transverse flow and process modeling on the polymer

*e-mail : [email protected]/08/194-10+2002 Polymer Society of Korea

194

Macromolecular Research, Vol. 10, No. 4, pp 194-203 (2002)

Transverse Flow and Process Modeling on the Polymer Composite with 3-Dimensionally Stitched Woven Fabric

Geon-Woong Lee, Sang-Soo Lee, Min Park, Junkyung Kim, and Soonho Lim*

Polymer Hybrid Research Center,Korea Institute of Science and Technology, Seoul 130-650, Korea

Received May 8, 2002; Revised July 1, 2002

Abstract : In resin infusion process(RIP), the fiber and the resin are in contact with each other for an impregnationstep and often results in flow-induced defects such as poor fiber wetting and void formation. Resin flow characteristicsin transverse direction and process modeling for woven fabric were studied, and the process modeling was appliedto the manufacturing of hybrid composite materials. This study also considered the compressibility of woven fabricsin a series of compression force, and it was fitted well to an elastic model equation. Void formation was varied withthe processing conditions in the stage of manufacturing composites using RIP. It was concluded from this study thatproper combination of pressure build-up and dynamic heating condition makes important factor for flow-inducedcomposite processing.

Keywords : polymer composite, resin film infiltration, process modeling, wettability.

Introduction

The resin infusion process(RIP) was originally developedfor autoclave processing using a vacuum bag/tooling com-bination for shaping parts. Due to a quite low viscosity ofthermosets at elevated temperature before cure, resin infusioninto a dry fibrous preform becomes an attractive manufac-turing technique especially for 3-D composite structures. Inaddition, RIP is easily adaptable to the fabrication of eithermonolithic or honeycomb sandwich structures with varioustypes of matrices. Furthermore RIP, more versatile processinvolving resin infiltration into a dry fibrous preform, hasovercome the problems encountered in conventional ResinTransfer Molding (RTM) such as low fiber content, the needof expensive match molds, long distance for resin to infiltrateout the fibrous preform, and void formation.1,2

In transverse flow, compression of the reinforcementoccurs with resin flow simultaneously. Therefore, the analysisof flow in transverse direction should be performed carefully.Compression is a common type of deformation happenedduring many composite processes. In RTM and RIP, thefibrous preform is preplaced in the mold cavity while themold is open. The resin is injected into the fibrous preformafter the mold is closed. In these processes, some compres-sion of the reinforcement occurs when the mold is closed.3

In addition, the applied pressure compresses fibrous pre-form as well as facilitates resin infiltration.

A combination of external pressure and resin pressuremay also be present in many cases of autoclave processing.4

In RIP, however, external pressure is transferred to thefibrous preforms and resin layers simultaneously. In addition,the fibrous preforms are compressed and as a result, bothporosity and pore structure have changed, which wouldaffect flow permeability and capillary pressure.

The resin flows in the stitched structure composed ofwoven fabric skins urethane foam core and transverselystitched glass bundles is depicted schematically in Figure 1.Transverse and longitudinal flow are expected when fillingthe hybrid sandwich structure with resin. The resin flow inthe longitudinal direction is known to be faster than that ofthe transverse direction. Thus, the resin flow through thestitching fibers is faster than that through the fibrous pre-form. After the resin passes the upper and the lower skinstructure, it starts to flow rapidly into the stitched fiberwithin foaming material.

Transverse or longitudinal flow through woven fabrics isrelated to physical conditions such as lay-up structure offiber beds, interfacial properties between fiber and resin,viscosity, compressibility, etc. However, the processingvariables are only mechanical pressure and temperature. Toinvestigate the effects of complicated factors into the twoprocessing variables, the precise analysis of the factors andtheir effects are needed.


