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The experimental determination of prepreg tack and dynamic stiffness
Composites: Part A 43 (2012) 423–434
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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
The experimental determination of prepreg tack and dynamic stiffness R.J. Crossley ⇑, P.J. Schubel, N.A. Warrior Polymer Composites Group, Division of Materials, Mechanics and Structures, Faculty of Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 6 October 2011 Accepted 15 October 2011 Available online 24 October 2011 Keywords: A. Prepreg B. Tack C. Adhesion D. ATL
a b s t r a c t A new peel test has been developed which quantifies the tack and dynamic stiffness of uncured prepreg. The test is designed to simulate the automated tape lay-up (ATL) and automated fibre placement (AFP) processes. It includes a pressure controlled application stage, where contact time is inversely proportional to peel rate. The use of a thin film allows stiffness to be isolated from peel resistance in a continuous two stage test. A repeatability study revealed consistent results with 16% standard deviation. Tack and stiffness variability has been observed across roll width and between faces in commercial hand layup prepregs. The overall tack and stiffness values for commercial hand lay-up prepregs were found to be inconsistent with the levels specified by manufacturers. A temperature increase revealed inconsistent effects on tack between materials. The contradictory results were rationalised by observing failure modes. The two failure modes observed appeared equivalent to those found in pressure sensitive adhesive (PSA) peel. The shear storage modulus of the prepreg resin was compared to the PSA Dahlquist criterion and found to support the principle of contact efficiency. However, the actual value for the criterion is expected to be a function of prepreg specific conditions such as resin content, fibre distribution and surface pattern. Ó 2011 Published by Elsevier Ltd.
1. Introduction Prepreg consists of reinforcement fibres pre-impregnated with a resin matrix and has been in use for many years [1]. Traditional prepreg hand lamination involves cutting plies into the required shape, removing the backing paper and placing into a mould. Pressure is manually applied to ensure the ply conforms to the mould surface. Tack levels are formulated such that the material will remain in place throughout the lamination process and can be repositioned if necessary. For hand lamination, tack and stiffness levels are relatively flexible due to the skill of experienced laminators. Stiffness is estimated based on fibre areal weight (FAW) and tack is generally specified on material datasheets as ‘low’, ‘medium’ or ‘high’ based on subjective touch tests. The hand lamination process is labour intensive, lacks consistency and would benefit considerably from automation [2]. However, considerable technical difficulty is attributed to the prepreg tack level which must be low to allow backing paper removal but remain high enough to hold the lay-up together [3]. New materials are now being developed for low cost mainstream automated tape laying (ATL) methods where a sensitivity to tack and stiffness is shown to dictate lay-up performance [4]. Therefore, it has become necessary to further define and quantify tack and stiffness. The term tack is used in the composites industry ⇑ Corresponding author. E-mail address: [email protected] (R.J. Crossley). 1359-835X/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.compositesa.2011.10.014
to describe instantaneous adhesion before the resin has cured [5]. Tack is also further defined within the pressure sensitive adhesives (PSAs) industry as the instantaneous adhesion associated with short contact times typically resulting in adhesive rather than cohesive failure [6]. Historically, tack may be defined as the adhesion which occurs quickly at room temperature, without special surface preparation, under finger application pressure. The definition is now extended to include instantaneous adhesion which occurs within the operating conditions of ATL. In the ATL process, prepreg tape and backing paper is fed from a reel and pressed onto the mould surface by a compaction tool. During lay-up the prepreg is expected to leave the backing paper, and be retained by tack to the mould surface [7]. A simplified force diagram (Fig. 1.1) shows that undesirable peel of the prepreg from the mould surface, resulting in lay-up failure, may occur if the tack to the backing paper exceeds tack to the mould surface. The stiffness of prepreg is also beneficial in aiding release from the backing paper as it is forced around the compaction shoe. Therefore, ATL may be represented by a continuous peeling process where application time is inversely proportional to feed rate. In ATL, AFP and hand lay-up the first ply is often considered the most difficult due to the smooth surface often treated with release agents which results in poor mould adhesion. Therefore, both tack to the mould surface and dynamic stiffness are considered the most significant characteristics of prepreg. Stiffness is also applicable to hand lay-up as it gives an indication of the ease of which it may conform to a contoured mould surface.
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Fig. 1.1. A Cincinnatti V4 ATL delivery head and a simplified force diagram of the ATL process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Standardised methods of determining prepreg stiffness [8] neglect polymer relaxation and rely on the weight of the prepreg to provide the measured deformation which complicates comparisons between alternate resin systems and FAWs. Several standardised methods for determination of PSA tack exist [9,10], however, results are difficult to quantify in relation to lay-up conditions. The probe test has been used extensively in the PSA industry [6] and has been adopted in various prepreg studies [11–13]. The probe test consists of a disc of resin between two circular surfaces. The surfaces are typically brought together under controlled force or to reach a predefined gap. Force is recorded for separation by a controlled rate. Stress is defined using the area of the probe surface [6]. However, the actual contact area is likely to differ and requires complex equipment for accurate definition [14]. A number of failure modes are observed occurring either in the bulk, possibly by fibrillation, or at the surface by crack formation and propagation [15]. The actual failure stress and type is believed to be a complex interaction between the probes surface and the nucleation of cavities [16]. The volume of air pockets trapped during the application stage and a change in stress distribution are considered to have an effect [17]. The probe test has also been found to be sensitive to changes in surface roughness caused by fibre patterns and resin impregnation distribution within the bulk specimen [18]. Therefore, the probe test is considered to be less suitable for the characterisation of ATL prepreg tack due to: Increased sensitivity to surface and bulk air pockets and cavitations. Failure within the bulk, which is allowed in the probe test, that is likely to be prevented by the continuous fibres during ATL. The inability to relate results to the peel mechanism. A peel test method is also used extensively in the PSA industry for adhesive tapes [6]. British standard methods are available [19] and have been utilised in the characterisation of prepreg [8]. However, the peel method is often considered inferior to the probe test as: The individual stages of separation cannot be isolated, limiting its analytical ability.
