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Producing thermoplastic matrix sheet composites: two processes.

The use of thermoplastic matrix sheet composites has expanded rapidly during the last five years. These materials are characterized by excellent mechanical properties, high impact strength, and rapid part fabrication (see PE, March 1988, p. 51), particularly for large parts. While there are many techniques for manufacturing such composites, only two have reached commercial status: melt impregnation Fig. 1) of a random fiber mat; and formation of an impregnated mat by deposition from an aqueous slurry Fig. 2). In both cases, a double-belt press is used for continuous consolidation of the polymer and fibers into a cohesive composite sheet.

Because the technical considerations are different for each of these processes, it is worth analyzing each method from the point of view of the controlling parameters. This article examines the melt penetration considerations of the melt impregnation process and the heat transfer considerations of the slurry deposition process.

Melt impregnation

In the melt impregnation process, preheated reinforcement is contacted on either side by the extruded polymer and then consolidated in the double belt press. Depending upon the design of the composite sheet, there may be only one reinforcement layer or as many as six. The reinforcement mats can be constructed with fibers of almost any length and can have a wide range of thicknesses, with porosities ranging from 0.90 to 0.40. The primary technical consideration in this process is the rate at which the polymer penetrates the reinforcing mat.

Melt impregnation into a fiber mat can be examined by considering the flow of a liquid through packed beds. The basic equation describing this flow is Darcy's Law, which states that

v = K [Delta]P/[eta]h

where v is the volumetric flow rate of the polymer per cross-sectional area of fibers; K is the permeability of the fiber structure; [Delta]P is the pressure drop through the fibers, il is the viscosity of the polymer; and h is the thickness of a reinforcement ply.

A commonly used model to predict the permeation constant, K, is the KozenyCarmen equation,

K = d e3[.sup. ] /180(1-e)[.sup.2]

where d is the diameter of the fibers and e is the porosity, or void fraction, of the fiber mat. This equation has been shown to provide an accurate measure of the flow of liquids through a uniaxially oriented fiber bed, both along and transverse to the fiber direction. It should, therefore, provide a reasonable estimate of the flow through a random mat.

In Fig. 3, the permeation coefficient of 13.5- Km diameter glass fibers (the diameter of most E-glass fibers) is shown to change by several orders of magnitude as the porosity of the glass mat increases from 0.40 to 0.90. Glass mats can be manufactured to provide almost any porosity within this range. The 40-wt% glass fiber reinforcement commonly used in composites occupies approximately 20 vol% of a polypropylene composite. This means that the porosity of the glass mat can be as high as 0.80.

The K values, determined from the porosity, are used to calculate the rate of penetration into the glass mat by Darcy's Law. The required degree of penetration of the polymer is one half the thickness of the fiber layer since the polymer penetrates the reinforcement from both sides. The time required to achieve complete penetration determines the length-velocity relationship of the double belt press. Figure 4 shows the calculated time required to fully impregnate a 0.43-mm-thick glass mat from both sides with a molten polymer having a 20,000-poise viscosity under a pressure of 1.4 MPa as a function of mat porosity. Because of the exponential relationship between porosity and the permeation coefficient, the time required for impregnation drops rapidly as the porosity increases.

Figure 5 shows that the maximum belt velocity of a composite passing through a double belt press during impregnation increases rapidly as the porosity of the mat increases. This is a result of the reduced residence time required for impregnation, arising from the lowering of the permeation coefficient. Viscosity and pressure were the same as above, and the press was assumed to be 6 m long, with 4.6 m allocated for impregnation and 1.4 m for cooling.

The impregnation times and belt velocities shown in Figs. 4 and 5 are proportional to polymer viscosity and belt pressure. In commercial practice, polymer viscosity is determined by balancing the properties required in the composite with processing considerations. From a processing point of view, low viscosities are most desirable. However, good impact strength, a highly desirable property in a composite, is attained with high molecular weight and high viscosity polymers. Therefore, the choice of polymer grade is usually the highest viscosity polymer that can be processed to meet the required production rate.

Melt viscosity as a function of shear rate for three grades of polypropylene is shown in Fig. 6. When the melt index of the polymer is above 12, impact properties begin to deteriorate. Polymers with a melt index below 5 have melt viscosities that are too high, in most cases, to allow the desired production rates. Because viscosity is an exponential function of temperature, increasing the processing temperature can improve the processing rate for low melt index polymers. However, the temperature increases are limited by the stability of the polymer and downstream cooling considerations.

Increases above a few MPa in pressure in order to increase the melt impregnation rate are difficult to achieve in practice-mainly because the polymer flows along the path of least resistance, which is usually transverse to the desired flow direction. The rate of polymer flow between the reinforcing layers is estimated to be ten times the rate of flow through the fibers. Therefore, the use of high pressure has been rejected in favor of higher processing temperatures and longer penetration times.