Macromol. Res., Vol. 10, No. 4, 2002 195

In this study, resin flow characteristics in transverse direc-tion were analyzed by the measurements of permeabilityand capillary pressure. The process modeling was followedby experiments including compression tests and transverseinfiltration into woven fabric. The existence of a lot of voidin final products is one of the factors lessening properties ofthe products, so it is important to decrease the void contentof the composite. In order to make the overall analysis ofthe resin infusion process and to examine the effects ontransverse resin flow and void formation during processing,the systematic study for the factors affecting processingvariables was performed. Finally, the stitched composites ofhybrid sandwich structure were manufactured on the basisof modeling using vacuum bag modified RIP.

Theoretical Backgrounds

Transverse Permeability and Capillary Pressure. Theresin flow through the fibrous preform can be described byDarcy's law. In resin infiltration experiments, the systemconsists of two distinct layers: the fibrous preform and theresin layer which may be placed above or below the fibrouspreform. Assuming a linear pressure distribution in thefibrous preform, the resin penetration thickness into thefibrous preform may be described as a function of time, andthen by rearranging subdivided pressure gradient intoapplied(Pm) and capillary(Pc) terms, the resin penetrationthickness is expressed as:

(1)

where, Yp is the resin penetration thickness in transversedirection, Ky is the transverse permeability, µ is resin viscos-ity, φ is apparent porosity. As shown in equation (1), thepermeability can be calculated from the resin penetrationthickness as a function of time. Similarly, the capillary

pressure can be calculated from the resin penetrationthickness.5,6

As a first approximation the capillaries between the fibersmay be regarded as being cylindrical and uniform in diame-ter. For a capillary with circular cross section, the pressuredifference is a function of the surface tension(γ lv) and thediameter of capillary.7 The equivalent diameter of thefibrous bed is defined by the cross-sectional area normal toflow divided by the perimeter presented to the fluid. Thefibrous preform, however, is usually anisotropic. In addition,the pores of a fibrous preform are different in size andshape. These anisotropic/geometric configurations may becombined in a dimensionless shape or form factor(F), whichdepends on the flow direction. For one-dimensional resinflow, the capillary pressure is described in a generalizedform as follows:

(2)

where, Df is the diameter of filament. For unidirectionalfibrous preform, F assumes the value of 4 for resin flowalong the fiber direction, while F assumes the value of 2 forflow perpendicular to the fiber alignment. For the complexfiber alignment such as the woven fabric preform, F may bedetermined only indirectly by measurement of permea-bility.6

Compressibility and Fiber Deformation. There are twodistinctive sources of compression in composite laminationprocess: one is the mechanical pressure applied externallywhen the mold is closed, the other is the resin pressureapplied when the resin infiltrates into the reinforcements.Gutowski et al. 4,8-10 proposed a model for the resin flow andfiber deformation in the processing of unidirectional fibercomposites. The deformation of parts of fiber betweencontact points was modeled according to the theory forelastic beams of circular cross section. Thus, one can derivethe non-linear stiffness behavior of the fiber mat and, thatshows good agreement with experiments. The proposedmodel equation by Gutowski is expressed as:

(3)

where, Vo is the original fiber volume fraction, Va is theavailable fiber volume fraction, and As is the spring con-stant. In general, fibers which are misaligned and cross overwill result in a lower available fiber volume fraction(Va)

.8 Several researchers properly identified the compressibility

equation in the molding process of fiber reinforcement, andthey handled it primarily in an empirical way for conve-nience. Samarasinghe et al. analyzed the consolidation in a

Yp2 2Ky

µφ-------- Pm∆ Pc∆+( )t=

Pc∆ Fγ θcosDf

------------------ 1 φ–( )φ

---------------=

P As

Vf

Vo

----- 1–

1Vf

---- 1Va

-----– 4----------------------=

Figure 1. Resin flow path in the hybrid sandwich structure.

S. Lim et al.

196 Macromol. Res., Vol. 10, No. 4, 2002

simple empirical method.11 They found out the experimentaldata for the effective pressure of the fibers and the volumefraction of the liquid were fitted to a logarithmic relationwhich has been found valid for a large number of normallyconsolidated soils. Toll and Manson12 proposed a micro-mechanical model which yielded the relation for planarfiber networks. They also proposed a more general empiricalrelation of the power-law type with two empirical constantas follows.