It does not define or include the application of the adhesive tape to the rigid substrate. The adhesive is typically applied in an uncontrolled secondary process following manufacturer’s guidelines. The application method prevents the study of short application times and contact conditions. There is considerable difficulty in separating bending from adhesive forces [20]. A new peel test was developed to address these issues whilst simulating the ATL process. The floating roller method [19] was modified to include a compaction roller which allows the simultaneous pressure controlled application and peel of prepreg. An additional thin covering film was used to allow stiffness bending effects to be characterised and separated from peel resistance, quantifying tack in a single two stage test. The value obtained for tack is peel resistance without the effects of bending and is believed to be analogous to the work of adhesion found in probe testing (Fig 1.2). Although this method remains unsuitable for the analysis of the various stages of tack separation it is considered an acceptable quantity by which to compare materials. The method was then used to characterise existing hand lay-up and ATL prepregs. Although the test may be reconfigured to measure backing paper tack, it is neglected during this study. Residual tack to the backing paper is required to maintain prepreg in position during cutting operations. The tack to the backing paper is therefore considered at present to be the minimum amount which allows retention during the cutting operation but release during lay-up. The backing paper is also applied at the time of manufacture, and although some peel rate and temperature dependence is likely to remain, as with most pre-applied PSA peel tests [6], the interface is well established and is expected to be less sensitive to the effects of such variables, Backing paper tack appears appropriate under certain lay-up conditions and the maximum variability and difficulty in ATL lay-up appears to occur in achieving tack to the mould surface. Therefore, this study focuses on the effect of mould surface to prepreg tack. The effects of resin type, temperature, feed rate and the variation in commercial prepreg were also investigated. Maximum ATL feed rates of 50,000 mm/min are typical, however, an initial feed rate of approximately 500 mm/min is typical when starting a new ply to the mould surface. Therefore, although equipment constraints
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Fig. 1.2. Comparison of typical probe test results (left [6]) with force measurements taken along the peel front (right [21]). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
limit test feed rates to 1000 mm/min results remain applicable to these starting conditions which are often most problematic.
2. Method The prepreg samples were pulled through the spring loaded rollers which provide an application force against a rigid substrate which simulates the mould surface. The first two rollers are used to guide the plate. The second two rollers provide the peel and application force. Application and 90° peel occurs instantaneously against the fixed top roller with the compaction force applied by the spring loaded bottom roller. Results are recorded for two sections in a continuous test. Stiffness is recorded for the first section of the test where the sample has a thin film covering the tack surface. The film is absent for the second section of the test where peel resistance is recorded (Fig. 2.1). By subtracting the average stiffness from the average peel resistance a value for tack was found with minor adjustment for the absence of the thin film. The correction value for films was typically less than 1 N found by measuring the rolling resistance of the film independently. Bearing and rolling losses (0.7 N) were calculated by measuring subsequently
Fig. 2.1. The operating principle of the prepreg tack and stiffness test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
narrower width films. They were then subtracted from all results and the rolling resistance of films to yield correction values. The new peel test equipment was mounted in a universal test machine (Fig. 2.2). Peel resistance and stiffness was then measured continuously over a predetermined peel distance using a load cell. Extension and load values were recorded for later analysis. To allow for temperature changes the test rig was enclosed in an oven. The specimen size was chosen to exceed the 115 mm recommended [19] for PSA continuous peel and to maintain plate stability. A specimen length of 300 mm was chosen requiring a 250 mm test plate. To allow changes in temperature the test rig was mounted within an oven with limited space. A 140 mm long test plate was the maximum length allowed within the oven space resulting in a 215 mm long sample (Table 2.1). A specimen width of 75 mm was chosen, exceeding the 25 mm British standard recommended value. The extra width minimises the impact of small randomly positioned irregularities across the width of the prepreg and increases the recorded tack load in relation to experimental noise. The increased width also emphasises any large irregularities which span a significant portion of the prepreg width. Such irregularities serve to highlight variability in the prepreg production,
Fig. 2.2. Tack and stiffness test rig mounted in a universal test machine.
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Table 2.1 Rigid substrate and prepreg specimen sizes with peel and stiffness measurement distance. Temperature chamber?
Oven
None
Sizes (mm)
L
W
L
W
Specimen
215
75
300
75
Results Length of stiffness Length of continuous peel
50 80
60 140
Recommended for PSA [19] W
N/A 115
storage and handling which may affect ATL performance and were avoided in the study of variable effects. The width is also convenient as it minimises the number of cuts required to test standard ATL roll widths of 75, 150 or 300 mm.
Table 2.2 Variables found in ATL production in comparison to adjustable test parameters. Variable
Production range
Test range
Limitation
Temperature
0–80 °C
20–80 °C
Oven capabilities
Relative humidity
0–95%
40–60%
Environmental chamber required
0.01–1000 mm/min
Test M/C
Application pressure
130–650 N
25–250 N
Springs, construction costs
Mould surface
Unlimited
Any plate and prepreg combination up to 6 mm thick
Mounting bracket, clearance and construction roller clearance
ATL tape thickness
0– > 1 mm
Feed rate
2.1. Variables The test rig was designed with features that allow the variables found in ATL production to be investigated (Fig. 2.3). The values represent those found commercially with practical limitations imposed by test equipment (Table 2.2). Jacking screws are used to control the compaction force up to a maximum of 250 N. This force is within the lower half of the 130–650 N specified by ATL manufacturers [7]. The compaction force was limited by the roller bearings associated rolling resistance and springs. Larger springs would require larger shaft diameters and bearings to suit the higher loads. This would cause a deviation from the British standard specifications for roller diameter [19] and a significant increase in construction costs. Temperature was adjusted by fitting the rig within a temperature chamber. Feed rate was adjusted via the universal test machine, limited to a maximum of 1000 mm/min. The solid substrate plate can be made from any rigid mould material and treated with release agents to simulate mould conditions. Adjustable clearance between the rollers allows for up to 6 mm thickness of rigid substrate and prepreg material. 2.2. Compaction roller calibration The sprung roller was calibrated in order to quantify the prepreg application force by inserting a rigid L shaped plate connected directly to the load cell via the fabric grips (Fig. 2.4). The L shape plate rests directly on the sprung roller. The jacking screws were then pre-tensioned to give the starting point of calibration. Subse-
Fig. 2.4. The peel test rig configured for compaction roller force calibration.
quent turns were made to give a force per number of screw turns. The test was repeated and showed a linear relationship. A chart was then produced to give the number of turns for the required compaction force used during testing (Fig. 2.5). During the test the first sample was loaded followed by pre-tensioning of the screws and the required number of turns to give the chosen compaction force.
3. Analysis
Fig. 2.3. The peel tack and stiffness test rig design features.
Results for a medium tack prepreg showed an increase in rolling resistance at the transition of the stiffness to peel resistance section of the test (Fig. 3.1). Average values for both sections of the test were calculated over the applicable area (Table 3.1). Care was taken to exclude any unreasonable peaks resulting from surface defects in the prepreg or backing paper defects, such as bubbles or folds. The values for stiffness and peel were expressed as force per unit width (N/75 mm). Standard deviation for a single sample was calculated (Eq. (3.1)) and represented by error bars on line or scatter plots where a single sample represents each point. Where possible, batch testing was carried out to overcome the variability in tack testing using a minimum of three samples. The batch standard deviation was calculated (Eq. (3.2)) and displayed as error bars on bar charts. The sample deviation gives an indication of the uniformity of peel. In a single sample a lower deviation indicates steady state peel. An unusually high deviation could indicate the stick-slip condition.
