Manufacture of aramid fibre reinforced nylon-12 by dry powder impregnation process

PII: S1359-835X(98)00021-9

Composites Part A 29A (1998) 933–938 1359-835X/98/$ - see front matter q 1998 Elsevier Science Ltd. All rights reserved.

Manufacture of aramid fibre reinforced nylon-12 by dry powder impregnation process

M. Rath, S. Kreuzberger and G. Hinrichsen* Technical University of Berlin, Institute of Nonmetallic Materials, Polymer Physics, Englische Straße 20, 10587 Berlin, Germany (Received 12 September 1997; revised 12 December 1997; accepted 15 January 1998)

The present paper deals with the manufacturing of composites, consisting of continuous aramid fibres and nylon12, by applying the so-called dry powder impregnation process. The fibres are pre-impregnated by the deposition of fluidised powder particles on aramid fibre tows which are consolidated by heat and pressure to form continuous prepregs. Features of the nylon-12 powders which are important for the dry powder impregnation process are discussed. Material, tool and processing parameters influencing the achievement of impregnation and therefore the fibre volume content of the prepregs are outlined. q 1998 Elsevier Science Ltd. All rights reserved. (Keywords: dry powder impregnation; E. prepreg manufacture; fibre reinforced composite; A. aramid fibre; nylon-12)

INTRODUCTION A great variety of unique processing and manufacturing techniques have been developed for the production of thermoplastic composite materials1–4. The suitability of impregnation techniques for the manufacture of specific fibre–matrix combinations is currently being investigated at the Technical University of Berlin5–7. Dry powder impregnation is identified in this paper as a suitable technique for the manufacture of thermoplastic prepreg tapes. This technique is based on the powder coating technique, which uses a fine dispersion of polymer particles in the fibre tow in which the charged powder particles are electrostatically deposited onto the fibre surface8,9. In the present study, experimental results are presented on the influence of various production parameters on the fibre volume content of aramid fibre reinforced prepregs during manufacture by the dry powder impregnation process. Manufacture was conducted with nylon-12 powder in a mechanically agitated fluidised powder bed. MATERIALS The aramid fibre TWARON 1056 is produced by Akzo Nobel, and the high modulus (HM) type is used in this work. Owing to the importance of powder size, four nylon-12 powders with different average particle size, commercially used in the fluidisation coating technique, were taken into consideration (Vestosint 1111, 1118, 2175, and 3158, Hu¨ls AG, Marl). Nylon-12 powders were selected because * Corresponding author

impregnation should operate well with these materials, resulting in an approximately fully wetted fibre tow with low viscosity melts, such as nylon-1210. Characteristic properties of the materials used are summarised in Table 110,11. Fluidisation A stream of gas is passed upwards through a perforated bottom plate (distributor) into an impregnation chamber including a settled powder bed. At a gas velocity u mf high enough to separate the particles from each other, the particles become freely supported and behave like a boiling fluid in the ascending gas. The velocity u mf is called ‘minimum fluidisation rate’12. Helping to disrupt large clusters of particles formed by interparticle forces and to realise uniform fluidisation and bed expansion, especially for cohesive powders which are difficult to fluidise12, it is important to agitate the powder layer. This procedure is successfully used in the fluidised bed dip coating technology and is identified as an effective method for the manufacture of composites4. The material to be coated is heated beyond the melting point of the polymer coating compound, then fully dipped into a bath of fluidised powder particles. The polymeric material melts and adheres to the surface of the article which is subsequently transferred to an oven where the plastics forms a homogeneous coating of constant thickness. Geldart et al.13 described a simple method to quantify the flowability of powders by measuring the leakage of powder particles through an outlet connector as a function of time. A schematic illustration of the arrangement used to estimate the flowability of powders is presented in Figure 1.