(4)

Equation (4) is proposed on the basis of elastic behaviorof fibrous preform and this relation seems to fit the experi-mental data of compression of layered woven fabric in spiteof its simplicity.

Experimental

Materials. Epoxy resin was used to analyze transverseflow. The resin system was formulated same amounts ofliquid YD-128 and solid YD-011 epoxy resin(both from KukDo Chem. Co.) were mixed 10 phr of dicyandiamide (DICY)as a curing agent. As a reinforcement, WR580A (HankookFiber Co.) which is E-glass woven fabric was used. Theproperties of materials used in this work are listed in Table I.

Permeability and Capillary Pressure. To visualize theresin flow in transverse direction, the mold consisting of asyringe and a piston was made of transparent acryl as shownin Figure 2. Metal mesh plates were placed in the upper andthe lower side of woven fabrics, which make good penetra-tion of fluid into fiber mats. Experimental procedure was asfollows. Fiber mats were cut into samples of diameter of 5 cmusing a sharp-edged punch die made of stainless steel. YD-128 and YD-011(50/50) were mixed together at the elevatedtemperature. Stacked ten plies of the woven fabrics wereplaced into the mold, and the heights were adjusted to con-trol porosity. The resin and the apparatus were preheated for40 min in a convection oven at fixed temperature. After suf-ficient preheating, the resin was filled into the mold underthe constant pressure. The partially impregnated glass fibermats were taken off to measure the weight of impregnatedresin and the depth of penetration was calculated.

The pressure was controlled by nitrogen gas. Appliedpressure was set to 0, 20, 40, 60, and 100 KPa. The porosi-ties of the fiber mats were fixed to 0.25, 0.3, 0.35, and 0.4by manually placing metal spacers. The low porosity waschosen in order to protect the compression of the fiber matsduring flow experiments under applied pressure. Also thepressure range was confirmed as not to change the porosityin the compressibility test. The impregnation was carriedout at 100oC. Transverse permeability and the capillarypressure were measured experimentally.

Compressibility of Woven Fabric. Porosity can be changedduring compression of fiber mats. The degree of compress-ibility of 10 plies and 20 plies of woven fabric with 5 cmdiameter was measured. The woven fabric mats were cut intothe same size. Since the fabric mats were easily deformedduring cutting, a sharp-edged punch die made of stainlesssteel was used. The compression tests were conducted usinguniversal test machine(UTM), and experiments were per-formed by three methods. The first one is to measure thethickness and the stress at constant crosshead speed of 0.5mm/min. The second one is to observe the stress changeafter compression. The third one is to investigate the effectof interlayer space on the compressibility of fiber mats. Thistest was performed by the second compression after elimi-nating the first applied load.

Resin Viscosity and Surface Tension. The viscosity profileas a function of temperature was measured with Brookfield

P cVfn=

Table I. The Basic Properties of Matrix Resin and Glass Woven Fabric

Resin System Reinforcement

YD-128 YD-011 DICY WR580A

E.E.W(g/eq) 185~192 450~500 21 22 Filament Diameter (µm)

Viscosity at 25oC (Pa�s) 12.5 solid Solid 1150 Roving Tex (g/km)

Specific Gravity at 20 oC 1.16 1.18 - 6.3Ý6.3 Cloth Density (tow/inch)

Melting Temp. (oC) - 60 202 580 Surface Density (g/m2)

Figure 2. Schematic diagram of equipment for measuring trans-verse resin flow.



viscometer. The surface tension of matrix resin was mea-sured by Wilhelmy method, introducing the platinum ringwhose contact angle had been already known.13 TheDynamic Contact Angle Analyzer, DCA322(Cahn Co.) wasused to measure the surface tension.

