam-chest moulding process. The tensile properties of EPP samples increases at higher steam pressure. A similar phenomenon was observed in EPS bead processing, where a high tensile strength and a high degree of inter-bead fusion was obtained at high moulding pressure [152]. The tensile strength of EPP moulded samples also increased significantly due to the development of crystals in the inter-bead areas during the cooling cycle of the steam-chest moulding process [119,153]. EPP bead size is another important parameter, which affects the inter-bead bonding and improves the mechanical properties of moulded products [119].

8.2.5. Trends EPP is in a state of constant development and getting closer to the customer. Previously EPP was mainly used in the automotive industry as structural material in the application as cores for crash bumpers or for tool boxes in the car boot. For those applications the specific advantages of EPP as low density and good energy dissipation at impact are harvested. Today's trends aim towards higher functionality. For example hinges, snap fits or fasteners make the material fit for new applications as furniture. Challenges are the steam nozzle imprints and its technical appearance (Fig. 6). Therefore, the development of multi-material systems is facilitated [154]. An EPP foam-core can be combined with a layer of TPE for decoration. The connection of both components can be achieved in an online process. However, if a

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

coating is desired, adhesion between the coating and EPP is still challenging making a surface treatment necessary [155]. Another approach to modify the properties of EPP is hybridisation [156]. So, the EPP beads are combined with metal bead foams in order to create a hybrid material with highly elastic behaviour at low stress (behaviour dominated by EPP) and high energy dissipation at high stress (behaviour now dominated by metal foam). The purpose is to produce better crash bumpers for cars to increase passenger safety while reducing weight. The development of EPP is not at the end, but very dynamic and rapidly 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 a wide range of properties, filling the gap between thermoplastics and elastomers. They combine the properties of elastomers with easy processability of thermoplastics. TPEs differ according to structure, rheological and thermal properties. There has been significant amount of work on the foaming of TPEs, however there is very limited literature on the processing of TPE bead foams. Thermoplastic polyurethanes (TPUs) consist of a phaseseparated molecular structure of rigid hard segment (HS) domains dispersed in a soft segment (SS) matrix, which provides TPUs with a unique combination of strength, flexibility and processability. However, the application of TPUs is mostly limited to its high hardness and cost. Hence foaming could be a desirable way of reducing density and thereby cost. TPUs do not still have a widely used foaming method and is one group of TPEs under a lot of research.

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Expanded TPU bead foams (ETPU) and its product have been developed recently [157]. One example is the shoe sole of the Adidas “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 properties and a high rebound effect. Generally softer grade TPUs with shore hardness between A 62 and A 80 are preferred to produce ETPU beads for industrial applications. The advantage is that softer TPUs have a lower concentration of HSs and hence a lower melting point with much better flowability. Thus lower processing temperature and pressure are required during the processing of ETPU beads. Furthermore, lower steam pressures are required during the sintering step of the ETPU beads, which reduces the sintering cost. The softness of the beads also makes adhesive-bonding of the ETPU beads more effective. The ETPU beads can be produced both in a batch setup using a high-pressure autoclave as well as a continuous setup using foam extrusion and an underwater pelletizer. During the processing of ETPU beads in a high-pressure autoclave setup, the TPU micropellets are fed into the high-pressure autoclave with suspension medium and saturated with the blowing agent. The autoclave is then heated to the impregnation temperature, which is generally near the softening point of the TPU. The blowing agent impregnates into the SS and results in the swelling of the TPU. The impregnation time is generally from 0.5 to 10 h [157]. The impregnation with blowing agent near the softening point of TPU results in the rearrangement of the existing HS crystalline domains. After completion of the saturation cycle, the autoclave chamber is depressurized, which results in production of ETPU beads. During the depressurization step, which results in cooling of the TPU material new HS crystallites are formed in the microstructure. The HS crystallites in the TPU microstructure can be effectively utilized as heterogeneous bubble nucleating agents to produce microcellular ETPU bead foams [158].

9.3. Biopolymer-based Among different biopolymers, polylactide acid (PLA) has gained a lot of interest for bead foaming applications. The interest is due to its high potential to substitute EPS bead foam products used in packaging and commodity products [168]. The production of EPLA beads has been commercialised and the technique is similar to the processing of EPS beads. In this technique, the PLA micro-pellets are saturated with blowing agent below PLA's glass transition temperature and the pre-foaming is done in a pre-expander machine using steam or hot air. The expanded EPLA beads are then sintered together using a steam-chest moulding machine. Prior to sintering, the pre-foamed PLA beads are coated with a special coating to improve the sintering of the EPLA beads during the steam-chest moulding process. The thermal and mechanical properties of EPLA beads processed with the technique described above are similar to the EPS bead foam product compared at the similar density. The technique of utilizing the double melting peak crystals (similar to the EPP beads) is believed to be a promising method to improve the crystallization kinetics of PLA and also the sintering of EPLA beads. It is believed that the presence of dissolved gas will significantly improve the crystallization kinetics of PLA and it will be beneficial for the production and sintering of EPLA beads [169]. The crystals formed during the processing of EPLA beads would also enhance the poor foaming behaviour of PLA. The generated crystals during the saturation process will promote the cell nucleation by acting as heterogeneous nucleating agents. Furthermore, the connection of PLA molecular chains through the generated crystals will improve the low melt strength of PLA and consequently increase the expandability of EPLA beads by minimizing the gas loss and cell coalescence. 10. Conclusion

