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15th International Conference on Composite Structures ICCS 15

A. J. M. Ferreira (Editor) FEUP, Porto, 2009

Innovative processing methods with thermoplastic PET-foam

Moritz Pieper, Lars Massüger, Roman Gätzi

Alcan Airex AG Industrie Nord, CH-5643 SINS, Switzerland

email : [email protected], web page: http://www.alcanairex.com

Key words: sandwich composite structure, core materials, thermoplastics, thermobonding.

Summary. Nowadays sandwich composite structures are mainly produced with thermosets. The AIREX core materials are designed for such processes but with the most recent product, the thermoplastic PET-foam AIREX® T90/T92, more innovative processing methods can be realized. While thermoplastics tend to dominate unreinforced plastics manufacturing, they are still not widely used in the composites industry. This paper will discuss innovative processing methods with AIREX® T90/T92 foam also in combination with other composite materials; topics are thermoforming of foam, all-thermoplastic sandwich and thermobonding facings to AIREX® T90/T92. The advantages of engineering thermoplastics include weight saving, impact resistance, potential of short cycle times and recycling.

1 Introduction Sandwich composite designs, wherein a lower density core has been laminated between two relatively thin but high tensile strength facings, are successfully employed in a wide variety of applications that require lightweight, yet structurally strong and stiff constructions. To name a few, structural sandwich composites are used in marine, wind energy rotor blades, aerospace, transportation and industrial applications. [1] As one of the world wide leading companies in core materials, Alcan Airex present, beside a wide product range of PVC-, PEI-, PUR-foam or Balsa wood, the second generation of Polyethylene-terephtalate (PET) foam AIREX® T92. This foam is produced in a continuous extruded process and provides a high quality, low standard deviation and a high production volume. The foam is based on an environmentally friendly blowing agent and the core material can be recycled easily. The PET-foam AIREX® T90 and T91 (predecessor of AIREX® T92) is approved since many years by all conventional sandwich composite production processes. Conventional processes to sandwich composites are hand lamination, vacuum infusion, autoclave manufacturing and prepreg curing. This paper present experiments of innovative processing methods that are now possible with AIREX® T92 and its exciting properties, pure thermoplastic basis and recyclability.

Moritz Pieper. Lars Massüger. Roman Gätzi

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2 Thermoforming

2.1 Why thermoforming Sheets of Airex foam are sliced out of rectangular blocks of dimensions up to ca 2.5 x 1.2 meter to the desired thickness to be used in a structural sandwich. Many sandwich parts are not planar but 2 or even 3-dimensional, like boat hulls, windmill blades, or train front ends. There are several possibilities to shape the foam. Some foams are cold bendable to a certain thickness and low degree of deformation and can easily be used to build 2-dimensional sandwich parts. Foam sheets can also be converted to scored foam, having small foam cubes bonded to a cloth. In this way even 3 dimensional shaped sandwich parts can be produced. Since the slits in the scrim foam must be filled with resin or foam bonding adhesive when processing to a sandwich part, more resin is needed and therefore an increase in specific weight results. Furthermore, there is a risk of not completely filling all the gaps. This results in stress concentration in the core, which can lead to premature failure of the sandwich part. The most efficient way to eliminate these problems is to thermoform the foam sheet to the desired end shape as the slits are not needed.

2.2 Thermoforming process Foam materials are stiffer at room temperature, they will soften at elevated temperatures, allowing for easy deforming into whatever shape is desired. By cooling the foam down to room temperature again the foam gets stiff and keeps the shape given. This general thermoforming process could be structured in 3 main processing steps, the heating-, forming- and cooling phase. A typical thermoform process of foam is, to heat the foam sheets in a convection oven and form it in a mould. However, the temperature must have the optimal thermoforming temperature. Because foam sheets are rather good thermal isolators the foam has to be dwelled at this temperature for a fairly long time mainly depending on the foam sheet thickness until the foam sheet reaches a uniform temperature. Afterwards, the foam sheet is taken from the oven and immediately formed in the mould. The foam must then be cooled down to approximately 60°C or at least below the softening point to freeze the shape before the pressure is released [2].


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Figure 1: Conventional thermoforming process Nowadays there are many other types of thermoform technologies on the market, that offer new ways of processing foam materials. The following chapter will give a short overview about possibilities of heating, forming and cooling.

2.2.1 Heating process The heating of foam is one of the critical steps of the whole process. Is the temperature too low, the foam wouldn’t keep the shape desired or even brake during forming. Is the temperature to high, then the foam cells will collapse and the homogeneous structure of the foam is destroyed. Depending on the thermoforming temperature and the processing cycle, 3 different heating strategies can be named as followed.