Manufacturing of Composite Specimens. Polymer com-posites were manufactured by RIP. Resin films with a thick-ness of 0.3, 0.5, and 1 mm were obtained by using doctorblade in order to control resin ratio to the fiber mats. Thetwo different experimental methods of manufacturing com-posite specimens were used. The first one is to investigatehow many voids exist in the specimen; 6 plies of WR580Awoven fabric were used. An autoclave modified hot-press asshown in Figure 3(a) was used on manufacturing the speci-mens. The lay-up procedure was similar to a conventionalautoclave process, except that a resin layer and a fibrouspreform were placed separately instead of using prepreglayers. These resin films and fibrous preforms were placedin a vacuum bag composed of releasing film and steel platesas caul plate and tool plate. The vacuum line was installedwith a breather mounted on top of the layered part. Since theapplication of vacuum was critical in eliminating voids inthe final composite structures, vacuum pump was used fromthe beginning of the process. The processing time and therate of increasing applied pressure were varied to observethe void formation. The distribution of entrapped voids inthe final specimen was investigated with a microscopicmethod.

The second one is the hybrid polymer composite. Thehybrid materials composed of glass fiber and PU foam, as

shown in Figure 3(b), were placed between the resin films.Specially, PU foam was sandwiched with glass woven fabricsand this sandwich structure was stitched with yarn in orderto bind together. The PU foam used in this work has closed-cell structure, which protects impregnating resin into thefoam inside. Therefore, stitching fibers could induce resinflow through the stitching hole and improve mechanicalproperties. The mechanical pressure was applied at the pointof increasing viscosity after the sufficient impregnatingwithin woven fabrics and stitching yarns.

Results and Discussion

Analysis of Transverse Flow. The resin penetration lengthwas measured at isothermal condition during flow visual-ization experiments of epoxy resin system, whose flowdirection is transverse to the glass woven fabrics. As theporosity increased, the permeability increased rapidly. Therelationship between permeability and porosity is predictedby Kozeny-Carman14,15 equation as shown in Figure 4. Fromslope of the line, the Kozeny constant can be determined,which represents the characteristics between the fiber matsand the resin system. Transverse permeability at high porosityis affected by macro pores, while at low porosity this isaffected by micro pores. The relatively uniform distributionof pores is formed at low porosity. That is caused by fiberrearrangement at high compression pressure. So, character-istics of fibrous preform are thought to be well defined atthe low porosity. In the figure, large deviation was seen athigh porosity. Therefore, Kozeny constant was measured at

Figure 3. Composite manufacturing by resin infusion process.(a) autoclave modified RIP and (b) stitched hybrid composite.

Figure 4. Permeability as a function of porosity showing linear-ity as predicted by the Kozeny-Carman equation in the range oflow porosity.

S. Lim et al.


low porosity. Kozeny constant was 6.5, which was reasonablevalue compared to other investigators’ results performedusing unidirectional fibers. Kozeny constant is the universalvalue regardless of the types and the arrangements of fibers,so it could be used to compare the characteristics of wovenfabrics with unidirectional fibers. In the previous works,4,5,11,16

Kozeny constant for transverse flow was from 11 to 18, andfor longitudinal flow from 0.35 to 0.7 in case of unidirectionalfibers. Woven fabrics have the structure of crossed fiberbundles, so there are macro pores to produce macro flowpaths. For this reason, the Kozeny constant is smaller thanthat of unidirectional fibers and this value should be appre-ciated to simulate the real system. Characteristic propertiesof woven fabric used in this experiment are shown in Table IIand compared with previous results. First, Kozeny constantis smaller compared to that of unidirectional fibers in longi-tudinal flow, because permeability is very large due to thegeometry having macro pores. Nevertheless, the ratio oflongitudinal to transverse Kozeny constants is very small.Then, micro voids could be formed to the transverse directionin impregnation of thick-part products. Therefore, for theoptimum design of RIP, it is necessary to arrange the resinfilm to shorten flow path.

