Development of a Rapid Thermoplastic Impregnation Device

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Development

of a

Rapid Thermoplastic Impregnation Device

Proefschrift

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 8 december 2008

te 12.30 uur

door

Teun WEUSTINK

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Dit proefschrift is goedgekeurd door de promotoren: Prof. dr. ir. A. van Keulen

Prof. dr. ir. R. Marissen

Samenstelling van de promotiecommissie:

Rector Magnificus, Voorzitter

Prof. dr. ir. F. van Keulen Technische Universiteit Delft, promotor Prof. dr. ir. R. Marissen Technische Universiteit Delft, promotor Prof. dr. ir. R. Akkerman Universiteit Twente

Prof. dr. D.J. Rixen Technische Universiteit Delft Dr. ir. H.E.N. Bersee Technische Universiteit Delft Dr. ir. K.M.B. Jansen Technische Universiteit Delft

c

2008 by Teun Weustink

No part of the material protected by this copyright notice may be reproduced or utilized in a any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission of the author.

Printed by PrintPartners Ipskamp

ISBN: 978–90–9022623–1

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Chapter 1 Introduction

1.1 General introduction

There is a constant search for materials which combine low weight and high strength and stiffness. In aerospace industry minimizing the weight of an air- or spacecraft has always been a motive for the search for new and lighter materials, which combine high strength and stiffness. Especially a high stiffness is an important issue in aerospace structures for the prevention of buckling and/or flutter. Moreover, even ground transportation can benefit from weight reduction. As an order of magnitude, it can be estimated that one kilogram mass reduction in a passenger car may translate into 3 kilogram fuel

consump-tion reducconsump-tion during the car’s lifetime [1, 2]. Composite materials offer a low weight in combination with high strength and stiffness.

Strictly speaking, a composite material consists of two or more materials where the combination of materials yields unique properties [3]. Already in the ancient Egypt a composite was created by reinforcing clay with straw for the construction of walls [4]. The straw acts as reinforcement for the wall against rupture and facilitates the drying pro-cess of the clay. However, the concept of composite materials is not invented by mankind; they are present in nature. Wood is, for example, a ‘natural’ composite material where the cellulose fibers are embedded in lignin, a kind of natural glue [5]. Clams are compos-ites of calcium-based minerals in a protein matrix called conchiolin. Other, man-made examples of composite materials are reinforced concrete and fiber-metal laminates. It is clear, that the term ‘composites’ covers a broad range of composed materials. Nowadays, there is a tendency to define composite materials as fiber reinforced plastics (FRP). This new, ‘modern’ definition of composites started in the 1940s with the development of glass fiber-polyester products [6].

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the high costs of the raw materials and laborious manufacturing of products. Another rea-son is the difficulty of modeling composite materials due to their generally non-isotropic and inhomogeneous behavior, which is not only due to the composition of the material, but also to the manufacturing process. These limiting factors are slowly diminishing, since a lot of research has been focused on modeling and manufacturing of both the composite it-self and composite products and structures. This has resulted in improved analysis models and more reliable, efficient and economical processes. For example, the price of carbon fibers is still decreasing [7]. Since composite materials and their product manufacturing become cheaper, they are also used on a large scale in automotive (doors, motor supports, underfloors), leisure (fishing rods, hockey sticks, bicycle frames and forks) and civil en-gineering (bridge panels). Various components in current aircrafts are already made of composites, typical examples are floorbeams and the airbus tailplanes.

Currently, Boeing is building the 787 ‘Dreamliner’, an airplane suited for 200–290 passengers, depending on the configuration. The majority of the primary structure, in-cluding the wing and fuselage, will be made of composite materials. Boeing expects a reduction of 20% fuel consumption compared to a competitive conventional airplane.

The intention of Boeing is a sign that analysis, manufacturing and production of structures of composite materials is sufficiently understood and has become economically viable.

Another example is the development of SpaceShipOne within the first private manned space program. SpaceShipOne is designed and built by Scaled Composites [8] and its primary structure is made of carbon fibers with epoxy. This Spaceship can carry up to three people and is airborne launched. It has already reached space, which is officially located at a height of100 kilometers, twice within the span of 14 days and has received

the X-prize of10 million dollars for this achievement [8].

Fiber reinforced plastics can be sub-divided in fibers reinforced with thermoset and thermoplastic polymers. The fibers can either be discontinuous or continuous. The main differences between the matrix materials are the high viscosity and reversibility of so-lidification and melting of thermoplastics compared to thermosets. Viscosities of molten thermoplastics are in the order of _{500 − 5000 Pa·s while typical values for thermosets} are _{100 Pa·s [9]. The application of thermoplastics as impregnate came into use in the} 1980s, primarily because of the ability to form them by heating at low pressure [10]. Other advantages using thermoplastics over thermosets are an often higher toughness, an unlimited shelf-life, short production cycles, a high environmental resistance, good impact properties and high level of recyclibility of the final product [11]. However, the main disadvantage is the high viscosity of thermoplastics which hampers impregnation of fibers.

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impregnation quality, which is often referred to as the ‘speed-quality’ dilemma.

The manufacturing process of a fiber-reinforced product can roughly be divided into methods where impregnation and forming is combined in a single process and methods where pre-impregnated half-products are used. The latter option seems to have the largest potential for application on large industrial scale, where the first method is rather slow and labor intensive. Processes using pre-impregnated half products which are re-heated are, amongst others, Glass Mat Thermoplastics (GMT), injection molding and filament winding. The quality of the pre-impregnated half products is decisive for the quality of the final product. Examples of pre-impregnated half-products, the so called pre-pregs, are tapes, plates and granules. Granules or pellets are made by chopping impregnated fiber bundles at a length above the critical fiber length up to 10 mm or even longer. These gran-ules are used in injection molding. In the automotive industry this process is referred to as the technique of Long Fiber reinforced Thermoplastics (LFT). Notice that ‘long’ is a rel-ative definition. Industry is constantly searching for cheaper and better pre-impregnated half-products, implying a huge commercial niche.

The problem in a thermoplastic filament winding process is, that the speed of the pro-cess is limited by the heating rate of the pre-preg materials, which are usually tapes. Nu-merous studies have been carried out for efficient heating of thermoplastic tapes [12–14]. The most important heating methods used are heating by direct flame, hot gas, infra-red and laser. Using the laser-heating method and a pre-heating oven, high winding speeds can be reached; a maximum speed of140 m/min is reported in [12]. However, the

opera-tional and purchase costs for the laser-heating method is high, process control is difficult and hence the choice in winding pattern is limited. Heating by hot gas is cheaper, but process speeds reported are low, a maximum value of18 m/min is mentioned in [12], but

more realistic values where still a good product quality can be obtained are in the order of 5 m/min [15]. Hence, also in the manufacturing of thermoplastic products the

speed-quality dilemma exists for economically viable products, even if good speed-quality pre-preg materials are used.