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Fig. 2.5. Compaction roller calibration results and jacking screw settings.
Fig. 3.1. A typical five sample test result where stiffness and peel resistance are recorded allowing average tack to be calculated. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3.1 Typical extension range of measurement areas. Extension range
Ambient (mm)
With oven (mm)
Total Stiffness Peel resistance
200 20–50 80–180
130 10–35 55–110
effect appeared exaggerated where very low values of tack were recorded overall. In such cases a comparison with experimental error is required. The average experimental batch standard deviation is taken for all levels and then divided by the average value (Eq. (3.4)). If the error value approaches or exceeds the effect then the effect is most likely to be a product of experimental noise. Expression of the effect of variables
Batch deviation was expressed to determine the deviation in average tack between samples and typically appeared higher where individual samples exhibit unsteady peel.
rs
ð3:1Þ
where rs is the standard deviation in a sample; ls the average tack or stiffness value; x the tack or stiffness values; n is the number of values.
rb
ð3:3Þ
Error ¼
Av g:std:dev : 100% Av g:v al
ð3:4Þ
4. Observations and discussion 4.1. Reducing uncertainty
Standard deviation in a batch of samples
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X ¼t ðxi lb Þ2 n i¼1
Max:v al Min:v al 100% Max:v al
Expression of overall experimental error
Standard deviation in a single sample
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X ¼t ðxi ls Þ2 n i¼1
Effect ¼
ð3:2Þ
where rb is the standard deviation in a batch; lb is the average batch tack or stiffness value; x is the average sample tack or stiffness values; n is the number of samples. In order to compare the effect of multiple level experiments directly the effect of the variable under consideration is defined by the maximum change as a percentage of the maximum recorded value for (Eq. (3.3)). This method proved effective for results where large changes of tack were recorded. However, the value of the
Observations during test development highlighted sources of error which were controlled in order to achieve reliable results (Table 4.1). Prepreg tack was found to be sensitive to temperature fluctuations which were limited to ±0.5 °C. A thorough cleaning schedule was followed for rigid substrates where cleaning and handling procedures were found to significantly affect experimental results. Disposable nitrile gloves were worn at all times as sweat residue deposited by finger prints on test could be seen in resin deposition patterns (Fig. 4.1). Any residue left by solvents or cleaning agents was completely removed from the rigid plates. Handling of the prepreg was minimised to prevent the transfer of body heat to the sample or rigid substrate.
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Table 4.1 A summary of observed experimental errors. Cause
Effect
Control
Environmental Temperature fluctuations
Severe
Use a temperature regulated room or environmental chamber
Relative Humidity fluctuations Set up and procedure Temperature fluctuations through excessive Handling Sweat or grease residue on rigid plates Excess resin on test rig Plate oscillations during testing Material Uncontrolled sample roll location Uncontrolled sample face Rips, tears, folds or defects Resin remains on backing film
Low
Severe
Minimise handling, use rubber gloves, allow sample to cool after handling
High
Avoid skin contact with rigid plates, use rubber gloves Clean with acetone Ensure the rigid plate is always positioned only 10 mm from the end of the prepreg
Med Low
High
Only samples cut from the same position along the width and on the same face should be compared
High
Chose only uniform samples in good condition Reject samples where resin remains on the backing paper after peeling
Severe
surface at the start of the test did not significantly affect final tack results. Three time delays of 0 (minimum possible loading time), 2 and 10 min were used. The results showed no noticeable effect on prepreg tack and stiffness exceeding experimental error (Table 4.2). However, a negligible rise in stiffness was attributed to the formation of excess resin squeezed out in front of the compaction roller during the dwell period. A raised pull force was recorded during this initial stiffness stage which was mostly outside the area of analysis. Therefore, although the effects appeared negligible the dwell time was minimised. 4.3. Radius of bending Isolating stiffness from peel resistance requires significant additional calculations and measurements in traditional peel testing [20]. Stiffness is now measured during the first stage of the new test, where a thin film covers the tack surface. However, there is a potential inaccuracy caused by a change in bending radius. The radius of bending has been seen to change between the stiffness and peel sections of the test when the peel force becomes elevated. For the stiffness section the sample typically follows the radius of the roller. During the peeling section the prepreg may be retained on the plate surface extending away from the roller, resulting in a smaller bending radius (Fig. 4.3). The presence of the fibres ensures that the change in radius remains small in all but the highest tack situations, at which point the bending force is considerably lower than tack. Therefore, the effects of a change in radius are considered negligible in the comparison of prepregs with similar stiffness. 4.4. Negative tack
Fig. 4.1. A finger print on the rigid substrate present before testing is revealed by lack of resin adhesion after the test.
The position of the rigid plate in relation to the prepreg was also seen to add a degree of fluctuation to results. Leaving an excessive plate overhang caused the plate to lift and drop within the clearance of the guide rollers. This occasionally led to oscillations in the recorded force and contributed to causing an unsteady peel condition. Therefore, for consistency, the rigid plate was consistently positioned approximately 5–10 mm from the end of the prepreg sample. Samples where resin has remained attached to the backing paper after peeling were excluded. Patches of lost resin were seen as an area where tack was reduced (Fig. 4.2). Defects in the prepreg material were found to have a detrimental effect on results, with lumps bumps, folds or tears showing up as artificial peaks in force as they passed through the rollers. Such defects are likely to be less influential on prepreg tack during actual lay-up where ATL machines typically use compliant rollers. Therefore, every effort was made to test uniform samples with the absence of defects.
Negative tack values were recorded for very low tack prepreg material. A component of the negative tack value is due to the bending resistance of the covering film which is present in the stiffness test but absent from the peel resistance section. Therefore, when stiffness is removed from peel resistance a negative value is found. To account for the bending stiffness films are calibrated by measuring rolling resistance through the rig. The relevant calibration value for the film used (Table 4.3) is then added to the average peel resistance. A small negative value for tack (typically < 2 N) has been observed in some results after the backing film correction has been made. Negative tack values may signify that surfaces are repellent, observed as zero tack in practice. Therefore, all negative tack values are considered negligible and are regarded as zero tack. These additional small negative tack values could be attributed to: Unusually high average stiffness values attributed to imperfect bending or folds, most often seen in stiff samples. Efforts should be made to avoid anomalous peaks in rolling resistance when analysing results. Frictional interaction between the film and the prepreg during bending which is not included in the calibration. Changes in the bend radius between the stiffness and peel section. A difference in the measured film stiffness and the actual film stiffness when applied to a prepreg ply. 5. Results and discussion 5.1. Repeatability study
4.2. Dwell time A dwell time test was conducted to ensure that the length of time that the material rests under gravity on the ridged plate
A blind trial was conducted on nine epoxy glass fibre prepreg batches of the same stock with a tenth tackier control batch were supplied randomly by the manufacturer. A total of 30 samples
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Fig. 4.2. Dry resin patches on the prepreg (top left), lead to reduced tack levels during the test observable by lost resin patches on the test plate (top right), bubbles and folds (bottom) were also responsible for artificially high peaks in tack readings.