The effect of capillary pressure is thought to be strong dueto large amount of micro pores, although the moving distanceis short in transverse flow rather than in longitudinal flow.Experimental results showed that the capillary pressure in-creased as porosity decreases. This capillary pressure affectsresin penetration and void formations. It is very difficult toeliminate entrapped voids in the impregnation stage. Thus,the prediction of capillary pressure with porosity changeinduced by external pressure is very important in the RIPprocess. In order to obtain the change of capillary pressurewith porosity, the proposed equation (2) was used and theresults were shown in Figure 5, which showed a linear rela-tionship in the range of measured porosity. The value ofslope in the figure is Fγcosθ. From the experiments for thetemperature dependence of surface tension and contact angle,the changes in capillary pressure with temperature could bepredicted. The surface tension of the resin system was 43dyne/cm, and the static contact angle was 40 at the experi-mental temperature. The shape factor calculated from equa-

tion (2) and Figure 5 was 2.4, which was similar to the valueof 2 for the unidirectional fiber mats. Ahn et al.6 obtainedshape factor of 1.8 in similar experiments using carbon fibermats. The fiber mat was a dense system composed of 3,000filaments of 7µm diameter leading to smaller shape factorthan that of unidirectional fibers. However, WR580A wovenfabric had loosened woven geometry, which gave us reason-able shape factor of 2.4. From the generalized relationshipbetween porosity and permeability or capillary pressure, theeffects of flowing variables in RIP could be predicted.

Analysis of Compressibility. The change in porosity offibrous preforms induced by mechanical pressure is calledcompressibility, which is used to predict changes of perme-ability and capillary pressure by applied pressure during pro-cess. In this study, we carried out the experiments to analyzethe compressibility, by changing the number of layers, bychanging the pressure applying method and by re-compression.Changing the number of layers was used to investigate theeffect of interlayer space, and changing the pressure applyingmethod was used in order to compare woven fabrics andunidirectional fibers, and the re-compression was used toinvestigate the elastic recovery. Figure 6 shows the results ofcompressibility in case of woven fabric. From this figure,the pressure differences between 10 and 20 plies are relativelylarge in the first run. However, the pressure differences inre-compression (second run) are so small. It seems that thecompression occurs in interlayer space at first, and then uni-formly in tows. Moreover, relatively large deviation inpressure at unsteady-state condition, where the thickness ofwoven fabric decreases linearly with time, was shown incomparison with the deviation at the steady-state condition.

Table II. Comparison of Kozeny Constant(k) for FiberAlignment

Flow DirectionWoven Unidirectional

WR580Aa Lam & Kardos Gutowski

Transverse (ky) 6.5 11 17.9

Longitudinal (kx) 0.06 0.35~0.68 0.7

kx / ky 9 × 10-3 5 × 10-2 49 × 10-2

aBased on apparent porosity in woven fabric.

Figure 5. Capillary pressure as a function of porosity showinglinearity as a predicted model equation (2).



At the initial stage it showed similar results, after that itshowed rapid increase of pressure. These results indicatethat the compression is applied continuously until the stressrelaxation occurred by the interlayer rearrangement. How-ever, these results were very different from those of previousworks using unidirectional fibers. In the unidirectionalfibers, fiber filaments were slipped to rearrange and theporosity changed rapidly in the initial stage of compression.As the rearrangement comes to hexagonal packing, the fiberscan lead to a rapidly increasing load. To apply equation (3)proposed by Gutowski8,9 for unidirectional fibers to the wovenfabrics, Va' should be assumed. Theoretically, maximumfiber volume fraction of unidirectional system is 0.907 and0.785 at hexagonal and square packing, respectively. Althoughthe inside status of fiber bundles is locally hexagonal, pres-ence of macro pores between tows reduces the maximumfiber volume fraction obtainable in woven fabrics. At thispoint, we can give a clear physical meaning for the porosityof woven fabric by following equation.17

(5)

where, φM is macro porosity, φm is micro porosity. Figure7(a) shows the tows in the square position, which is similar tounidirectional system, (b) is similar to hexagonal unidirectionalpacking. Therefore, the maximum fiber volume fraction hasa range between 0.712 and 0.823. Figure 8 shows the resultsof model equation8,9 applied to woven fabric. In this figure,Va is assumed for the three cases, which were proposed fromthe above results in woven fabric system. The comparison ofmodel equation by Gutowski and experiments showed bigdifferences. Therefore, the maximum fiber volume fraction

is smaller than theoretical value, so the Gutowski equationcannot be applied to woven fabric systems.