9.2. Polyesters Foaming of polyesters as for example polyethylene terephthalate (PET), polybutylene terephthalate (PBT) or polylactideacid (PLA) is challenging, since they possess disadvantageous rheological properties for foaming like low melt elasticity and low viscosity [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 biobased bead foams. Foams of PET are commercially available since the 1990s from many companies, for example by Airex AG (AIREX T92), Armacell Benelux S.A. (ArmaFORM PET) and BASF SE (Kerdyn). They are mainly used for foam-cores in sandwich applications. Extrusion foaming is the mostly used production method and allows the production of foams with densities between 30 and 400 kg/m3 [160]. In contrast to extrusion foams, bead foams made of PET are not available on the market yet. However, patents were filed since 2011 [161e164]. In the past, studies on the foaming behaviour of PBT were done by extrusion foaming by Jeong and Xanthos [165,166] and finally achieved a density of 330 kg/m3. Recently, the bead foaming capability of PBT was studied for expanded PBT using extrusion with an under-water pelletizing system [167]. The lowest density achieved for the processed EPBT beads was 280 kg/m3. The effects of processing parameters like knife-speed, water pressure and temperature as well as the viscosity of the PBTgrade were investigated. A variation of processing conditions showed moderate influence on bead shape and cellular morphology. In contrast to that, the viscosity was much more important. Increased viscosity lead to a better bead shape and cellular structure.

This review-paper outlined the work done on bead foams both from an industrial and scientific viewpoint and showed the trends and perspectives of state-of the-art materials and machinery, as wells as new material developments. Currently, EPS is the most widely used bead foam. It is used for commodity applications requiring cost effective part production in huge quantities as packaging and insulation. EPS has a high specific modulus and strength at the drawback of low elasticity. In contrast to EPS, the other two commonly used bead foams EPE and EPP are more expensive, but have a much higher elasticity making them suitable for the packaging of more sensitive goods. Especially EPP is gaining more and more attention as new fields for its application besides the traditional ones as structural material are found. Nowadays, EPP is used for furniture or multi-material combinations. An advancement of the recent past is ETPU, since thermoplastic polyurethane is one of the most elastic thermoplastically processable materials. Bead foams made of this material shine out with extraordinary elasticity making parts from ETPU an ideal candidate for damping high impact forces as found in shoe soles. ETPU bead foams will open new areas as the foaming reduces TPU hardness without the use of plasticizers. However, all state-of-theart bead foams have a low usage temperature in common. Therefore, current developments aim to tackle this issue by making technical thermoplastics like PBT ready for bead foaming. Bead foams made of bio-based polymers like PLA are also a key-focus of current research. Beside research on new materials or their optimisation, the other focus of investigation lies on the processing of bead foams. The trends in machinery show that steam-chest moulding machines are made ready for new types of bead foams requiring high

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steam temperature, which is critical since mould, steam chamber and injectors have to withstand much higher steam pressure. Optimisation towards lower energy consumption to facilitate an efficient production of huge quantities of bead foam parts is also a challenge to ensure efficient production. More efficient steaming and cooling cycles are key features for new steam-chest moulding machines. Although bead foams are quite mature in terms of time on the market, there are still many open scientific questions regarding the effect of their unique morphology on properties, physical phenomena during steam chest moulding and how to achieve suitable welding properties. Acknowledgement We thank our colleagues Kalaivani Subramaniam, Tobias Standau, Christian Trassl, Amir Fathi, Peter Schreier, Julia Gensel and Janina Lauer for numerous fruitful discussions and for support. The supports of the German Research Foundation (DFG and SFB 840) 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 calorimetry HS hard segment SS soft segment TG glass transition temperature Tm melting temperature TPE thermoplastic elastomer References [1] Beverte I. Deformation of polypropylene foam Neopolen® P in compression. J Cell Plast 2004;40:191e204. [2] Schellenberg J, Wallis M. Dependence of Thermal properties of expandable polystyrene particle foam on cell size and density. J Cell Plast 2010;46: 209e22. [3] Lee EK. Novel manufacturing processes for polymer bead foams. University of Toronto; 2010. [4] Koleski MIP, Prates LB. WO 2008019458 A1-expanded polypropylene foam, 2007. [5] Sasaki H, Sakaguchi M, Akiyama M, Tokoro H. US 6,313,184 B1 expanded polypropylene resin beads and a molded article, 2001. [6] Hossieny N, Ameli A, Park CB. Characterization of expanded polypropylene bead foams with modified steam-chest molding. Ind Eng Chem Res 2013;52: 8236e47. [7] Winterling H, Sonntag N. Rigid polystyrene foam. Kunstst Int 2011;10: 18e21. [8] Trassl C, Altst€ adt V. Particle foams: future materials for lightweight construction and design. Kunstst Int 2014;2:73e6. [9] Lee ST, Ramesh NS. Polymeric foams e mechanisms and materials. CRC Press; 2004. [10] Altst€ adt V, Mantey A. Thermoplast e Schaumspritzgießen. Hanser Verlag; 2010. [11] Kundra P, Upreti SR, Lohi A, Wu J. Experimental determination of composition-dependent diffusivity of carbon dioxide in polypropylene. J Chem Eng Data 2011;56:21e6. [12] Sato Y, Takikawa T, Takishima S, Masuoka H. Solubilities and diffusion coefficients of carbon dioxide in poly(vinyl acetate) and polystyrene. J Supercrit Fluids 2001;19:187e98. [13] Zhang Q, Xanthos M, Dey SK. Parameters affecting the in-line measurement of gas solubility in thermoplastic melts during foam extrusion. J Cell Plast 2001;37:284e92.

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