− Thermal convection (convection oven, hot air radiator) − Thermal conduction (heating plate teflon coated) − Thermal radiation (infrared radiator, ceramic plate radiator, halogen radiator)

Best thermoforming results with Airex foams can be done by applying the following temperatures to the foam before forming. These values will give an approximate temperature, but it always depends on the part thickness. Representative trails are necessary. Foam Formation Temperature range AIREX® T90/T92 Polyethylene terephthalate PET 155-175°C AIREX® R82 Polyetherimide PEI 205-215°C AIREX® R63 Linear polyvinyl chloride PVC 90-110°C AIREX® C70 Crosslinked polyvinyl chloride X-PVC 115-135°C AIREX® C52 Polyurethan PU 140-160°C Table 1: AIREX core material and their thermoforming temperatures

Pressing + Cooling Heating of foam Demoulding

Oven Mould

Foam

Shaped foam


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2.2.2 Forming process During a change in the shape of a foam due to an applied force, the strain that will effect on the foam cells would be due to local tensile-, compression- and shear forces. Depending on the base material of the foam, there are different reactions of the cells in extreme deforming conditions. The more homogeneous a material is in its raw material, the better are the properties for heating and forming the foam. The loading could be done in different ways, depending on the cycle time, accuracy needed and complexity of the shape. A shaping mould made of wood, metal or GRP can be used with a pressure given medium like a vacuum membrane or a load introduced with weight. In most industrial processes there are moulds used made of metal with lower tool and upper punching tool. Very large and thick foam parts can be thermoformed using the creep forming procedure. The foam already loaded by weight is fixed on the mould and placed in an air circulation oven. The oven is then heated to the desired thermoform temperature and dwelled until the foam adapted to the shape of the mould. Depending on the forming process the foam could be cooled by using a cooling media like water or air flowing through the mould, or cooling it only by room temperature. Because of the shrinkage of the material at the temperature exposed, the cooling process creates inner stresses in the foam. Therefore the effect of a spring back must always be considered into the mould.

2.3 Thermoforming of AIREX ® T90/T92 In all processes of thermoforming described above, the new AIREX® T92 is one of the most uncomplicated and easy processing foams on the market. With its quick heating and cooling properties, this foam opens doors for the short cycling time processing industries. Just as well the deformation into very complex shapes is actually possible and has nearly zero spring back effect. The deformation of AIREX® T92 with compression- and shear forces are analyzed in the following with the processing methods compression moulding and deep drawn thermoforming.

2.3.1 Compression moulding Out of a planar geometry the foam was compressed into a mould after heating it in a convection oven to 170°C. After a short cooling time, the formed sample was removed without any spring back and with a homogeneous surface.


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Figure 2: Compression moulded AIREX T92 specimen The foam was compressed in several sections, which means that the density there is certainly higher and the foam cells are extremely deformed. The cell-structures at the compressed cross-sections of the sample have been microscopically investigated and compared to reference samples of the identical type of foam and sheet.

Figure 3: AIREX T92 reference (left) & compressed (right) Other trials also clearly showed that the cell walls do not get crushed during the load cycle, even at the rather low temperature of 105°C. Therefore no damage is induced, which could lead to weakening of the material. Additionally, mechanical properties are even expected to have improved, as the density of the foam is increased by the hot moulding process.

2.3.2 Deep drawn thermoforming Deep drawn thermoforming is a sheet forming process in which a thermoplastic sheet is drawn into a forming die by the mechanical action of a punch. The combination of advanced production possibilities and the advanced foam materials offer new ways of producing thermoformed units in short cycle times and constant quality. For exact thicknesses within closed tolerances it is suggested to thermoform in a closed mould.

thermoformed geometry

initial geometry


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The thermoforming parts with AIREX® T92.100 in 10mm thickness were produced in cooperation with the thermoforming company Plastika Balumag AG [3]. The foam sheet with a dimension of about 800mmx 500mm was positioned with a clamping frame on a open area where two halogen fields heated the foam from both sides to the desired temperature. With the halogen spots, a homogenous temperature distribution on every section of the sheet is possible, because each little spot can be adjusted separately. After the 30 seconds heating cycle at 170°C, the halogen radiator run backwards and the mould was closed within 3 seconds with the 80°C heated lower and upper punching tool. A cooling time of 20 seconds is enough before removing the component showed below in figure 4.