Woven fabrics are the systems showing perfect elasticityas shown in Figure 6. In addition, the woven fabric showsthat the fiber slippage hardly occurs due to cross-weavingstructure, so general elastic model was used to decide exper-imental variables. To linearize the experimental results, nat-ural logarithm was used. As a result of many investigations,our experimental results agreed with the equation (4), andthe compressibility was expressed as follows

(6)

Conclusively, it was confirmed that the woven fabrics

φ φM φm+( ) φMφm( )–=

Pm 1512Vf14=

Figure 6. Porosity change as a function of applied pressure inwoven fabric.

Figure 7. Representation of idealized woven and unidirectionalpacking. (a) square and (b) hexagonal.

Figure 8. Comparisons of unsteady compression data and modelequation applied to woven fabric.

S. Lim et al.


showed elastic behaviors which are attributed to cross-weaving structure of fiber bundles.

Process Modeling. Only pressure and temperature, whichare dependent on time, are processing variables in RIP.However, it is impossible to analyze process without under-standing mutual effects of factors affecting impregnationstage. The effects of processing variables were consideredto have three categories as follows. The first is the mechanicalpressure, which changes the porosity of fabric layers leadingto the changes of capillary pressure and permeability. Thesecond is temperature, which changes the viscosity of resin,so surface tension and contact angle are changed. These alsoaffect permeability and capillary pressure. The third is thecombined effects of temperature and pressure.

In isothermal condition, permeability and capillary pressurecan be predicted by the model equation stated above. Per-meability, which is a main factor governing macroflow,decreases while capillary pressure increases as porositydecreases. However, as porosity decreases, the filling timeincreases. From this point, it is necessary to maintain highpermeability by increasing porosity rather than increasingcapillary pressure in case of constant inlet pressure.

As mechanical pressure change becomes linear in isother-mal condition, each processing variable and their effectswere investigated. Figure 9 shows porosity, permeability,capillary pressure, and penetration length with time, at thepressure increasing rate of 0.1 atm/min. The value is pre-dicted by equation (1), (2), (6). Porosity and permeabilitydecreased with time. However, capillary pressure increasedto become a sufficient driving force for the stage of impreg-nation. Penetration length was linearly increased at initialstage, and the slope was lowered as the compression pro-gressed. When increased mechanical pressure makes theporosity reach the critical value, permeability was loweredto prevent the resin from penetrating in spite of increasedcapillary pressure. This means there is a critical penetrationlength.

To analyze the temperature effect, viscosity change withtemperature before curing was fitted using Arrhenius typeequation with two empirical constants. From the experimentalresults, the equation of viscosity profile is as follows.

(7)

Temperature dependence of capillary pressure is analyzedusing equation (2). The change of surface tension with vary-ing temperature is expressed by Guggenheim equation18

derived for low molecular weight liquids and has been alsofound to be applicable to polymers.

(8)

where γo is the surface tension at 0 K, and Tc is the critical

temperature. Obtained parameters were γo = 91.3 dyne/cm,Tc = 807.7 K, showing the similar values to the experimen-tal results. The variation of contact angle(θ) according totemperature depends on the relative magnitudes of surfaceentropy of two phases. In general, at ordinary temperaturerange, the values of -dθ/dT is quite small, about 0.05 deg/oC.18

Therefore, temperature dependence of contact angle can benegligible in this section. Figure 10 shows viscosity, surfacetension, capillary pressure, and penetration length at con-stant pressure and constant porosity with the dynamic heat-ing rate of 5oC/min. The value is predicted by combinedequation (1) to (8). Penetration length increased linearly.However, there are differences in penetration lengthbetween Figure 9 and Figure 10 after initial stage. Probablythis is due to decreased viscosity with increased permeabilityunder dynamic heating condition. Penetration length withtime using model equation is shown in Figure 11, when me-chanical pressure was applied under non-isothermal state.Penetration length increased rapidly at initial time, and thenlinearly increased at the latter period. Nevertheless, comparingwith Figure 10, at constant pressure under heating condition,the penetration length was shorter, possibly by the decreasedpermeability with increased pressure. Consequently, it is