The thermoplastic filament winding process can only be of widespread commercial interest if the winding speeds can be increased without loss of impregnation quality or lack of adhesion between layers of the filament-wound product. A potential advantage of thermoplastic filament winding over thermoset or dry winding is the possibility of creating large deviations from the geodesic line. Strictly speaking, a geodesic trajectory on a surface is defined as a trajectory without curvature in the tangent plane to the trajectory. This means that there is no force generated in the tangent plane which could provoke deviation of the trajectory if the generated friction forces are not sufficient. Deviations larger than those based on friction can be established if the impregnated fiber bundle actually solidifies to the previous layer at the nip-point. In this way, the winding pattern provides a large freedom in design which, consequently, allows for manufacturing of more complex products.

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prod-fiber bundle thermoplastic polymer

impregnation device

tapes or granules in situ process

Figure 1.1: Scheme showing the manufacturing process of thermoplastic reinforced (half-) products.

ucts on a commercial basis is the impregnation process. In Figure 1.1 a scheme is

dis-played showing the manufacturing process of thermoplastic reinforced (half-) products. The technological challenge is found in the development of an impregnation device which can produce good impregnated fiber bundles at high processing speeds. Such an impreg-nation device is not only applicable for the manufacturing of pellets and tapes, but also for in situ filament winding, since the re-heating problem of pre-pregs is eliminated.

Thermoplastic impregnation is performed either by mixing of thermoplastics in a solid state prior to melting, using precursors of low viscosity or melt impregnation [16]. Com-mingled yarns and powder impregnation are examples of the first mentioned method. These methods are expensive and still do not solve the problem of heating at a sufficient speed, but the impregnation quality is generally high. The basic idea behind the applica-tion of precursors of low viscosity is to impregnate at low viscosity and then ‘upgrade’ the matrix material. This ‘upgrading’ can be performed by using thermoplastics which increase their molecular weight at high temperature, for example PPS. Solvent methods, where the thermoplastic material is dissolved and the solvent is later removed, are also used. These approaches are laborious, and the flexibility in terms of material selection is limited, since the thermoplastic should be able to increase its molecular weight at high temperature or be suited for the solvent technique. Moreover, solvent evaporation causes air pollution. Melt impregnation offers a better perspective, since in principle all ther-moplastic materials can be used and no pre-shaping of the polymer, like grinding as in powder impregnation, has to be performed. Thus, an extruder is sufficient to deliver a melt. However, the high viscosity of thermoplastic polymers requires special attention for the construction of an impregnation device.

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Figure 1.2: Prototype impregnation device (length40cm) with tapered, convex and

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Extruder Impregnation Device

Pulling Machine

Pre-tensioner

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1.2 Description of the project

A prototype design of a thermoplastic melt impregnation device for rapid impregnation of fiber bundles is developed in 1999 at the Delft University of Technology in co-operation with DSM. This prototype, called the ‘Rollerbox’, is depicted in Figure 1.2 and is

de-scribed in [17]. In Figure 1.2 the initial fiber bundle trajectory through the impregnation device is shown; during the project the trajectory and lay-out of the ‘Rollerbox’ has been changed several times. An overview of the test set-up in the laboratory is shown in Fig-ure1.3. The impregnation device consists of a heated, steel container which can be closed

and is fed with molten thermoplastic by an extruder. There are 4 convex, tapered and

powered rotating rolls present, together with static cylindrical pins. In Figure1.2 it can be

seen that there are3 static, cylindrical vertical pins present, even as 2 horizontal

cylindri-cal pins which are mounted on the cover. The verticylindri-cal pins are also used for stabilizing the fiber bundle during the impregnation process. The reason for the rotating rolls is that the tension in the fiber bundle theoretically can be reduced as compared to a situation with the same rolls fixed. It is also expected that the impregnation quality is not negatively affected by rotating rolls and can even be beneficial. In this way, good impregnation of the fiber bundle can be achieved without exceeding the maximum breakage strength of the fiber bundle. A more thorough description of this phenomena is presented in Chap-ter 3. It is assumed that the convex design of the rotating rolls improves spreading of

the fiber bundles which facilitate impregnation. Suspension on one side and the tapering allows broken filaments to be removed from the impregnation process. This principle is called the self-cleaning capacity. The prototype emerged in 1999 from a number of in-ternal studies at Delft University of Technology [14, 18, 19]. The project started in 1994. The aim, in terms of processing speeds, is to reach 90 meters/minute while maintaining

good impregnation. The preliminary experimental results of the ‘Rollerbox’ are promis-ing [17] in terms of lowerpromis-ing tension in the fiber bundle and impregnation quality. The experimental results for tension reduction from [17] are reproduced in Figure 1.4. The

results presented here are performed at a fiber bundle speed of 30 meters/minute using

Stamylan R

112MN40 polypropylene (PP) and Vetrotex p375 glass fibers (2400) tex for 3 different throughput settings of the extruder. The ratio rf is introduced which defines

the ratio between the speed at the surface of the rollsvroll divided by the processing speed

of the fiber bundlevfiber). Hence, a ratiorf = 0 implies no rotating rolls, where rf = 1

means that there is no relative velocity between fiber bundle and surface of the roll. On the vertical axis the measured pulling force is displayed. The figure demonstrates that the pulling force of the fiber bundle, thus the tension in the fiber bundle, significantly decreases with increasing ratiorf. From Figure 1.4 can be concluded that powered rolls

do lower the tension, but that the tension does not consistently decreases for increasing ratio rf. A potential explanation for this behavior is that the polymer feeding rate is too

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0 0.2 0.4 0.6 0.8 1 0 20 40 60 80 100 120 Speed ratio v roll/vfiber [−] Pulling force [N] 1.33 grams/s 1.50 grams/s 1.66 grams/s

Figure 1.4: Pulling force as a function of the speed ratio rf = vroll/vbundle between

the speed at the surface of the rolls vroll and speed of the fiber bundle vfiber for three

polymer feeding rates. The results are reproduced from [17]. Experiments are carried out atvfiber = 30 meter/minute with the ‘Rollerbox’ at three different throughput settings of

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understood yet. The results presented in Figure 1.4 are performed in a closed container, making the actual impregnation invisible and turning the ‘Rollerbox’ into a blackbox.

Taal [20] replaced the steel cover of the ’Rollerbox’ by a cover with glass windows which allows for observation of the impregnation process. This type of information can only be gathered if the ‘Rollerbox’ is not entirely filled with thermoplastic polymer. In this particular setting, the static pins on the cover, see Figure 1.2, are not present any

more. Figure 1.5 shows the ‘Rollerbox’ equipped with a transparent cover in process

condition. The ‘Rollerbox’ is surrounded by porous bricks for insulation. Details on the experimental set-up are given in Chapter 4. Taal observed that the position of the

fiber bundle on the rolls is not stable in time and seems to ‘dance’. Spreading of the fiber bundle is also limited and his experiments in terms of tension reduction still are not consistent with the expectation of a lowering tension in the fiber bundle by an increasing speed ratiorf. In Chapter4 the explanation for this behavior is given.