Table 4.2 The effect of dwell time before the start of the test where material is allowed to rest under gravity on the rigid substrate surface. Dwell Time (Mins)
0
2
10
Stiffness N/75 mm
34.06 1.77 65.42 24.73
37.2 1.33 50.07 21.14
38.32 1.33 56.9 23.76
r Tack N/75 mm
r
Effect% Error% Effect% Error%
11.1 4 23.5 40.4
Fig. 5.1. Results of a repeatability experiment with nine same stock samples and a higher tack control sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4.3. The deviation in bending radius from the radius of the roller during the peel of high tack prepreg.
Table 4.3 Covering film calibration values.
Five samples were tested for each material on three occasions. The rig was removed from the universal testing machine between each occasion to allow for possible set up deviations. Stable and repeatable values were recorded for all stiffness values with minimal standard deviation (Fig. 5.2). Tack values also appeared stable and repeatable for unidirectional (UD) materials with reasonably low standard deviation. However, multidirectional materials with higher resin content showed inconsistent tack results with significant deviation between samples and batches. The results also show significant deviation from manufacturers specified tack levels. 5.2. Commercial prepreg variability
Backing film
Calibration value
Embossed polythene Red polythene Clear PET
4 N/75 mm 1 N/75 mm 0.3 N/75 mm
were tested in batches of three samples each (Fig. 5.1). The nine similar materials measured an average tack level of 4.8 N with a 0.8 N (16%) standard deviation, which was acceptable considering environmental fluctuations of 20% relative humidity and 1 °C in temperature. The 10th sample, a tackier material, was correctly identified registering a significantly higher reading of 17 N in line with expectations. The repeatability study was then extended to characterise several commercial hand lay-up prepreg materials with epoxy resins of various specified tack levels, fibre weight and architecture (Table 5.1).
Commercial prepreg rolls of up to 1.5 m wide are typically produced in a continuous process which is believed to result in uniform tack along the length but not necessarily across the width. Other deviations may also occur as a result of changes in surface texture caused by alternate fibre orientation or backing film patterns between faces. Therefore, alternate faces and roll width positions were tested to quantify any variability. 5.2.1. Roll position Samples from the previous repeatability experiment had their location on the roll recorded to allow for the distribution of prepreg tack across the roll width to be investigated (Fig. 5.3). All samples showed differences in average stiffness across the roll with two samples increasing towards the middle (Fig. 5.4). The glass triaxial material showed the greatest increase of up to 10 N stiffness towards the centre of the roll. UD and biaxial glass materials showed the least variation in stiffness across the roll width.
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Table 5.1 Specifications of the existing commercial prepregs tested. Ref.
Fibre weight (g/m2)
Fibre direction
Resin content by weight (%)
Specified tack level
GB600 GT1200 GUD1600 CUD600
600 1200 1600 600
Bi-axial ± 45° Tri-axial 0° ± 45° UD 0° UD 0°
45 38 32 32
Medium Medium High Low
All samples appeared to show some tack variability across the roll width with the exception of the CUD600 carbon samples which did not display any measurable tack (Fig. 5.5). The GT1200 sample showed a significant increase in tack towards the centre of the roll. The variations in tack and stiffness at roll width positions are generally attributed to the prepreg production method. Fibre distribution methods, roller position, pinch and tension settings may result in variations in impregnation across the roll due to variability such as bleed out at the roller ends. 5.2.2. Sample face Experiments were carried out to determine if tack is dependent on the prepreg face tested (Fig. 5.3). Three samples from each face were tested for each of the four commercial prepregs. Stiffness values remained reasonably consistent for all materials (Fig. 5.6). However, tack values were found to vary significantly between faces. The GT1200 triaxial sample showed the greatest variation between faces which is likely to be the result of a difference in fibre pattern and therefore the resin surface layer between faces. CUD600 displayed little variation due to its low tack properties. Overall, experimental standard deviation in stiffness and tack can be reduced by controlling the roll position and face tested. Small variations in tack between faces in other samples could be the result of the roll direction, with the inner face experiencing a difference in strain during production and storage causing differences in surface resin migration.
Fig. 5.2. A tack and stiffness repeatability study with 35 sample batches of four commercial hand lay-up prepregs tested on three occasions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.3. Specimen location for analysis of tack and stiffness variability as a function of roll position and face.