η Pa .s( ) 5.63 105–× 39920T

--------------- exp=

γ γo 1 TTc

-----–

11 9⁄

=

Figure 9. Predictions according to process model for increasingrate of 0.1 atm/min of applied pressure at constant temperature.



very important to control pressure and temperature so as toimpregnate easily in transverse direction. From the facts

described above, this study showed that process modelingfor the actual manufacturing process was possible from theanalysis of effects of processing variables on flow behaviors.In addition, overall process optimization to produce void-freefinal products would be expected.

Manufacturing of Composites. Composite specimens withepoxy resin and glass woven fabric were prepared usingRIP. Resin films and glass woven fabrics were mounted in avacuum bag composed of lay-up structures, which wassimilar to a conventional autoclave process. The moldedstructure was placed in the programmable hot press. Initialresin amount was 47 wt%, which is the excess amount com-pared to the general process. Heating rate was 5oC/min andthe system was maintained for 2 hrs after reaching 180oC.Resin is impregnated into fabrics with decreasing viscosityas temperature rises. The effects of pressurizing point and theincreasing rate of pressure were analyzed. The pressurizingpoints were 80oC and 120oC, and the increasing rate wasvaried. Cross-sectional areas of cured composite were ob-served to find out the degree of impregnation and entrappedvoid. For the specimen pressurized at 80oC with slowincreasing rate, large amount of resin was squeezed out soas not to fill the macro pores at surface of fabrics. In addition,there were macro voids between tows in the fabrics asshown in Figure 12. Viscosity and capillary pressure directlyaffect the resin flow, the latter plays a greater role in micropores than in macro pores. This means that resin fills insidethe tows of the fabrics faster than the macro pores betweentows. When the external pressure is applied at low viscosity,impregnated resin is squeezed out. And the volume shrinkageoccurs as cure reaction progresses, so macro voids were form-ed between tows. For the specimens pressurized at 120oC,

Figure 10. Predictions according to process model for increasingrate of 5oC/min of temperature at constant applied pressure of 20kPa, porosity of 0.35.

Figure 11. Predicted penetration thickness as a function of timefor dynamic heating condition at constant pressure.

Figure 12. Photographs of cross-section for the woven fabriccomposites.

S. Lim et al.


however, resins were filled in micro pores as well as inmacro pores. At 120oC, the viscosity increases slowly withprogress of cure reaction, and the pressure was appliedslowly to prevent the squeezing out of the resin. From theseresults, it was confirmed that the applying time of the pres-sure should be controlled to reach its maximum pressure atthe gel point of resin.

Sandwich structures were prepared with inserting PUfoam having thickness 3 cm into the layer of glass wovenfabric. They were stitched with strands of glass yarn throughthe transverse direction. Since the foam structure was notimpregnated with resin, stitching was introduced in order toinduce resin flow and to improve mechanical property. Thisstitching was known to create a complex resin flow mecha-nism in the composite processing. Therefore, the investigationof the resin flow in this special structure was necessary toprovide a better understanding of infusion phenomena intothe fibrous preform during RIP. Two strings of stitchingfiber bundle were used. Sufficient impregnation was carriedout at low viscosity range, and it was pressurized to get void-free products. From the process modeling as mentionedabove, the processing variables such as applied pressure andtemperature were controlled in manufacturing of stitchedsandwich structure. In the experiment, it was observed thatthe resin flows well both side of the laminated structure;transverse flow in upper and lower glass woven fabric,longitudinal flow in stitching fiber bundles. The resin wasimpregnated well at the structure of stitched fibers as shownin Figure 13. This sandwich form can be used in buildingstructure such as container wall parts, whose propertiesshould be light and stiff. Mechanical properties can be con-trolled by stitching density and enhanced material propertiesare also achieved by using this structure. In addition, stitchedsandwich structure may be substituted with honeycombstructure. Conclusively, it was found that the resin flowcharacteristics and process modeling could be applied todesign of manufacturing process using RIP with improvedproperties and void-free structural composites.