Figure 4: Deep drawn thermoformed AIREX T90/T92 specimen 3 Thermoplastic sandwich The strength and stiffness of a beam can be dramatically increased with very little added weight by adding a low density core between a material. In such sandwich structures, less material is required than with a solid structure and it acts much like an endless I-beam, where the skins are separated at a distance to provide stiffness. By increasing the sandwich thickness by two (double the core thickness), the panel will become 4 times as stiff and 2 times as strong at almost same weight. The advantage of a sandwich is that little weight is added when increasing the thickness. The selection of materials that are used for sandwich structures is very wide. In the following research thermoplastic skins in combination with thermoplastic foams like AIREX® T90/T91 are investigated and there comparisons to thermoset components are listed. Also some possibilities of producing thermoplastic sandwich structures are presented.


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3.1 Core materials Beside weight and stiffness, another advantage of a sandwich panel is that the core materials help to distribute and absorb impact. In common cores such as balsa wood or foam, the skin is in constant contact with the entire core; thus during an impact, the force is distributed over a wider area. But this is not true for all sandwich panels, for example honeycomb cores. They are often stiffer and stronger but have much lower impact resistance as the skin is not in full contact with the core.

3.2 Composite skins Most composites skins consist of two parts, a structural fabric and a resin matrix holding the fabric together. This structural fabric (glass-, carbon-, aramid fibre) is strong, lightweight, and when bundled up with thousands of other tiny fibres, it is incredibly strong. However, this fabric alone is not stiff or sturdy enough to support loading requirements. In order for the sandwich panel to be truly effective, the fabric skins must be “impregnated” and held together with a resin matrix. The resin matrix for composites can be broken down into two major categories, thermosets and thermoplastics. Thermoset resins generally come in liquid form, and when mixed with a catalyst, a chemical reaction occurs forming a solid. Thermoset molecules crosslink with each other during curing, thus once cured, they cannot change. A thermoplastic matrix has molecules that are generally not crosslinked, meaning, the material can be repeatedly melted and reused. Usually, no chemical change occurs when thermoplastic is cured. [4] In addition to the thermoplastic fibre-reinforced skins (e.g. Polystrand, SkinTec or Twintex), also self-reinforced skins (e.g. Curv) find more importance in sandwich structures. Self-reinforced thermoplastic skins capture the mechanical properties of highly oriented tapes in a matrix of the same material, e.g. polypropylene (PP). That means that the material is a 100% PP thermoplastic without fibres[5].

3.3 Comparison of thermoplastic and thermoset sandwich

3.3.1 Thermoset sandwich Pros Cons Easy to process and laminate Does not necessarily need heat to cure Generally stronger than thermoplastics Generally better suited to higher temperatures then thermoplastics

Often release emissions before cured Cannot be recycled or reclaimed easily Short workable pot life Less-than-perfect surface finish (shrinkage)

Table 2: Pros and Cons of thermoset sandwich structures


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Types of Themoset Resins Types of conventional core materials Epoxy Polyester Vinylester Polyurethane Phenolic

Balsa Polyurethane (PU) Crosslinked Polyvinyl chloride (PVC)

Table 3: Thermoset types of resin and conventional types of core materials

3.3.2 Thermoplastic sandwich Pros Cons High Impact Strength Attractive Surface Finish Recyclable No Emissions Can bond to other thermoplastics Can be moulded or shaped with reheat

Generally softens with heat

Table 4: Pros and Cons of thermoplastic sandwich structures Types of Thermoplastic Resins Types of thermoplastic core materials (foams) Polybutylene terephthalate (PBT) Polyethylene terephthalate (PET) Polycarbonate (PC) Polypropylene (PP)

Polyethylene terephthalate (PET) Linear Polyvinyl chloride (PVC) Polyetherimide (PEI)

Table 5: Thermoplastic types of resin and core materials

3.4 Production process All-thermoplastic composites sandwiches, with reinforced thermoplastic skins and thermoplastic core materials, are of interest for the short cycle processing industry with cycle times below 1.5 minutes. The tools and processing steps are similar to the thermoforming process descript above. Following chapter will describe two common processing methods for manufacturing such sandwich panel to 3 dimensional shapes.

3.4.1 Creep forming and skin bonding in an oven The assembling of the core and sheet materials is done before placing them into the mould. The mould material must be temperature resistant, as temperature is applied onto the whole system by heating it in a convection oven. The mould must be loaded with a constant pressure so that the core and skin is deformed when gradually reaching the necessary temperature. This constant pressure could be applied for example by a strong spring system. The melting


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temperature of the thermoplastic skin matrix must be under the foam stability temperature. The compression resistance of the core must be enough to create a sufficient adhesion between skin and core and that is only reachable with enough pressure on the facing. Just as well as the advantage of a high surface quality in consistence with thermoplastic skins can only be achieved with the backpressure of the core material. After the sandwich structure was held under pressure for some minutes in the closed mould, the component could be cooled down in or outside the oven and be demoulded afterwards.