5.3. Commercial prepreg repeatability A total of 21 samples per prepreg were tested. Consistent overall values for tack and stiffness were recorded between the 15 sample roll position study and the six sample face study (Table 5.2). The stiffness values appear reasonable, with CUD600 displaying the highest stiffness. This is most likely a result of the increased stiffness of carbon fibres in comparison to E-glass fibres. It would be reasonable to assume that the stiffness of glass fibre prepreg would be proportional to material weight. However, GT1200 shows increased stiffness in comparison to GUD1600. Therefore, fibre architecture and increased resin content may also contribute to increased stiffness. Repeatable values for tack were found which were inconsistent with specified tack levels (Table 5.2). Manufacturers typically specify tack values based on constituent resin tests with the absence of fibres. Multidirectional fabrics display the highest deviation. Therefore, It is likely that fibre direction and resin content also effect tack. They may change the resin layer thickness and surface pattern, which has also been shown to have an effect on probe tack test results [18]. A suction component may account for increased tack values recorded for multidirectional fabrics. This cavitation and suction effect may be less pronounced in UD prepregs due to long cavities normal to the peel front. The cavities, which follow the fibre pattern, may extend out beyond the peel and application zone. Therefore, they may be mostly open to the atmosphere preventing the suction effect from occurring. Unidirectional material showed the least variability and is used in future investigations
Fig. 5.4. Stiffness distribution across the 1.5 m width of commercial hand lay-up prepreg rolls. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
to reduce the relative experimental noise when comparing the effect of variables. 5.4. Effect of resin type Bespoke prepreg samples were prepared using unidirectional Eglass 200 g/m2 impregnated with 32%, by weight, resin content. Three resin types, formulated for their tack properties (‘High’, ‘medium’ and ‘low’), were impregnated using a consistent method and equipment. Other process variables were held constant with feed rate and compaction force set at 500 mm/min and 100 N. Five samples were tested for each resin type; error bars represent the batch deviation. A low measurable effect on stiffness is observed
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Fig. 5.5. Tack distribution across the 1.5 m width of commercial hand lay-up prepreg rolls. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
tack CUD600 specimen was tested for temperature effects. The CUD600 prepreg showed a similar stiffness decrease response to temperature. However, a contradictory tack response was found with a significant (97%) increase in tack compared to 23% error (Fig. 5.9). Newly developed wind energy ATL prepreg tape consisting of 400 g/m2 FAW E-glass fibre impregnated with 28% wt. low tack BPA epoxy resin (GUD400ATL) was also tested. This material showed increased uniformity in results allowing a single sample at each temperature to provide reasonable results at an increased number of temperature intervals. Testing of the new material revealed a peak in tack and continuously reducing stiffness with increased temperature (Fig. 5.10). The three differing responses to temperature in commercial and ATL prepreg cannot be rationalised easily without considering the failure mode. 5.6. Failure mode
Fig. 5.6. Tack and stiffness values between alternate faces of the prepreg roll. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(19.4%) with 8% uncertainty. The effects on tack show good agreement with manufacturer’s specified tack levels (Fig. 5.7). A 98.2% effect with 25.6% uncertainty shows that resin formulation has a significant effect on tack. This result appears to indicate that the resin properties have a dominant effect on tack provided a consistent fibre surface pattern and resin content is maintained. 5.5. Effect of temperature GUD1600 and CUD600 hand lay-up prepregs were tested in batches of three samples at five temperature intervals. An oven was used to heat the sample and rigid substrate. Temperature was measured using an IR thermometer focused on the tack area immediately after the test. Results for GUD1600 single level experiments appear to show a reduction in stiffness and tack with increased temperature (Fig. 5.8). Values for standard deviation reveal a significant level of certainty in the results (Table 5.3). The decreasing tack levels with increased temperature contradicted manufacturers expectations. Typically, when production becomes problematic due to low tack customers are advised to warm the prepreg, often resolving the issue. Therefore, the low
Two types of failure mode were observed during temperature tests. These failure types can be compared with those observed in PSA peeling [6,22]. Some PSA failure modes were not observed during prepreg testing. These failure modes are attributed to failure at the flexible PSA substrate which is not possible in prepreg due to the fibres being clamped. Interfacial failure was observed with the CUD600 low tack prepreg sample and resulted in minimal resin deposition at the surface. Cohesive failure is observed with GUD1600 samples and results in significant resin deposition and the possible formation of fibrils (Fig. 5.11). Both failure modes are observed in GUD400ATL prepreg over the temperature range. The peak in measured tack (Fig. 5.1) corresponds to the temperature at which transition in failure is observed (Fig. 5.12). Interfacial and cohesive failure modes appear to have opposing responses to increasing temperature. Tack during interfacial failure is increased by improved surface wetting and continues to develop into cohesive failure, where failure now appears to occur within the resin bulk. The bulk then reduces in strength as temperature is further increased. Failure between fibres within the bulk of the prepreg appears to be avoided most probably due to the continuous unidirectional fibres being clamped within the grip. A third failure mode is possible at the interface between fibres and resin but was not observed, possibly due to good impregnation which results in a non uniform resin–fibre interface boundary. Further rationalisation of failure modes and temperature response may be found by considering the viscoelastic response of the resin to increasing temperature. 5.7. Viscoelastic properties The suitability of a PSA resin for adhesive applications is typically determined by considering the viscoelastic properties. The most basic definition is that of ‘contact efficiency’ where the Dahlquist criterion states that a PSA must have a shear storage modulus of 3 105 Pa or below to be considered ‘contact efficient’ and a useful PSA [6]. Materials which fall above the criterion are considered too stiff to make adequate contact with the surface. The viscoelastic windows principle is used to further categorise PSAs according to their suitability for particular applications [23].
Table 5.2 Commercial stiffness and tack results compared to manufacturers specified values. Matl. ref.
GB600 GT1200 GUD1600 CUD600
Fibre type
E-glass E-glass E-glass Carbon
Resin content (%)wt.
45 38 32 32
Fibre weight (g/m2)
600 1200 1600 600
Stiffness (N/75 mm)
Tack (N/75 mm)
Position study
Face study
Position study
Face study
Manufacturers specified tack level
14.4 28.4 21.8 32.2
12 26.7 21.9 33.7
60 44.4 17.22 0
52.2 43.9 18.6 0
Medium Medium High Low
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Fig. 5.7. The effect of resin type on tack and stiffness of experimental prepreg at ambient temperature, 100 N compaction force and 500 mm/min feed rate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.10. Tack and stiffness of GUD400ATL glass prepreg tape in response to increasing temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.11. Interfacial (left) and cohesive (right) peel failure modes seen in CUD600 and GUD1600 prepreg at 500 mm/min, ambient temperature (20 °C). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.8. Tack and stiffness response of GUD1600 prepreg to temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 5.3 Standard deviation and effect of temperature on the tack and stiffness of GUD1600 prepreg. Temperature (°C)
16
18.3
22.3
27
30.1
Stiffness (N/ 75 mm)
39.37
25.97
19.18
16.36
15.64
r
5.59 55.73 3.26
3.26 35 2.98
0.48 14.49 1
0.75 2.68 1.12
1.53 3.93 0.16
Tack (N/75 mm)
r
[%] Effect
60.27
r
9.964 95.19 7.619
Effect
r
The viscoelastic properties of the prepreg were obtained by oscillatory shear rheology of resin only samples taken before impregnation using ø25 mm parallel plate system with 500 lm gap at 3 Hz. The results appear to support these PSA principles (Fig. 5.13). The low tack resin systems have a higher stiffness at ambient conditions resulting in interfacial failure. The high tack resin system displays cohesive failure at ambient conditions. On heating the low tack resin the viscosity is reduced and surface contact improves with transition to cohesive failure occurring around the point where viscosity drops below the Dahlquist criterion. However, the discrepancies found in the tack testing of commercial prepregs indicate that the actual modulus value for contact efficiency of prepregs is also likely to be a function of fibre surface patterns and impregnation conditions.