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Composite production by dry powder impregnation: M. Rath et al. The expansion ratio (relation between the bed height of the expanded powder, h, and that of settled powder, h 0) for different powder masses in the chamber is dependent on the gas velocity. The result is shown in Figure 2. The leaked powder mass as a function of the expansion ratio is presented in Figure 3. As illustrated in Figure 2, with increasing gas velocity the expansion ratio increases in relation to the powder mass in the impregnation chamber. Keeping the gas velocity constant the expansion ratio increases with the inserted powder mass. To avoid dust emission during the impregnation process a well defined amount of powder is used, because of suitable expansion ratios at low gas velocities and sufficient flowability of the agitated powder bed structure (see Figures 2 and 3). The results presented in Figures 2 and 3 agree with the observations of the expansion behaviour of flour for different gas velocities by Brakken et al.14. Figure 3 demonstrates that the powder particle discharge is very small for expansion ratios h/h 0 , 1.4, because of the collapsed flowability of the powder particles. Therefore, a reproducible impregnation of the fibre tow is impossible and such ratios can not be used for the dry powder impregnation process of Vestosint 3158.

thermoplastic composites can be divided into different categories, according to the techniques of pre-impregnation and consolidation of the prepregs. Reviews and descriptions for the different techniques have been provided by various authors15,16. One of the earliest references to the powder impregnation technique using thermoplastic matrices was the patent of Price17. The dry powder impregnation processes can be subdivided into three main categories, namely impregnation in: • a settled powder bed • a fluidised powder bed and • an electrostatic field.

DRY POWDER IMPREGNATION PROCESS Techniques for the production of reinforced polymers The impregnation processes used for the manufacture of

Figure 1 Schematic of the basic arrangement for estimation of the flowability of powders. 1, Fluidization gas; 2, Impregnation chamber; 3, Oszillation sieve; 4, Diffusor; 5, Fluidized bed of polymer particles; 6, Outlet connector; 7, Guide plate; 8, Leaked powder particles; 9, Precision balance

Figure 2 Powder expansion ratio as a function of the gas velocity for different powder masses

Figure 3 Discharged powder mass as a function of the expansion ratio for various observation times

Table 1 Characteristic properties of the materials Property Density Melting point Tensile strength Elongation at break Young’s modulus Filament diameter Filament number Mean powder size

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¹3

(g cm ) (8C) (MPa) (%) (GPa) (mm) (mm)

Twaron HM 1056[11]

Vestosint 3158[10]

1.45 Degradation temperature . 500 3150 2.0 121 12 5000 –

1.02 172–180 57 300 0.98 – – 12

Composite production by dry powder impregnation: M. Rath et al. Design of the laboratory equipment The dry powder process developed by us for the manufacture of prepregs consisting of aramid multifilament fibre tows and nylon-12 powder is shown schematically in Figure 4. The fibre tow is continuously unwound from a fibre spool (1) by a motor (2) using a constant pretension by which the motor torque is controlled using a speed controlling device. The fibre tow passes some guide pins (3) which widen the collimated fibre tow. The guide pins ensure that the fibre tow is spread to such an extent that each filament in the tow can be uniformly coated with powder particles. After passing a force-sensitive device (4) governed by closed loop controlling unit (5), the fibre tow reaches the impregnation bath (6) filled with fluidised powder and the impregnation pin (7) inserted in the impregnation tool. A stream of gas controlled by a gas flow counter (8) is passed through a diffusor (9) in a mechanically agitated powder bed. The gas pressure was measured below the diffusor using a manometer (10). The agitation of the powder bed was realised by an oscillating sieve (11) driven by another motor (12). After leaving the impregnation bath the pre-impregnated fibre tow passes through a controlled heating chamber (13, 14), in which the matrix material is melted. The fibre tow is finally impregnated and consolidated with pressure rollers (15),

then transported through speed-controlled tension rolls (16) and wound up (17).

EXPERIMENTAL Classification of the parameters for the dry impregnation process Vodermayer18 defined various tool, material and processing parameters which influence the fibre volume content in the continuous production of fibre reinforced polymers using the aqueous dispersion technique. A synopsis of the most important parameters for the dry impregnation technique is shown in Table 2. Some of the parameters were held constant during all experiments (see Table 3). In preliminary experiments it was observed that the fibre volume content remains practically constant, while the drawing velocity has been varied between 1 and 3 m min ¹1 (Figure 5). Therefore, to determine a reasonable value of the fibre volume content of the prepregs for constant width of the impregnation pin the obtained data for the five investigated drawing velocities were averaged. The drawing velocity is influenced by the limited torque of the speedcontrolled tension rolls (see Figure 4 (16)).