1.3 Problem description

There is still a lack of knowledge how the observed ‘tension reduction mechanism’, as demonstrated in Figure 1.4, works. The goal is the improvement of the ‘Rollerbox’ in

order to shift the traditional ‘speed-quality dilemma’ in thermoplastic impregnation and the demonstration of the potential of the newly designed impregnation device in the ther-moplastic filament winding process. More precise, the following sub-goals are defined within the present study:

• Understanding of the tension reduction mechanism using driven impregnation rolls. • Understanding of the (thermoplastic) impregnation mechanism.

• Design and construction of an improved ‘Rollerbox’.

• Assessment of the possibilities using the improved impregnation device in situ in

the thermoplastic filament winding process.

In order to reach these goals, use is made of experiments, modeling and literature research.

1.4 Outline of the thesis

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techniques for thermoplastic impregnation can be determined. Also the available product manufacturing techniques are provided, as well as the technical and commercial chal-lenges which arise from this overview.

Chapter 3: The theory of (thermoplastic) impregnation is dealt with. A prediction for tension build up in a fiber bundle over fixed pins and driven rolls is given, where it is shown theoretically that driven rolls reduce tension. This effect is demonstrated in experiments as well. Spreading experiments are carried out, where a good agreement with theory is found. Finally, implications of the theory for the design of a thermoplastic impregnation device are presented.

Chapter 5: A kinematic model for the prediction of geodesic filament trajectories over series of rolls or pins of arbitrary shape and location is developed. In this way, feasible filament trajectories for given configurations can be established. The calculated geodesic trajectory for the new, improved ‘Rollerbox’, the ‘Integrated Rollerbox’ is shown and discussed.

Chapter 4: Experimental results with the initial prototype and modifications to this device are presented. Inter Laminar Shear Strength (ILSS) test are performed on samples of in situ filament-wound cylinders for speeds up to 60 meter per minute. The

experi-mental results demonstrating the principle of tension reduction in the fiber bundle using a driven roll in a melt pool, are more thoroughly treated. The implications from all experi-ments for the design of a new, improved ‘Rollerbox’, are provided.

Chapter 6: The ‘Integrated Rollerbox’ which is an improvement of the ‘Rollerbox’ has actually been built. The design itself is described. Due to time-constraints in the current project, only preliminary results are discussed.

Chapter 7: An overview of the filament-winding process in general is given. An assessment of the possibilities and limitations for non-geodesic thermoplastic winding are described.

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Chapter 2 A review on fiber reinforced

thermoplastics

2.1 Introduction

Fiber reinforced thermoplastics are a sub-group of Fiber Reinforced Plastics (FRP), which on its turn is a sub-group of composite materials. The most common utilized fibers and thermoplastic polymers are presented. Impregnation techniques to merge fibers and ther-moplastic polymers and processes for product manufacturing are provided as well. In Gibson and M˚anson [16] an extensive review on the impregnation technology for ther-moplastics matrices up to the year 1992 is given. Merging of fibers and thermoplastic polymers results initially in half-products; examples are granules or pellets, tapes and plates.

The main problems in the field of fiber reinforced thermoplastics are(i) the

impreg-nation of fiber bundles because of the inherent high viscosity of thermoplastic polymers and (ii) re-heating of thermoplastic impregnated half-products. Good impregnation of

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polymers

rubbers thermoplastics

crystalline non–crystalline thermosets

Figure 2.1: Classification of polymers

discussed, including the principle of the ‘Rollerbox’ [17] and its improved successor, the ‘Integrated Rollerbox’, which are both subject of the present thesis. These devices are based on thermoplastic melt impregnation using fixed pins and powered, rotating rolls. Notice that the word ‘pin’ refers to a fixed impregnation object, while ‘roll’ is used for freely rotating or powered objects.

Finally, at the end of the chapter commercial and technological challenges for the field of fiber reinforced thermoplastics are provided.

2.2 Thermoplastic polymers

Thermoplastics are linear or branched polymers which can be heated, formed, and re-heated several times. Polymers are classified into three main groups: thermosets, rubbers and thermoplastics [21], see Figure 2.1. Thermosets are to a large extent cross-linked

polymers which are generally rigid and non-processable after cross-linking. They pos-sess a dense, three-dimensional molecular network and in most cases degrade when too much heat is applied. Rubbers are cross-linked as well, but exhibit elastomeric proper-ties and consequently can be stretched to a large extent, and more important, spring back quickly if the tension is released. This is possible by the fact that the chain segments between the links are very mobile, as if they are a melt. However, the lightly cross-linked macromolecular structure implies that it is impossible to melt rubber. Thermoplastics themselves can be subdivided in crystallizing and non-crystallizing polymers. The degree of branching and the regularity of the thermoplastic polymer are factors influencing the ability to crystallize. Complete crystallization does never occur which is the reason to indicate this polymer-group as semi-crystalline. Non-crystallizing amorphous polymers are mostly used as ‘glass-replacement’ polymers.

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Bulk (<100◦_C) _Applications

Polyethylene (PE) Bags, pipes, baskets, packaging Polypropylene (PP) Fibers, pipes, packaging foils Polystyrene (PS) Packaging foam, coffee-cups Polyvinylchloride (PVC) Pipes, cable coating, bottles Engineering (_{100 − 150}◦_C) _Applications

Polyamide (PA) Bearings, fibers, gears Polycarbonate (PC) Glass replacement Polyoxymetylene (POM) Gears, car-accessories Polyethylene Terephtalate (PET) Bottles, foils, fibers

Acrylonitril-butadiene styrene (ABS) Toys (LEGO R_{) , telephones} Specialty (>150◦_C) _Applications

Polyether Ether Ketone (PEEK) Tubes, screws, wear blades, electrical carriers Polyphenylene Sulfide (PPS) Exhaust gas return valves, carburator parts Polyetherimide (PEI) Printerframe

Polyethersulfon (PES) Battery housing Polytetrafluorethylene (PTFE) Bearings, coatings

Table 2.1: Classification of the most important thermoplastic polymers in terms of their continuous use-temperature, including their abbreviation and examples of application.

engineering and specialty or high performance polymers, which have continuous use-temperatures lower than100◦_{C, between}₁₀₀◦_{C and}₁₅₀◦_{C and above}₁₅₀◦_{C, respectively.}

The most common used thermoplastics, including their abbreviation and applications, are listed in Table2.1. This table is based on [3,22,23] and does not have the intention to treat

the applications exhaustively, it merely gives an overview of possible and common prod-ucts. For physical, mechanical and thermal properties the reader is referred to [23, 24]. In general, beneficial properties of thermoplastics are high environmental resistance, impact strength, high level of recyclability and reformability as compared to thermosets [11]. Also, product-cycle times are low, since no time consuming curing, in, for example, au-toclaves is needed.