6. Conclusions
Fig. 5.9. Tack and stiffness response of CUD600 prepreg to temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
At present, a standardised definition and procedure for the determination of tack and stiffness is undefined and considered low importance, mostly because experienced hand laminators are able to cope with a range of prepreg properties. However, automated methods such as ATL have demonstrated increased sensitivity to these properties. Therefore, a new test has been developed allowing manufacturers to compare prepreg on two important characteristics. A value for stiffness and tack is obtained signifying ease of forming and retention of the prepreg to the mould surface. The new test is an extension of an existing British standard peel method. The new method now includes a pressure controlled application stage where contact time is inversely proportional to peel rate, simulating the ATL process. Values for both stiffness and tack in a continuous two stage test are obtained. ATL production variables such as feed rate, compaction force and mould
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Fig. 5.12. The observed change in failure mode in GUD400 ATL prepreg tape from interfacial (left) through transition at 27 °C to cohesive (right) showing increasing resin deposition with increasing temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
were likened to those found in PSA peeling and the shear storage modulus of the prepreg resin was compared to the PSAs Dahlquist criterion. The prepreg appears to follow the principles of the criterion where higher modulus (stiffer) resins display poor contact. As the stiffness is reduced contact is improved, failure eventually switches to the bulk which becomes progressively weaker as temperature increases further. Although the criterion principle is supported it is likely to be a function of prepreg and mould surface conditions rather than a single value such as that set by Dahlquist for PSA applications. References
Fig. 5.13. Shear storage modulus of the prepreg resins in comparison to the Dahlquist criterion for PSA contact efficiencys. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
surfaces can be adjusted to allow for the future investigation of their effects. A repeatability study revealed consistent results with 16% standard deviation and the successful identification of a tackier control sample. Repeatability in the measurement of tack and stiffness was found in the testing of commercial hand lay-up prepregs. Tack and stiffness variability was observed across the roll width and between faces in commercial hand lay-up prepregs. Variability in further experiments may be reduced by controlling the roll position and face tested. The overall tack and stiffness values for commercial hand lay-up prepregs did not match manufacturers specified tack levels of ‘low’, ‘medium’ and ‘high’. Multidirectional fabrics with medium tack resin systems were found to have higher tack than unidirectional fabrics with high tack resin systems. Although, tack results for equal unidirectional fibre prepregs impregnated with ‘high’, ‘medium’ and ‘low’ tack resin systems did agree well with manufacturers tack expectations. Therefore, results support the existing evidence that tack could also be affected by the surface resin pattern of the fibres and resin volume fraction. Therefore, tack should be specified based on prepreg tack properties, which include fibre surface and impregnation effects, rather than resin only tack properties. The effects of a temperature increase revealed inconsistent results between materials. Low tack prepregs showed increased tack while high tack prepregs showed a reduction in tack. Newly developed ATL prepreg exhibited a peak in tack. The contradictory results were rationalised by observing the failure mode. Interfacial failure was observed in low tack samples with cohesive failure in high tack samples. Interfacial failure was observed to transition to cohesive failure in the ATL prepreg with the transition point corresponding to a peak in measured tack. The two failure modes
[1] Rudd CD, Long AC, Kendall KN. Liquid moulding technologies. Cambridge, England: Woodhead Publishing Ltd.; 1997. [2] Newell GC, Buckingham RO, Khodabandehloo K. The automated manufacture of prepreg broadgoods components – a review of literature. Compos Part A – Appl Sci Manuf 1996;27(3):211–7. [3] Goode K. Automated ply lamination. Composites, vol. 21. ASM International; 2004. ASM Handbook. [4] Crossley RJ, Schubel PJ, Warrior NA. Challenges in automated turbine blade production; Automated tape layup (ATL) of wind energy grade materials. In: EWEC 2010. EWEA: Warsaw, Poland; 2010. [5] Putnam JW, Seferis JC, Pelton T, Wilhelm M. Perceptions of prepreg tack for manufacturability in relation to experimental measures. Sci Eng Compos Mater 1995;4(3):143–54. [6] Benedek I, Feldstein MM. Fundamentals of pressure sensitivity. London: CRC Press; 2009. [7] Grimshaw MN. Automated tape laying. Composites, vol. 21. ASM International; 2001. p. 480–5. ASM Handbook. [8] Banks R, Mouritz AP, John S, Coman F, et al. Development of a new structural prepreg: characterisation of handling, drape and tack properties. Compos Struct 2004;66(1–4):169–74. [9] Anon. Tack measurement for pressure sensative adhesives – Determination of rooling ball tack. 1999 BSI, Report No. BS EN 1721:1999. [10] Anon. Tack measurement for pressure sensative adhesives – Determination of loop tack. 1999 BSI, Report No. BS EN 1719:1999. [11] Ahn KJ, Seferis JC, Pelton T, Wilhelm M. Analysis and characterization of prepreg tack. Polym Compos 1992;13(3):197–206. [12] Seferis JC, Meissonnier J. Development of a tack and drape test for prepregs based on viscoelastic principles. Sampe Quarterly-Soc Adv Mater Process Eng 1989;20(3):55–64. [13] Putnam JW, Hayes BS, Seferis JC. Prepreg process-structure-property analysis and scale-up for manufacturing and performance. J Adv Mater 1996;27(4): 47–57. [14] Tordjeman P, Papon E, Villenave JJ. Tack properties of pressure-sensitive adhesives. J Polym Sci Part B-Polym Phys 2000;38(9):1201–8. [15] Lin YY, Hui CY, Conway HD. A detailed elastic analysis of the flat punch (Tack) test for pressure-sensitive adhesives. J Polym Sci Part B-Polym Phys 2000; 38(21):2769–84. [16] Gay C, Leibler L. Theory of tackiness. Phys Rev Lett 1999;82(5): 936–9. [17] Chiche A, Pareige P, Creton C. Role of surface roughness in controlling the adhesion of a soft adhesive on a hard surface. Comptes Rendus De L Academie Des Sciences Serie Iv Physique Astrophysique 2000;1(9): 1197–204. [18] Dubois O, Cam JBL, Beakou A. Experimental analysis of prepreg tack. Experim Mech 2009. Online. [19] Anon. Determination of peel resistance of high-strength adhesive bonds Floating roller method. 1995 BSI, Report No. BS EN 1464:1995. [20] Derail C, Allal A, Marin G, Tordjeman P. Relationship between viscoelastic and peeling properties of model adhesives. Part 1. Cohesive fracture. J Adhesion 1997;61(1–4):123–57.
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[21] Kaelble DH. Peel adhesion: micro-fracture mechanics of interfacial unbonding of polyers. Transac Soc Rheol 1965;9(2):135–63. [22] Aubrey DW, Sherriff M. Peel adhesion and viscoelasticity of rubber–resin blends. J Polym Sci Part A-Polym Chem 1980;18(8):2597–608.
[23] Chang EP. Viscoelastic windows of pressure-sensitive adhesives. J Adhesion 1991;34(1–4):189–200.