Table 2 Classification of parameters for the dry powder impregnation technique Tool parameter

Processing parameter

Distance of impregnation pins Drawing velocity a Contact angle Tension of fibre tow a Number of impregnation pins Gas velocity Diameter of impregnation pins a Width of impregnation pins a Depth of fluidised powder layer above the impregnation pin a a

Material parameter Filament diameter Particle size a Flowability of powders Expansion behaviour of powders Powder particle size distribution

Parameters investigated in this paper

Figure 4 Sketch of the dry powder prepreg process. 1, Fibre spool; 2, Motor; 3, Guide pins; 4, Force-sensitive device; 5, Motor torque controlling unit; 6, Impregnation bath; 7, Impregnation pin; 8, Gas flow counter; 9, Diffusor; 10, Manometer; 11, Oszillating sieve; 12, Motor; 13, Heat controlling unit; 14, Heating chamber; 15, Roll nip; 16, Tension rolls; 17, Winder

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Composite production by dry powder impregnation: M. Rath et al. Using the rule of mixture the fibre volume content of the prepregs can be easily determined gravimetrically requiring densities and masses of the two components. mF ·rM (1) vF ¼ mF ·rM þ (mc ¹ mF )·rF

constant the fibre volume content increases with the applied tension. A fibre volume content in the range of 20–45% can be adjusted using an impregnation pin with a diameter of 10 mm and width in the range 5–100 mm.

where v F is fibre volume content (%), m F is fibre mass per unit length (g), m C is composite mass per unit length (g), r M is the density of the matrix (g cm ¹3) and r F is the density of the fibre (g cm ¹3).

Table 3 Parameters which have been held constant during manufacturing

DATA AND RESULTS

Number of impregnation pins Contact angle (deg.) Filament diameter of (mm) Twaron HM 1056 Temperature of heating (8C) device

1 180 12 300

Position of the impregnation tool in the fluidised powder bed The local position of the impregnation tool within the fluidised powder bed influences the dispensation of powder particles in the nip of the impregnation pin. The fibre volume content as a function of the bed height of the expanded powder above the impregnation pin is presented in Figure 6. The chosen position defines the thickness of the fluidised powder layer between the impregnation pin and the surface of the powder layer. As can be seen from Figure 6 the fibre volume content decreases with increasing thickness of the fluidised powder layer above the impregnation pin. Care must be taken to ensure the powder bed height in the impregnation chamber is sufficiently high, because a restricted flowability of the powders for bed heights lower than 1.0 cm above the impregnation pin leads to irregular dispensation of powder particles, so that an entirely immersed impregnation pin during the impregnation process can not be guaranteed. For a reproducible impregnation process during manufacture it is necessary that a fluidised and bubbling powder layer of 2 cm above the impregnation is realised.

Figure 5 Fibre volume content as a function of drawing velocity for different widths of impregnation pin. Roving tension 2 N

Width of the impregnation pin A second decisive parameter influencing the fibre volume content of the prepregs is the width of the impregnation pin. The function of the impregnation pin is to open up and spread the fibre tow to as large a volume as possible to facilitate the penetration of powder particles into the free space between the filaments, so that powder impregnation can occur throughout the complete fibre tow. The influence of spreading on the fibre volume content was investigated by variation of the width (b i) of the impregnation pin and the tension of the fibre tow, while the diameter of the pin was held constant (d ¼ 10 mm). A schematic of the impregnation pin is shown in Figure 7 and the results of the measurements are presented in Figure 8. An increase of the width of the impregnation pin in the range 5–34 mm results in a decrease in the fibre volume content and levels up to a constant value for pins of 34– 100 mm width. In addition, the tension applied to the fibre tow also has a remarkable influence on the fibre volume content of the prepregs. Holding the width of the pin

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Figure 6 Fibre volume content as a function of the depth of the fluidised powder layer above the impregnation pin