In Table 2.1, the classification based on continuous use-temperature is roughly the

same when the classification based on tensile and flexural properties is made [24]. How-ever, it should be mentioned that polyethylene is both the raw material for plastic bags, but also, although with an ultra high molecular weight, for the Dyneema R

high performance fiber. Its application is found in, for example, bullet-proof vests [25]. The difference be-tween the plastic bag and Dyneema R _{fiber is the orientation of the molecular structure.} In case of the bag, the molecules have a rather randomly and unorganized distribution, similar to the appearance of cooked spaghetti. The molecules in the Dyneema R

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lined up in an organized way, giving the material its high performance.

2.3 Reinforcing fibers

The properties of thermoplastics themselves can be excellent, but generally they are not suited for the construction of large, heavily loaded constructions or products. Adding reinforcing fibers results in a composite material with superior properties compared to the properties of the individual fibers and thermoplastics. The reinforcing fibers can either be continuous or discontinuous.

Common fibers used for reinforcement are aramid, boron, carbon and E-glass [6]. The most widely used fiber is undoubtedly the E-glass fiber, primarily because of the low costs. Glass fibers are available in different forms; as chopped strands, rovings, yarns and fabrics. The basis for all these products are a number of filaments (typical diameter in the order of_{7 − 20 µm) which are coated by a sizing. The purpose of a sizing} is twofold: protection of the filament during processing and improvement of adhesion between filament and resin. A strand is a gathering of filaments, a roving or fiber bundle is a collection of strands and a yarn is defined as twisted strands [26].

Carbon fibers are the most used for severely loaded primary structures, while the use of aramid is limited by its low compression strength and poor bonding to resins. Boron is expensive and difficult to handle. E-glass fibers are by far the most cheapest, but due to research, development and increasing production scale the price of carbon fibers is still decreasing [7].

2.4 Speed-quality dilemma

The two main problems in fiber reinforced thermoplastics are the impregnation of the fiber bundles and re-heating of already impregnated material at sufficient, economical processing speeds. The high viscosity of thermoplastics compared to thermosets hampers impregnation, while the low conductivity of thermoplastics hampers re-heating processes. The viscosity of thermoplastics lies in the range of _{500 − 5000 Pa·s, whereas typical} values for thermosets are _{100 Pa·s [9]. Good impregnation of fiber bundles is defined} as the situation where all the individual filaments within the fiber bundle are wetted by matrix material and the dispersion of filaments is homogeneous over a cross section of the fiber bundle. A discussion on a proper definition of impregnation quality is provided in Chapter ??.

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impreg-nation quality is generally referred to as the ‘speed-quality dilemma’ [17]. This implies that there exists an optimal point in terms of processing speed, which depends on the re-quired (half) product quality. Even when pre-impregnated half-products are used, their optimal point is not shifted sufficiently, because of the limitations related to re-heating. In situ impregnation would eliminate the re-heating problem, but is only effective if the optimal point of the ‘speed-quality dilemma’ for the impregnation process can be shifted substantially.

2.5 Manufacturing techniques

For the manufacturing of most fiber reinforced thermoplastic products the production process is composed of two steps: first, the impregnation of the fibers resulting in a half-product and, second, (re)heating of the half-half-product to produce the final half-product. This route is in fact not efficient, since re-heating has to be done. For injection molding the two–step approach is inevitable, but for continuous production techniques such as fila-ment winding, in situ impregnation would be extremely efficient. However, producing fiber reinforced thermoplastic products using in situ impregnation is already possible, but the ‘speed-quality’ dilemma is the limiting factor in commercial applications. In the following sections the manufacturing techniques for half-products and products are presented. Melt impregnation is the best technique for in situ impregnation and manu-facturing processes, but to overcome the speed-quality dilemma technical solutions are necessary. Proposed solutions from (patent-) literature are presented and reviewed, in-cluding the principle of the ‘Rollerbox’ [17] and its successor.

2.6 Half-products

The techniques for thermoplastic impregnation of fiber bundles are divided into three categories; melt impregnation, processes where the thermoplastic is inserted in the fiber bundle in solid state and processes involving low viscosity precursors [16]. In Gibson and M˚anson [16] an extensive review on the impregnation technology for thermoplastics matrices up to the year 1992 is given. However, since than, especially in the field of melt impregnation, many new techniques have been proposed.

2.6.1 Melt impregnation

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basics of impregnation theory are provided. A more thorough discussion can be found in Chapter 3.

By integration of Darcy’s law [27], the penetration depths in a porous medium in one

direction is obtained as

s = s

2K∆pt

η , (2.1)

wheret is the impregnation time, ∆p the pressure difference in flow direction, η the

vis-cosity of the fluid andK is the permeability coefficient. The permeability coefficient on

its turn is a function of the fiber diameter, fiber packing and flow direction. In order to increase the degree of impregnation, a number of conditions for an impregnation pro-cess must be satisfied simultaneously. These conditions are formulated by inspection of Equation (2.1) and are

• high pressure difference in flow direction (∆p), • high permeability (K),

• long impregnation time (t), • low viscosity (η).

These are the conditions which must, theoretically, be fulfilled at the same time. The question is which technical solutions are necessary and how to merge all these aspect and to translate them into an efficient impregnation device suited for industrial application. A very important issue is that fiber bundle placement must be easy and can be carried out in a short amount of time, without damaging it significantly.

A low viscosityη is obtained by heating the polymer and a small penetration depth s

is created by spreading the fiber bundle. Spreading is achieved by tensioning the bundle which can be achieved by pulling it over a curved surface. In this way, also pressure on the fiber bundle is generated for a high pressure difference in flow direction∆p.

As already stated, many impregnation techniques and apparatuses have been devel-oped and patented, claiming a good degree of impregnation. Some are to a certain extent successful, others have fallen into oblivion. In the present sub-section a classification is made for the most commonly applied melt impregnation principles: melt pool impreg-nation using fixed pins, freely or driven pins (rolls) or no pins at all and the principle of specific melt application. For each principle examples from (patent-) literature are provided and discussed.