Contents lists available at SciVerse ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
The experimental determination of prepreg tack and dynamic stiffness R.J. Crossley ⇑, P.J. Schubel, N.A. Warrior Polymer Composites Group, Division of Materials, Mechanics and Structures, Faculty of Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 6 October 2011 Accepted 15 October 2011 Available online 24 October 2011 Keywords: A. Prepreg B. Tack C. Adhesion D. ATL
a b s t r a c t A new peel test has been developed which quantifies the tack and dynamic stiffness of uncured prepreg. The test is designed to simulate the automated tape lay-up (ATL) and automated fibre placement (AFP) processes. It includes a pressure controlled application stage, where contact time is inversely proportional to peel rate. The use of a thin film allows stiffness to be isolated from peel resistance in a continuous two stage test. A repeatability study revealed consistent results with 16% standard deviation. Tack and stiffness variability has been observed across roll width and between faces in commercial hand layup prepregs. The overall tack and stiffness values for commercial hand lay-up prepregs were found to be inconsistent with the levels specified by manufacturers. A temperature increase revealed inconsistent effects on tack between materials. The contradictory results were rationalised by observing failure modes. The two failure modes observed appeared equivalent to those found in pressure sensitive adhesive (PSA) peel. The shear storage modulus of the prepreg resin was compared to the PSA Dahlquist criterion and found to support the principle of contact efficiency. However, the actual value for the criterion is expected to be a function of prepreg specific conditions such as resin content, fibre distribution and surface pattern. Ó 2011 Published by Elsevier Ltd.
1. Introduction Prepreg consists of reinforcement fibres pre-impregnated with a resin matrix and has been in use for many years [1]. Traditional prepreg hand lamination involves cutting plies into the required shape, removing the backing paper and placing into a mould. Pressure is manually applied to ensure the ply conforms to the mould surface. Tack levels are formulated such that the material will remain in place throughout the lamination process and can be repositioned if necessary. For hand lamination, tack and stiffness levels are relatively flexible due to the skill of experienced laminators. Stiffness is estimated based on fibre areal weight (FAW) and tack is generally specified on material datasheets as ‘low’, ‘medium’ or ‘high’ based on subjective touch tests. The hand lamination process is labour intensive, lacks consistency and would benefit considerably from automation [2]. However, considerable technical difficulty is attributed to the prepreg tack level which must be low to allow backing paper removal but remain high enough to hold the lay-up together [3]. New materials are now being developed for low cost mainstream automated tape laying (ATL) methods where a sensitivity to tack and stiffness is shown to dictate lay-up performance [4]. Therefore, it has become necessary to further define and quantify tack and stiffness. The term tack is used in the composites industry ⇑ Corresponding author. E-mail address: [email protected] (R.J. Crossley). 1359-835X/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.compositesa.2011.10.014
to describe instantaneous adhesion before the resin has cured [5]. Tack is also further defined within the pressure sensitive adhesives (PSAs) industry as the instantaneous adhesion associated with short contact times typically resulting in adhesive rather than cohesive failure [6]. Historically, tack may be defined as the adhesion which occurs quickly at room temperature, without special surface preparation, under finger application pressure. The definition is now extended to include instantaneous adhesion which occurs within the operating conditions of ATL. In the ATL process, prepreg tape and backing paper is fed from a reel and pressed onto the mould surface by a compaction tool. During lay-up the prepreg is expected to leave the backing paper, and be retained by tack to the mould surface [7]. A simplified force diagram (Fig. 1.1) shows that undesirable peel of the prepreg from the mould surface, resulting in lay-up failure, may occur if the tack to the backing paper exceeds tack to the mould surface. The stiffness of prepreg is also beneficial in aiding release from the backing paper as it is forced around the compaction shoe. Therefore, ATL may be represented by a continuous peeling process where application time is inversely proportional to feed rate. In ATL, AFP and hand lay-up the first ply is often considered the most difficult due to the smooth surface often treated with release agents which results in poor mould adhesion. Therefore, both tack to the mould surface and dynamic stiffness are considered the most significant characteristics of prepreg. Stiffness is also applicable to hand lay-up as it gives an indication of the ease of which it may conform to a contoured mould surface.
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Fig. 1.1. A Cincinnatti V4 ATL delivery head and a simplified force diagram of the ATL process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Standardised methods of determining prepreg stiffness [8] neglect polymer relaxation and rely on the weight of the prepreg to provide the measured deformation which complicates comparisons between alternate resin systems and FAWs. Several standardised methods for determination of PSA tack exist [9,10], however, results are difficult to quantify in relation to lay-up conditions. The probe test has been used extensively in the PSA industry [6] and has been adopted in various prepreg studies [11–13]. The probe test consists of a disc of resin between two circular surfaces. The surfaces are typically brought together under controlled force or to reach a predefined gap. Force is recorded for separation by a controlled rate. Stress is defined using the area of the probe surface [6]. However, the actual contact area is likely to differ and requires complex equipment for accurate definition [14]. A number of failure modes are observed occurring either in the bulk, possibly by fibrillation, or at the surface by crack formation and propagation [15]. The actual failure stress and type is believed to be a complex interaction between the probes surface and the nucleation of cavities [16]. The volume of air pockets trapped during the application stage and a change in stress distribution are considered to have an effect [17]. The probe test has also been found to be sensitive to changes in surface roughness caused by fibre patterns and resin impregnation distribution within the bulk specimen [18]. Therefore, the probe test is considered to be less suitable for the characterisation of ATL prepreg tack due to: Increased sensitivity to surface and bulk air pockets and cavitations. Failure within the bulk, which is allowed in the probe test, that is likely to be prevented by the continuous fibres during ATL. The inability to relate results to the peel mechanism. A peel test method is also used extensively in the PSA industry for adhesive tapes [6]. British standard methods are available [19] and have been utilised in the characterisation of prepreg [8]. However, the peel method is often considered inferior to the probe test as: The individual stages of separation cannot be isolated, limiting its analytical ability.