Slurry Deposition

In the slurry deposition process, a polymeric powder and discontinuous reinforcing fibers are mixed with water to form a random slurry. The solids are filtered from the slurry with a moving screen that passes through the headbox, producing a randomly dispersed mat of the polymer and fibers. The mat is then dried and consolidated in a double belt press (not shown in Fig. 2). Since the polymer is intimately dispersed within the fiber mat structure, impregnation of the polymer into the fibers is not the key technical concern. Rather, it is providing heat to melt and coalesce the polymer particles into a homogenous matrix.

Glass mat reinforced sheets produced by the slurry deposition process typically have a glass content between 25 and 40 wt%. Below 20% glass, it is difficult to obtain a coherent sheet with a uniform distribution of fibers. Above 40% glass, processing into a sheet becomes much more difficult, with extensive anisotropy occurring. The fibers used in this process are between 3 and 20 mm long. Shorter fibers do not provide the desired mechanical properties, while longer fibers are difficult to randomly disperse in the slurry.

Consolidation of the dried plies into composite sheets is most effectively accomplished by continuous lamination in a double belt press. Because the dried mats are highly porous and excellent insulators, the most important parameter affecting consolidation of slurry-formed mats is heat transfer prior to consolidation. A typical dried mat is 1 to 2 mm thick and has a density range of 0.2 to 0.5 g/cc. Thermal conductivity of a 2-mm-thick, 30-wt% glass fiber polypropylene mat having a density of 0.5 g/cc was measured to be 2.66 x 10[.sup.-4] cal/cm-sec- degrees C. The thermal conductivity of a fully consolidated sheet was determined to be 7.5 x 10[.sup.-4] cal/ cm-sec- degrees C

Consolidation is accomplished by slightly compressing several sheets prior to and during heating, and then applying a higher pressure once the polymer has been melted. Preheating the mats prior to applying pressure has been shown to be more effective than applying pressure and then heating. Using the analysis of unsteady state heat transfer between fixed temperature surfaces, it is found that the time required to bring the center of a 6-mm stack (three 2-mm-thick mats of 0.5-g/cc density) from room temperature to 200 degrees C between metal platens at 205 degrees C is 175 sec, while only 30 sec are required to heat the core of the fully densified sheet. This points out the benefits of preheating the mats prior to passing them through the double belt press.

Once the material is heated above the melting point of the polymer, consolidation into a cohesive composite occurs quite rapidly, since the polymer has already permeated throughout the fiber structure. Compressing the mats at this point forces the molten polymer droplets to coalesce into a continuous matrix. The primary consideration at this stage is to allow sufficient holding time for the joined polymer droplets to weld together and for the compressed fibers to relax. A strong polypropylene weld was reported to have been produced at a melt contact time of 15 sec.

The consolidation time can be reduced by adapting a hybrid heating procedure. If a modest pressure is applied to the mats as they are heated, the high void mat structure will collapse as the polymer becomes viscous above its melting or softening point. The pressure must be low enough so as not to induce lateral flow or stress in the fibers, but still high enough to cause the structure to collapse as the polymer melts. A simulation of this process using a 1.4 MPa pressure on the glass fiber polypropylene mats described above indicated that the time to heat the core of the sheet to 20 degrees C could be reduced to 90 sec (Fig. 7). The minimum residence time in the double belt press is thus reduced by 85 sec and the consolidation rate increased to 305 cm/min, a rate comparable to that realized in the melt impregnation process.

If the sheet is preheated prior to entering the double belt press, consolidation times can be quite short (on the order of 10 sec) and the corresponding throughput rates very fast ([similar to] 4000 cm/min). A factor limiting the speed is the tendency of consolidated slurry-formed mats to "loft" upon reheating prior to part formation. Because the tendency to loft decreases with consolidation time under heat and pressure, some finite holding time is required. This time for relaxation of the composite structure to eliminate lofting has not yet been quantified. Conclusions

The slurry deposition technique appears to be a viable alternative to melt impregnation for large volume production of thermoplastic matrix sheet composites. The limiting step in the melt impregnation process is the rate at which the polymer flows into the fiber mat. This rate can be described by Darcy's Law, using the Kozeny-Carmen equation to calculate the permeation constant. Examination of these equations shows that the rate of impregnation is directly proportional to the pressure, inversely proportional to the melt viscosity, and exponentially proportional to the porosity of the mat.

On the other hand, slurry-deposited mats are already impregnated, but highly porous in the preconsolidated state. The rate of consolidation of these materials is dependent upon the rate at which they are heated. Preheating the unconsolidated mats under modest pressure can significantly reduce the time required for consolidation. Under ideal circumstances, the rate of consolidation of slurry-formed composites can be significantly faster than the melt impregnation of glass fiber mats. The primary unknown is the melt contact time required to eliminate "lofting" of the consolidated slurry-formed mats.

(Figures Omitted)
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Author:Bigg, Donald M.
Publication:Plastics Engineering
Date:Oct 1, 1990
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