Melt pool

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0000000000 0000000000 0000000000 0000000000 0000000000 1111111111 1111111111 1111111111 1111111111 1111111111 0000000000 0000000000 0000000000 0000000000 0000000000 1111111111 1111111111 1111111111 1111111111 1111111111

Figure 2.2: Cross-head die which is attached to an extruder. The fiber bundle is pulled through the die [16].

should be as small as possible because the thermoplastic polymer is prone to thermal degradation. Therefore, the size of the container or pool should be as small as possible. In Figure 2.2, a cross-head die is depicted which is mounted on an extruder. The fiber bundle is pulled through the die. The impregnation quality for this type of techniques is poor, since the impregnation time is too short and the fiber bundle is not spread at all. The only pressure squeezing the thermoplastic polymer into the fiber bundle, is the pressure of the thermoplastic material itself. This uniform pressure compacts the fiber bundle and makes impregnation even more difficult. This technique originates from wire-coating extrusion. An extension to this technique is described in [28], where a pressurized chamber (_{1 − 800 bar) is used with a minimal length of 300 mm. The process generally} results in a core of un-wetted filaments surround with a jacket of wetted filaments.

000000000 000000000 000000000 000000000 000000000 000000000 000000000 000000000 111111111 111111111 111111111 111111111 111111111 111111111 111111111 111111111 (a) (b) (c)

Figure 2.3: Specific melt application: (a) a fixed pin with a radial slit before the impreg-nation zone [29]. (b) a fixed pin with radial slits in the impregimpreg-nation zone [30]. (c) a porous impregnation wheel free to rotate [31].

Specific melt application

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in the worst case is never processed. In Figure 2.3 three techniques from patent literature for melt impregnation with specific applied melt are displayed [29, 30, 32]. In all three cases the melt is directly applied at the impregnation location. Techniques (a) and (b) in Figure 2.3 use a fixed cylindrical pin, where in case (a) the melt is applied before the impregnation zone and in case (b) the melt is applied via radial slits in the impregna-tion zone. The impregnaimpregna-tion zone, which is the region where the distance between fiber bundle and surface is small, is for case (b) situated in an arc-shaped slit. Using concept (a) a high degree of impregnation is obtained up to a process speed of 10 meters per

minute [10]. Case (c) in Figure 2.3 represent a porous impregnation wheel which is free to rotate [31, 32]. The melt is applied via a porous metal wheel made by sintering. This impregnation technique is tested in situ for the manufacturing of filament wound cylin-ders. An ILSS-test is used to determine the critical strength of the matrix material and the bonding between filaments and matrix [33]. The results in terms of ILSS-values for the in situ application of the porous impregnation wheel are good up to a process speed of15

meter per minute. Above this speed a drastic drop in terms of ILSS-value is observed. It is difficult to tell whether the quality is due to the porous impregnation wheel or by the filament winding process. A higher processing speed increases the tension in the fiber bundle which increases the compaction force on the filament wound product. The com-paction roll even further improves the quality of the product and possibly the apparent impregnation quality of the impregnation process itself. In addition, cleaning of slits and porous wheels can be a problem.

Instead of applying the melt directly between fiber bundle and surface, it can also be applied on the fiber bundle which is located on a surface. Accordingly, it is guided over so-called coating rolls of cylindrical shape. Kneading cylindrical rolls improve the im-pregnation quality at a later stage [34]. In [35], the melt is applied to the fiber bundle specifically before a freely rotating, heated roll by means of non-convex shaped pins or rolls. Multiple pins and rolls can be used to increase impregnation quality. In [36], a fiber bundle is suitably tensioned, aligned and spread by pulling it over a series of spreader surfaces. The spread bundle is then guided over a heated spreader surface where ther-moplastic polymer powder is fed into the nip between spreader surface and fiber bundle. Multiple heated spreader surfaces can be used, where the spreader surfaces are prefer-ably cylinders which can be either fixed, freely rotating or driven. This technique is not classified as powder impregnation since the powder forms a melt before impregnation.

The positive aspect for all techniques for specific melt application in Figure 2.3 is that a pressure difference is built up between both sides of the bundle driving the melt into the fiber bundle. At the same time, specific melt application reduces the risk of degradation of the polymer. For case (a) and (b) the pressure is generated by tension, for the case of the porous impregnation wheel (c), the pressure is generated by pressurizing the melt.

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the impregnation time, but also the fiber tension which can eventually lead to a situation of fiber breakage before the desired level of impregnation is reached.

Figure 2.4: Principle of a melt pool with fixed pins

Melt pool with fixed pins

Many thermoplastic impregnation techniques are based on the concept of a melt pool with fixed impregnation pins. In Figure 2.4 the concept is displayed. The idea is to generate pressure between the pins and the fiber bundle. The pressure is induced by the tension in the fiber bundle, which also spreads it. The pressure squeezes the thermoplastic mate-rial in the fiber bundle. The opposite approach is that the fiber bundle is pressed into the thermoplastic material. Applying a sequence of impregnation pins increases the impreg-nation time and allows for impregnating the fiber bundle from two sides. By increasing the number of pins, the pressure and spreading increases. Numerous studies are dedicated to melt-pool impregnation using static pins [10, 27, 37–40]. Consequently, in patent lit-erature there are also many patented techniques either using impregnation pins or other shaped impregnation tools [41–45]. In [41], no individual pins are used, but the fiber bun-dle follows a narrow ‘w’-shaped impregnation trajectory, see Figure 2.5. The three tips of the ‘w’-shape act as impregnation pins and the walls of this impregnation trajectory also contribute to an increased impregnation quality. In [42], the pressure under the fiber

polymer supply

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side view

top view impregnation bath

Figure 2.6: Melt pool with fixed pins: the fiber bundle is guided trough a bath of polymer and then over a drop-shaped impregnation surface for spreading of the fiber bundle and squeezing the polymer inside the fiber bundle [42].

polymer supply

Figure 2.7: Melt pool with fixed pins: A narrow and curved impregnation trajectory is formed by semi-cylindrical shaped pins [43].

bundle is generated by pulling the fiber bundle over a drop shape to obtain a maximum spreading of the fiber bundle, see Figure 2.6.

In [43], an impregnation apparatus is described where a narrow, curved impregnation trajectory is formed by semi-cylindrical pins, see Figure 2.7. This impregnation appara-tus consists of two parts on which the semi-cylinders are mounted in order to simplify the fiber bundle input at start-up. In [44], static individual impregnation pins are used where extra static pins are placed close to the impregnation pins in order to improve the impreg-nation quality by locally increasing the pressure. In [45], fixed individual impregimpreg-nation pins are used in a melt pool, where the melt in the pool is pressurized, see Figure 2.8. The fiber bundle is pre-heated before inserting it into the melt pool.

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pressure pre-heating

Figure 2.8: Melt pool with fixed pins: Fixed impregnation pins are used in a pressurized melt pool. The fiber bundle is pre-heated [45].

so that either the process velocity must be reduced (if possible) or fiber breakage occurs.

Figure 2.9: Melt pool with fixed and non-fixed pins which can either be freely rotating or powered [17, 46–49].

Melt pool with non-fixed pins

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fiber bundle is pushed away, there is no load transfer possible between roll surface and fiber bundle and hence no tension reduction is achieved. This principle is described in Chapter 4.