Figure 7 Schematic diagram of the impregnation pin

Composite production by dry powder impregnation: M. Rath et al. Collimated filaments allow fewer particles to penetrate than an open fibre tow which includes a greater accessible volume for powder particles, and hence the fibre volume content increases with decreasing width of the impregnation pin. It has to be mentioned that the standard deviation of the fibre volume content varies with the width of the impregnation pin in the range 5–14 mm and levels down to about 2% for greater widths, i.e. using small pin widths the impregnation process is characterised by considerable fluctuations which cause uncertainties in the fibre volume content of the prepregs. Diameter of the impregnation pin

fluidise because of high interparticle forces and, therefore, their use for the dry powder impregnation process is restricted. On the other hand, impregnation of individual fibres becomes more difficult with larger particles14. In the past, powder particles with diameters in the range 15– 150 mm have been successfully applied4. Our experiments were conducted using an impregnation pin of 34 mm width and 30 mm diameter and five steps of roving tension. In this investigation the average particle size (x 50) of the nylon-12 powders was determined with a Sympatec Helos particle size analyser. The powder was suspended in aqueous dispersion and stirred to break up aggregates of powder particles. The results of the size measurements are presented in Table 4.

The diameter of the impregnation pin defines the length on which the fibre tow is in contact with the surface area of the impregnation pin. A sketch of the impregnation tool is shown in Figure 9, and the dependence of the fibre volume content of the produced tapes on the diameter of the impregnation pin is presented in Figure 10. As can be seen, the fibre volume content increases monotonically with the pin diameter; roving tension has a minor but significant influence on this quantity. The guide pins open the fibre tow and spread it to a sufficient width, so that polymer particles are allowed to penetrate between the filaments. The period of time for which the fibre tow is in contact with the impregnation pin rises with increasing impregnation pin diameter, and a certain amount of powder particles already ‘fixed’ between the filaments are squeezed out and, therefore, the fibre volume content is increased. Particle size It has been pointed out by Iyer and Drzal15 that for optimum impregnation it is advantageous that the dimensions of the powder particles should be approximately identical with the diameter of the fibre, which is difficult to achieve in practice. Particle size is restricted by economic limitations and the cost of the manufacturing method used to prepare the powder. Very fine particles are difficult to

Figure 9 Schematic diagramm of the impregnation tool. 1, Fluidized powder bed; 2, Impregnation pin; 3, Guide pins; 4, Fibre tow; 5, Distributor; 6, Oszillating sieve

Figure 8 Fibre volume content as a function of the width of the impregnation pin. Tension of the roving has been varied in the range 2– 10 N

Figure 10 Fibre volume content as a function of the diameter of the impregnation pin. Tension of roving has been varied in the range 2–10 N

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Composite production by dry powder impregnation: M. Rath et al. • Continuous manufacture of aramid fibre reinforced nylon-12 prepregs, with different fibre volume contents in the range 20–50%, can be realised by the developed dry powder impregnation process. • Various parameters, such as the width and diameter of the impregnation pin, tension of fibre tow and average particle diameter, influence the fibre volume content. • The production of high performance prepregs consisting of aramid fibre reinforced nylon-12 can be warranted by the presented dry powder impregnation process.

ACKNOWLEDGEMENTS Figure 11 Fibre volume content as a function of the average diameter of the powder particles. Tension of roving has been varied in the range 2–10 N

Table 4 Average powder particle size Powder Vestosint 3158 Vestosint 1118 Vestosint 2175 Vestosint 1111

x 50 (mm) 12 31 64 86

The authors thank Hu¨ls AG, Marl, for supplying the appropriate nylon-12 powders.

REFERENCES 1. 2. 3.

As can be obtained from Figure 11, the fibre volume content decreases with increasing average diameter of the powder particles. In the region of fine powders (average particle diameter in the range 12–30 mm) the fibre volume content remains nearly constant and decreases strongly for coarse powders (average particle diameter: 64–86 mm) resulting in a matrix-rich composite. In the case of a broad distribution of powder particle diameter one can imagine that the larger particles, which have penetrated into the roving and are fixed to the filaments, ‘open’ the fibre tow for smaller particles. This effect may lead to an additional impregnation of the fibres which results in a decrease of the fibre volume content15. The quality of the powder impregnation was evaluated using an x-ray refraction method developed by Hentschel et al.19. The set of these results influencing the quality of the powder impregnation for different diameter of the impregnation pin and powder particle size will be presented in a further paper.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

CONCLUSIONS • Nylon-12 powders are successfully fluidised by passing a stream of compressed air in a mechanically agitated powder bed. The expansion takes place beyond a lower limit of the gas velocity.

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16. 17. 18. 19.

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