Conclusions

Resin flow characteristics in transverse direction and pro-cess modeling for woven fabric were studied, and the processmodeling was applied to the manufacturing of hybrid com-

posite materials. The results may be summarized as follows:The change of permeability with porosity called Kozeny

constant for the woven fabric showed smaller value than thatof unidirectional aligned fiber. It was considered that thedifferences of Kozeny constants were due to the structure ofwoven fabric containing macro pores. For the capillarypressure, there was a linear relationship for the range ofmeasured porosity from which the form factor in wovenfabric system was determined. The results of compressibilityfor the woven fabrics were found to fit well to an elasticmodel equation.

When the mechanical pressure was increased at constantrate under isothermal condition, the permeability became sosmall that the resin cannot penetrate the woven fabrics. How-ever, penetration length increased linearly with time becauseof decrease of resin viscosity in dynamic heating condition.Consequently, the optimization of real process could bedesigned by using process modeling, and quality of finalproduct could be controlled. Void formation was varied withthe processing conditions in the stage of manufacturingcomposites using RIP. The pressing condition and the heatingcondition are important for the good impregnation of resinand the manufacture of void-free product. Especially, it wasshown that longitudinal and transverse flow analysis andprocess modeling could be applied directly to the manufac-ture of hybrid products with foam materials and stitchingfibers.

References

(1) S. B. Shim, K. J. Ahn, and J. C. Seferis, Polym. Compos., 15,453 (1994).

(2) Sang-Beom Shim, Autoclave and Resin Transfer Processesfor Thermoset Matrix Based Composites, Ph.D. Dissertation,University of Washington, Seattle (1995).

(3) C. Lekakou, M. A. K. B. Johari, and M. G. Bader, Polym.Compos., 17, 666 (1996).

(4) T. G. Gutowski, SAMPE Quarterly, 16(4), 58 (1985).(5) J. G. Williams, C. E. M. Morris, and B. C. Ennis, Polym. Eng.

Sci., 14(6), 413 (1974).(6) K. J. Ahn, J. C. Seferis, and J. C. Berg, Polym. Compos., 12,

146 (1991).(7) P. C. Hiemenz, Principles of Colloid and Surface Chemistry,

Marcel Dekker Inc., New York, 1986.(8) T. G. Gutowski, T. Morigaki, and Z. Cai, J. Compos. Mater.,

Figure 13. Photograph of stitched hybrid composite after extracting PU foam.



21, 172 (1987).(9) T. G. Gutowski, Z. Cai, S. Bauer, D. Boucher, J. Kingery, and

S. Wineman, J. Compos. Mater., 21, 650 (1987).(10) T. G. Gutowski, J. Kingery, and D. Boucher, SPE Ann. Tech.

Conf. Proceeding, 32, 1316 (1986).(11) R. C. Lam and J. L. Kardos, Polym. Eng. Sci., 31, 1064

(1991).(12) S. Toll and J. A. E. Manson, FRC Papers, 25, 1 (1994).(13) C.-M. Chan, Polymer Surface Modification and Character-

ization, Hanser Pub., Munich, 1994.(14) P. C. Carman, Flow of Gases through Porous Media, Butter-

worths, London, 1956.(15) F. A. L. Dullien, Porous Media; Fluid Transport and Pore

Structure, Academic Press, New York, 1979.(16) G. W. Lee, N.-J. Lee, J. Jang, J.-D. Nam, and K.-J. Lee, Com-

pos. Sci. Technol., 62, 9 (2002).(17) G. W. Lee, Resin Flow and Process Modeling in Flow-

induced Composite Processing, Ph.D. Dissertation, SeoulNational University, Seoul, South Korea (1999).

(18) S. Wu, Polymer Interface and Adhesion, Marcel Dekker, NewYork, 1982.

transverse flow and process modeling on the polymer

Documents