Comparison melt impregnation techniques

In Table 2.2, a comparison of the theoretical expected impregnation quality, based on Equation(2.1), for different thermoplastic impregnation techniques is provided. For

con-venience, Equation(2.1) is repeated

s = s

2K∆pt

η . (2.2)

The comparison is made using the assumption that each technique is extended to the moment fiber bundle breakage is imminent and is performed at the same processing speed. The impregnation quality is assessed, ranking from the worst case ‘0’ till the best ‘+ + + + +’. The way the terms in Equation (2.1) are influenced by the impregnation principle, are

listed in the column ‘Factors’. For extra information, the references to the figures for each principle are also mentioned. Each impregnation technique is assigned a numberN, see

Table 2.2. In the following overview, the impregnation techniques(N) are discussed and

their efficiency assessed. For all techniques the viscosityη is lowered by heating. The

permeability coefficientK is not taken into account in the assessment.

1 Quality 0: The fiber bundle is pulled trough a cross-head die, where only a high

pressure in the melt is generated. No attempt is made to reduces and increase he

pressure∆p.

2 Quality+: The thickness of the fiber bundle s is reduced, because tension is built

up. Also, pressure is generated, but this technique is limited in terms of impregna-tion time(t).

3 Quality+: The thickness of the fiber bundle s is reduced, because tension is built

up. Also, pressure(∆p) is generated, but this technique is limited in terms of

im-pregnation timet. The impregnation result will approximately be the same as for

Technique2.

4 Quality++: The thickness of the fiber bundle s is reduced, because tension is built

up. The tension built up will not increase as rapid as for Technique2 and 3, because

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by the pressure in the melt itself. Because of the less rapid tension built up, the impregnation timet is increased.

5 Quality++: Thickness of the fiber bundle s is decreased and both pressure (∆p)

and impregnation time(t) are increased. The tips of the ‘w-shape’ are quite sharp,

so filament damage or breakage is likely to occur. Hence, the tips should be suffi-ciently round.

6 Quality+ + +: Thickness of the fiber bundle s is decreased and both pressure ∆p

and impregnation timet are increased. Compared to Technique 5, the fiber bundle

is pulled over a smooth surface and the fiber bundle will spread too a higher extent. Disadvantage is that there is no polymer supply after the fiber bundle reaches the drop-shaped surface.

7 Quality+ + +: Thickness of the fiber bundle s is decreased and both pressure (∆p)

and impregnation time (t) are increased. Compared to Technique 6 there is a

con-tinuous presence of polymer, although spreading of the fiber bundle is expected to be less.

8 Quality + + ++: Thickness of the fiber bundle s is decreased and both

pres-sure (∆p) and impregnation time (t) are increased. Pre-heating the fiber bundle

reduces the ‘thermal shock’ for the fiber bundle the moment it enters the hot poly-mer, so the polymer is not cooled by the fiber bundle. The extra pressure in the melt is beneficial to squeeze the thermoplastic polymer inside the fiber bundle.

9 Quality+ + + + +: Thickness of the fiber bundle (s) is decreased and both

pres-sure (∆p) and impregnation time (t) are increased. This technique is extended

using driven of freely rotating rolls in an environment entirely surrounded by ther-moplastic polymer. Freely rotating rolls and especially driven rolls, increase the impregnation time(t) even more.

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degradation of the polymer should be reduced by narrowing the melt pool. Moreover, the mixing effect of the driven rolls prevents ‘dead spots’ with degrading polymer.

2.6.2 Low viscosity precursors

The main idea behind the method of low viscosity precursors is to impregnate the fiber bundle in a state of low viscosity and then ‘upgrade’ the resin to a higher viscosity. This upgrading can be done by using one of the following techniques [16]:

Polymerization

This technique relies on polymerization of monomers in situ, hence after impregnation. However, this technique has not yet achieved significant success.

Chain extension

Some thermoplastics, like PPS, increase their molecular weight upon heating by chain extension. The impregnation is carried out with the low molecular version of the thermo-plast and afterward the molecular chains are extended. It should be noted that undesired cross-linking effects can occur. In case of PEEK, the initial low molecular version, can be increased by chain extension by applying a reagent on carbon filaments before impregna-tion.

Solvents

The viscosity is reduced by using a solvent, which is a one of the simplest methods for reducing the viscosity. However, there are some difficulties. These are:

(i) The removal of the solvent after impregnation.

(ii) The polymers, which can be used for solvent processing, are susceptible to solvents. (iii) The material produced may exhibit a non-optimum fiber-matrix adhesion.

Despite the drawbacks, a range of PEI-based thermoplastics is successfully produced by the solvent technique. These materials are commercially available as ‘Cetex’ by Ten Cate Advanced Composites Group [50].

Plasticizers

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Schematic picture N Figure Quality Factors m el t p o o l 0000000000 0000000000 0000000000 0000000000 0000000000 1111111111 1111111111 1111111111 1111111111 1111111111 0000000000 0000000000 0000000000 0000000000 0000000000 1111111111 1111111111 1111111111 1111111111 1111111111 1 2.2 0 [-] S p ec ifi c m el t im p re g n at io n 2 2.3 (a) + _{s ↓, ∆p ↑} 3 2.3 (b) + _{s ↓, ∆p ↑} 000000000 000000000 000000000 000000000 000000000 000000000 000000000 000000000 111111111 111111111 111111111 111111111 111111111 111111111 111111111 111111111 4 2.3 (b) ++ _{s ↓, t ↑} M el t im p re g n at io n w it h fi x ed p in s 5 2.5 ++ _{s ↓, ∆p ↑, t ↑} 6 2.6 +++ _{s ↓, ∆p ↑, t ↑} 7 2.7 +++ _{s ↓, ∆p ↑, t ↑} 8 2.8 ++++ _{s ↓, ∆p ↑, t ↑} F re e o r d ri v en 9 2.9 +++++ _{s ↓, ∆p ↑, t ↑} Table 2.2: Comparison of the theoretical expected impregnation quality, based on Equation(2.1), for

different thermoplastic impregnation techniques. The comparison is made using the assumption that each technique is extended to the moment fiber bundle breakage is imminent and is performed at the same processing speed. The impregnation quality is assessed, ranking from the worst case ‘0’ till the best ‘+ + + + +’. The way the terms in Equation (2.1) are influenced by the impregnation principle, are listed in the

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2.6.3 Mixing of matrix in solid state

The technique of mixing the matrix in a solid state basically consists of inserting the ther-moplastic polymer in a solid state into the fiber bundle before heating takes place. The main idea is that the distance (s), see Equation (2.1), required for the flow of the

ther-moplastic medium upon heating is minimized. In fact, three techniques can be discerned: powder impregnation, commingled fibers and film stacking [16]. In powder impregna-tion, the thermoplast polymer is ground in very small particles and brought into the fiber bundle by means of pins or rollers. The thermoplastic can also be shaped in continuous or discontinuous filaments, preferably of the same size as the filaments in the fiber bundle, instead of in powder form. This is the commingled fibers technique; a detailed review can be found in [9]. In film stacking thin layers of polymer film fiber mats are stacked. The long cycle times needed for manufacturing products using these stacks make the process unattractive.