Figure 5: Creep formed specimen with thermoplastic foam and skins

Figure 6: Process of creep forming with skin bonding Using AIREX® T92 for this application, temperatures of 170°C – 180°C can be applied. For this temperature range, thermoplastic skins with PP matrix and woven skins like Twintex are an option.

3.4.2 Separate heating of skins This production process is used for most industrialised thermoplastic sandwich manufacture, due to its very low cycle times. With heating the skins separately by heating blades, infrared, or halogen radiation, a very high temperature can be applied without affecting the core material but for achieving the necessary matrix viscosity of the skins. This production process

Assembling Moulding Heating under pressure + Cooling Demoulding

Skins

Oven Mould

Core Sandwich


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allows using higher temperature skin matrix with lower temperature resistant core materials. The skin with best results in the trails was SkinTec of the company IQ-Holding with PBT matrix and a melting point of about 230°C. The heated skin must be consolidated on a very fast process with the cold foam material. An automated process is often necessary for this application; the high skin temperature must be kept constant until pressing them together with the foam material. Only by keeping the right parameters, a controlled adhesion between skin and core can be achieved, which is also decisive for an attractive surface finish. The foam itself often doesn’t need an extra heating, after assembling all layers, the foam absorb the necessary thermoforming temperature of the skins. Some restrictions apply depending on the foam thickness and the desired shape.

Figure 7: Thermoplastic sandwich produced with separate skin heating

Figure 8: Process of thermoplastic sandwich manufacturing with separate skin heating The application could be for interior, boxes, containers, generally all other sandwich composite applications where short cycle times are needed.

Assembling Moulding + Cooling Heating of skins Demoulding

Skins Heating blade Mould

Core

Sandwich


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4 Thermobonding of a sandwich panel Joining has proved to be a critical step in the process of manufacturing composite products because it can initiate a number of irregularities in the structure, which can result in weakening of the properties. In all composite sandwich structures a resin or other bonding element is necessary to join the facing to the core material. Such adhesion layers or resin matrix are creating weight and costs, they have long curing cycles and often release emissions. By the way they are often the hindrance of an easy recyclability of a sandwich structure. Fusion bonding is widely considered to be the ideal joining technique for thermoplastic composites. Known also as welding, it can be generally described as joining of two parts by fusing their contact interfaces, followed by cooling under pressure, which enables the bond to be made. With heating the PET-foam surface over its melting point to about 280°C, a PET-melt could be created that brings the adhesion to a facing compared to a hotmelt adhesion. The Following experiment will show a possibility of creating aluminium - PET-foam sandwich structure without adding any adhesion media to the whole compound.

Figure 9: Thermobonded sandwich panel with aluminium skin In this experiment the aluminium sheets were heated to 300°C with a heating blade, and then pressed onto the AIREX® T92 from both sides. The aluminium sheets were only brushed but not pre-treated any more. A constant temperature on the aluminium sheets must be applied during the low pressure consolidation to create a PET-melt between foam and aluminium skins. Some seconds of heating are necessary to liquidate 1.5mm of foam material to a compact PET-melt. Afterwards the compound was cooled under pressure to room temperature within some seconds. A good adhesion was proved when peeling off the skins from the core and destroying the panels by an impact load, showed in figure 10.


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Figure 10: Process of thermobonded sandwich panel with aluminium skins The advantages for this production process of a sandwich panel are the short production cycles, the loss of pre-treatment of the skins and weight saving. Better impact behaviour and recyclability are expected. Looking at an industrial production process, a continuous heating, pressing and cooling process could be imaginable with all skins of electrically conducting. 5 Conclusion Methods for processing PET-foams presented in this paper, have been investigated in very simple experiments. However, these basic tests are allowing a view into innovative processes, which are nowadays not really observed by the composites industry. New machineries, materials and linked know-how will open doors for many applications for composite materials. The demonstrated processing methods are all using the advantage of the pure thermoplastic material of AIREX® T90/T92. Either the exciting formability of PET-foam with applying temperature for thermoforming, or good combination with other thermoplastic materials like fibre reinforced skin of PBT matrix, or the adhesion force of melted PET for producing a sandwich panel without any glue. References [1] Feichtinger K, “Novel thermoplastic foam structural core material with enhanced thermoformability, fatigue endurance and elevated temperatures properties”, [2] Processing guideline of Alcan Airex [3] www.plastikabalumag.ch, Plastika Balumag AG, 6281 Hochdorf Switzerland [4] www.sandwichpanels.org [5] SkinTec PBT - www.iq-holding.com, Twintex PP - www.owenscorning.com, Curv - www.curvonline.com

Assembling Pressing + Cooling Heating of skins Demoulding

Aluminum Skins Heating blade Mould

Core

Sandwich

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