In conclusion, mixing of matrix in solid state is rather expensive, but the quality in terms of impregnation of the half-products and quality of the final products is good. A commercial example is TEPEX R

which is available with glass, carbon and/or aramid fibers with in principle any thermoplastic polymer. Sheets with a thickness range from

0.1 mm to 3 mm can be made using a double belt press, resulting in a high performance

half-product.

2.7 Products

Fiber reinforced thermoplastics gained interest in the early 1980s [10] and offer a number of advantages compared to the use of thermosets. The key advantages are a short cy-cle time, an often increased toughness, moisture and chemical resistance (in fact a good environmental resistance) and the potential for recycling [47]. In the present section an overview of the manufacturing techniques for thermoplastic reinforced products and in some cases examples of application are given. This overview is mainly based on [3,6,51]. In [3], an extensive elaboration on (thermoplastic) fiber reinforced processing techniques is provided.

An overview of the most commonly used production techniques for fiber reinforced thermoplastics is given. The production techniques are explained by schematic pictures and a comparison between the production techniques can be found in Table 2.3. Hereafter, the production techniques are individually discussed, subsequently the table in which the techniques are compared is presented.

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bundle or tape should be in a molten state. This can be done by using a heat source or, an in situ impregnation technique. Thermoplastic filament winding opens the way to more complex wound products, because of so–called online consolidation. This principle im-plies that the filament(s), fiber bundle or tape consolidate or freeze to the previous layer the moment they are placed. In this way, concave products can be manufactured and winding patterns with large deviations with respect to geodesic patterns can be obtained. These deviations can be larger than those based on friction as is the case for thermosets. In Chapter 7, the thermoplastic filament winding process and its potential is treated. Ex-amples of filament wound products are pressure vessels, storage tanks, golf shafts and wind mill blades. Common factor is that the product geometry is relatively simple and axisymmetric.

Pultrusion, see Figure 2.11, is a process where traditionally re-heated continuous pre-pregs are pulled through a die. The pulling tension increases rapidly with increasing processing speed, again because of the high viscosity of thermoplastic polymers. The surface quality of the final product is poor. In situ impregnation for this particular process seems to be of less interest, since in situ impregnation does not reduce the drag force in the die.

Roll forming, see Figure 2.12, is also a pultrusion process, but now the die is replaced by (a sequence of) powered, driven forming rolls. The friction with the now ‘rotating die’ is reduced. Speeds up to10 meter per minute for the manufacturing of a so-called

top-hat-section made from glass fiber reinforced polyamide and polypropylene are successfully demonstrated under laboratory conditions [52]. For this particular process, since die fric-tion is relieved, in situ impregnafric-tion could be interesting.

Tape laying, see Figure 2.13, is a process where a pre-impregnated tape is placed on a mold by a tape placement head. Tape laying machines are generally large because of the integration of multiple functions in the tape head. The process is mainly used in aerospace industry for the manufacturing of large panels with moderate curvature such as wing skins.

Fiber placement, see Figure 2.14, is a process which combines the tape laying and filament winding process, where limitations and drawbacks from both methods are re-duced. Fiber placement does not rely on tension in the fiber bundle for consolidation and can use independent tensioned fibers.

Compression molding, see Figure 2.15, is performed using Glass Mat Thermoplas-tics (GMT), where the pre-impregnated and pre-heated glass mat consisting of randomly orientated fibers, is pressed between molds.

Hot press technique, see Figure 2.16, is a process where pre-preg are stacked and pressed between heated molds. Unlike Glass Mat Thermoplastics (GMT), the pre-pregs contain continuous oriented fibers.

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heater or

impregnation device consolidationroller mandrel

Figure 2.10: Principle of the filament or tape winding process. A pre-preg material is heated or a fiber bundle is impregnated in situ and is wound on an axisymmetric mandrel.

heater or

impregnation device die pulling rolls

Figure 2.11: Principle of the pultrusion process. A pre-preg material is heated or a fiber bundle is impregnated in situ and is pulled trough a die by two pulling rolls.

Diaphragm forming, see Figure 2.18, is a process especially developed for thermo-plastic pre-pregs and is not derived from thermoset processing techniques. Sheets from pre-preg materials are placed between two diaphragms and formed by heat and pressure against a mold. Pre-pregs with continuous fibers can be used, since the pre-preg is not clamped, but can slide between the two diaphragms. This process did not receive much commercial interest yet, but a lot of research is carried out on the production of complex parts.

Injection molding, see Figure 2.19, is the most used fiber reinforced manufacturing technique and finds its application on a large scale in automotive parts and consumer goods. Granules or pellets, which are chopped, impregnated fiber bundles with a length above the critical fiber length of 10 mm or even longer, are re-heated by an extruder

and pressed into a die. The process can be highly automated and products ranging from

5 grams up to 85 kilograms have been documented. In situ impregnation of fiber bundles

is not applicable for this particular process, however, a technique for rapid production of high quality granules or pellets is of large commercial interest.

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com-heater or impregnation device

pulling and forming rolls

Figure 2.12: Principle of the roll forming process. A pre-preg material is heated or a fiber bundle is impregnated in situ and pulled and formed by a sequence of rolls.

heater

processing head

product shape

Figure 2.13: Principle of the tape laying process. A pre-preg material is heated and placed on the product. The processing head is flexible and generally has many degrees of freedom.

heater processing head

mandrel

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00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 pressure pressure

sheet randomly oriented fibers

Figure 2.15: Principle of compression molding. A pre-heated sheet with randomly ori-ented fibers is placed between a mold.

00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 11111111111 pressure pressure sheet oriented fibers

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0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 vacuum autoclave vacuum bag

Figure 2.17: Principle of autoclave processing. This process is similar to the hot press technique, see Figure 2.16, but the pressure is generated by a vacuum bag and the heat by an autoclave. 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 00000000000000 00000000000000 00000000000000 00000000000000 00000000000000 11111111111111 11111111111111 11111111111111 11111111111111 11111111111111 clamp diaphragms vacuum vacuum pressure

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process pre-preg continuous fibers

in situ production speed

shape size

Filament winding (2.10) tapes yes yes slow to fast

axi-symmmetric

small to large

Pultrusion (2.11) tapes yes yes fast constant

cross-section

cross-section small to medium; no re-striction on length

Roll forming (2.12) tapes yes yes fast constant

cross-section

cross-section small to medium; no re-striction on length

Tape laying (2.13) tapes yes difficult medium simple to

complex

small to large Fiber placement (2.14) tapes yes difficult medium to

fast

simple to complex

small to large

Compression molding (2.15) sheet no no fast simple to

complex

small to medium

Hot press technique (2.16) sheet yes no fast simple to

complex

small to medium

Autoclave processing (2.17) sheet yes no medium simple to

complex

small to large

Diaphragm forming (2.18) sheet yes no slow simple to

complex

small to medium

Injection molding (2.19) pellets no no fast simple to

complex

small

Table 2.3: Global comparison for the most common production techniques for fiber reinforced plastics based on raw material, produc-tion speed, shape and size of the product. The pre-pregs forms are menproduc-tioned, whether continuous fibers are used and if the techniques

3

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00 00 00 00 00 00 00 00 00 00 11 11 11 11 11 11 11 11 11 11 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 extruder hopper final product

Figure 2.19: Principle of injection molding. Granules or pellets are introduced into a hopper and are molten by an extruder. Accordingly, the melt including fibers is injected into a mold.

pared for the following aspects:

• type of pre-preg material,

• possibility of using continuous fibers,

• possibility of in situ impregnation of fiber bundles, • production speed,

• shape and • size.

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2.8 Challenges and future directions

Fiber reinforced thermoplastics are already applied with success in all kinds of structures and components. They are applied, for example, in automotive (doors, motor supports, underfloors), leisure ( fishing rods, hockey sticks, bicycle frames and forks), civil engi-neering (bridge panels), offshore (drilling pipes) and aerospace (tailplanes, floorbeams, pressure tanks).

However, applications are mainly restricted to weight sensitive structures or products which require a high strength over weight ratio. In some cases, fiber reinforced thermo-plastics are selected for their chemical resistance, for example, in corrosive environments. This implies that the costs for using fiber reinforced thermoplastics are higher than for conventional materials such as metals, but that, for example, due to weight saving, the structure or component saves money during its operational life. If, for example, conven-tional metal parts in automotive industry are replaced by fiber reinforced thermoplastics, or by composite material in general, the final car is lighter and will consume less fuel during its lifetime. Also, the formability of fiber reinforced plastics in general is higher than for metals, which opens the way to complex shaped products. The anisotropy for FRP can be influenced for specific loading conditions.

Another aspect which restricts introduction on a large scale, although it could be a beneficial property, is the general non-isotropic behavior of composite materials, which complicates the analysis of composite structures. The fracture behavior and failure mech-anisms are difficult to model. Cost reduction and developing adequate analysis models for fiber reinforced plastics and for composites in general are the main challenges for the future [53].

In the field of fiber reinforced thermoplastics, cost reduction is obtained by increasing production speed while maintaining or reaching a reasonable impregnation quality. In the present chapter it has become clear that the high viscosity of thermoplastics hampers impregnation of fiber bundles, and that the low conductivity hampers heating of pre-preg material. Many techniques have been developed in the past, where the most promising method for merging fibers and polymers is the melt pool using fixed and non-fixed, driven rolls. This technique is subject of the present thesis and demonstrates that it can overcome the speed-quality dilemma. The commercial market for the production of granules or pellets for injection molding is enormous. The application of in situ impregnation with a filament winding process potentially allows for all kinds of new, more complex and concave shapes which can be produced at high processing speeds.

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Chapter 3 Thermoplastic impregnation of fiber

bundles

3.1 Introduction

Impregnation of fiber bundles with a thermoplastic polymer is a slow process. This is mainly because of the inherent high viscosity of thermoplastic polymers. Heating re-duces the viscosity, but the general low conductivity limits the rate of heating. Thermoset impregnation is much easier due to a significantly lower viscosity. However, applying thermoplastic polymers has the advantage of a high environmental resistance, a high im-pact strength, low product cycles, a high level of recyclability and reformability [11]. Moreover, consolidation can speed up the production process due to the absence of a curing cycle.

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even longer are realized. In conclusion, the potential market for thermoplastic products is large. The application of thermoplastics would be further increased if thermoplastic im-pregnation techniques, preferably in combination with a product manufacturing process, are improved.

For thermoset impregnation, pulling the fiber bundle through a bath with resin, some-times with a pin or wheel in it, is generally sufficient. For thermoplastics it is not suf-ficient, only at low processing speeds impregnation occurs. In the present chapter, the basic parameters which influence the impregnation are studied. The starting point will be an empirical relation derived by Darcy for a fluid flowing through a porous structure. An implication of Darcy’s law is that static pins seem necessary. However, the tension in the fiber bundle rapidly increases with an increasing number of pins. Reducing tension is realized by using driven rolls, thus reducing the drag force. In the present study, it is discovered that the principle of tension reduction using driven rolls works properly if fixed impregnation pins are located close to the driven rolls. This principle is explained in Section 3.4. Experimental results demonstrating the principle of tension reduction are presented, as well as a simple model confirming the effects observed.

Pins do not only induce the necessary tension to generate pressure between fiber bun-dle and pin, but also spread the fiber bunbun-dle if the filaments are under tension. Wilson [54] developed a simple analytical model to predict spreading of fiber bundles. The model of Wilson is verified in experiments, using the TuvRov R

4588 of 2400 tex with a filament

diameter of17µm.

The present work intends to evaluate all equations quantitatively by identifying the relevant parameters and their effect on impregnation. Finally, the design implications for a thermoplastic impregnation device for rapid impregnation of fiber bundles are provided.

3.2 Darcy’s law

Darcy proposed an empirical relation from experiments on water flow through sand beds [10]. The general formulation in3-dimensions yields

q_{= −}K

η ∇p, (3.1)

where q is the flux per unit area, _{η the viscosity of the fluid, ∇p the pressure gradient} vector andK the permeability coefficient of the porous medium. The permeability

coeffi-cient has to be determined in experiments. The flow through a fiber bundle perpendicular to the filaments is considered. The situation is considered as a one-dimensional problem. A sketch of a fiber bundle with a thickness b impregnated by distance s is depicted in

Figure 3.1. The fluid is a thermoplastic polymer with viscosity η. The pressure in the

Development of a Rapid Thermoplastic Impregnation Device

Development

of a

Rapid Thermoplastic Impregnation Device

Proefschrift

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 8 december 2008

te 12.30 uur

door

Teun WEUSTINK

Contents

Chapter 1

Introduction

1.1

General introduction

1.2

Description of the project

1.3

Problem description

1.4

Outline of the thesis

Chapter 2

A review on fiber reinforced

thermoplastics

2.1

Introduction

2.2

Thermoplastic polymers

2.3

Reinforcing fibers

2.4

Speed-quality dilemma

2.5

Manufacturing techniques

2.6

Half-products

2.6.1

Melt impregnation

2.6.2

Low viscosity precursors

2.6.3

Mixing of matrix in solid state

2.7

Products

2.8

Challenges and future directions

Chapter 3

Thermoplastic impregnation of fiber

bundles

3.1

Introduction

3.2

Darcy’s law