It does not define or include the application of the adhesive tape to the rigid substrate. The adhesive is typically applied in an uncontrolled secondary process following manufacturer’s guidelines. The application method prevents the study of short application times and contact conditions. There is considerable difficulty in separating bending from adhesive forces [20]. A new peel test was developed to address these issues whilst simulating the ATL process. The floating roller method [19] was modified to include a compaction roller which allows the simultaneous pressure controlled application and peel of prepreg. An additional thin covering film was used to allow stiffness bending effects to be characterised and separated from peel resistance, quantifying tack in a single two stage test. The value obtained for tack is peel resistance without the effects of bending and is believed to be analogous to the work of adhesion found in probe testing (Fig 1.2). Although this method remains unsuitable for the analysis of the various stages of tack separation it is considered an acceptable quantity by which to compare materials. The method was then used to characterise existing hand lay-up and ATL prepregs. Although the test may be reconfigured to measure backing paper tack, it is neglected during this study. Residual tack to the backing paper is required to maintain prepreg in position during cutting operations. The tack to the backing paper is therefore considered at present to be the minimum amount which allows retention during the cutting operation but release during lay-up. The backing paper is also applied at the time of manufacture, and although some peel rate and temperature dependence is likely to remain, as with most pre-applied PSA peel tests [6], the interface is well established and is expected to be less sensitive to the effects of such variables, Backing paper tack appears appropriate under certain lay-up conditions and the maximum variability and difficulty in ATL lay-up appears to occur in achieving tack to the mould surface. Therefore, this study focuses on the effect of mould surface to prepreg tack. The effects of resin type, temperature, feed rate and the variation in commercial prepreg were also investigated. Maximum ATL feed rates of 50,000 mm/min are typical, however, an initial feed rate of approximately 500 mm/min is typical when starting a new ply to the mould surface. Therefore, although equipment constraints
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425
Fig. 1.2. Comparison of typical probe test results (left [6]) with force measurements taken along the peel front (right [21]). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
limit test feed rates to 1000 mm/min results remain applicable to these starting conditions which are often most problematic.
2. Method The prepreg samples were pulled through the spring loaded rollers which provide an application force against a rigid substrate which simulates the mould surface. The first two rollers are used to guide the plate. The second two rollers provide the peel and application force. Application and 90° peel occurs instantaneously against the fixed top roller with the compaction force applied by the spring loaded bottom roller. Results are recorded for two sections in a continuous test. Stiffness is recorded for the first section of the test where the sample has a thin film covering the tack surface. The film is absent for the second section of the test where peel resistance is recorded (Fig. 2.1). By subtracting the average stiffness from the average peel resistance a value for tack was found with minor adjustment for the absence of the thin film. The correction value for films was typically less than 1 N found by measuring the rolling resistance of the film independently. Bearing and rolling losses (0.7 N) were calculated by measuring subsequently
Fig. 2.1. The operating principle of the prepreg tack and stiffness test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
narrower width films. They were then subtracted from all results and the rolling resistance of films to yield correction values. The new peel test equipment was mounted in a universal test machine (Fig. 2.2). Peel resistance and stiffness was then measured continuously over a predetermined peel distance using a load cell. Extension and load values were recorded for later analysis. To allow for temperature changes the test rig was enclosed in an oven. The specimen size was chosen to exceed the 115 mm recommended [19] for PSA continuous peel and to maintain plate stability. A specimen length of 300 mm was chosen requiring a 250 mm test plate. To allow changes in temperature the test rig was mounted within an oven with limited space. A 140 mm long test plate was the maximum length allowed within the oven space resulting in a 215 mm long sample (Table 2.1). A specimen width of 75 mm was chosen, exceeding the 25 mm British standard recommended value. The extra width minimises the impact of small randomly positioned irregularities across the width of the prepreg and increases the recorded tack load in relation to experimental noise. The increased width also emphasises any large irregularities which span a significant portion of the prepreg width. Such irregularities serve to highlight variability in the prepreg production,
Fig. 2.2. Tack and stiffness test rig mounted in a universal test machine.
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Table 2.1 Rigid substrate and prepreg specimen sizes with peel and stiffness measurement distance. Temperature chamber?
Oven
None
Sizes (mm)
L
W
L
W
Specimen
215
75
300
75
Results Length of stiffness Length of continuous peel
50 80
60 140
Recommended for PSA [19] W
N/A 115
storage and handling which may affect ATL performance and were avoided in the study of variable effects. The width is also convenient as it minimises the number of cuts required to test standard ATL roll widths of 75, 150 or 300 mm.
Table 2.2 Variables found in ATL production in comparison to adjustable test parameters. Variable
Production range
Test range
Limitation
Temperature
0–80 °C
20–80 °C
Oven capabilities
Relative humidity
0–95%
40–60%
Environmental chamber required
0.01–1000 mm/min
Test M/C
Application pressure
130–650 N
25–250 N
Springs, construction costs
Mould surface
Unlimited
Any plate and prepreg combination up to 6 mm thick
Mounting bracket, clearance and construction roller clearance
ATL tape thickness
0– > 1 mm
Feed rate
2.1. Variables The test rig was designed with features that allow the variables found in ATL production to be investigated (Fig. 2.3). The values represent those found commercially with practical limitations imposed by test equipment (Table 2.2). Jacking screws are used to control the compaction force up to a maximum of 250 N. This force is within the lower half of the 130–650 N specified by ATL manufacturers [7]. The compaction force was limited by the roller bearings associated rolling resistance and springs. Larger springs would require larger shaft diameters and bearings to suit the higher loads. This would cause a deviation from the British standard specifications for roller diameter [19] and a significant increase in construction costs. Temperature was adjusted by fitting the rig within a temperature chamber. Feed rate was adjusted via the universal test machine, limited to a maximum of 1000 mm/min. The solid substrate plate can be made from any rigid mould material and treated with release agents to simulate mould conditions. Adjustable clearance between the rollers allows for up to 6 mm thickness of rigid substrate and prepreg material. 2.2. Compaction roller calibration The sprung roller was calibrated in order to quantify the prepreg application force by inserting a rigid L shaped plate connected directly to the load cell via the fabric grips (Fig. 2.4). The L shape plate rests directly on the sprung roller. The jacking screws were then pre-tensioned to give the starting point of calibration. Subse-
Fig. 2.4. The peel test rig configured for compaction roller force calibration.
quent turns were made to give a force per number of screw turns. The test was repeated and showed a linear relationship. A chart was then produced to give the number of turns for the required compaction force used during testing (Fig. 2.5). During the test the first sample was loaded followed by pre-tensioning of the screws and the required number of turns to give the chosen compaction force.
3. Analysis
Fig. 2.3. The peel tack and stiffness test rig design features.
Results for a medium tack prepreg showed an increase in rolling resistance at the transition of the stiffness to peel resistance section of the test (Fig. 3.1). Average values for both sections of the test were calculated over the applicable area (Table 3.1). Care was taken to exclude any unreasonable peaks resulting from surface defects in the prepreg or backing paper defects, such as bubbles or folds. The values for stiffness and peel were expressed as force per unit width (N/75 mm). Standard deviation for a single sample was calculated (Eq. (3.1)) and represented by error bars on line or scatter plots where a single sample represents each point. Where possible, batch testing was carried out to overcome the variability in tack testing using a minimum of three samples. The batch standard deviation was calculated (Eq. (3.2)) and displayed as error bars on bar charts. The sample deviation gives an indication of the uniformity of peel. In a single sample a lower deviation indicates steady state peel. An unusually high deviation could indicate the stick-slip condition.
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427
Fig. 2.5. Compaction roller calibration results and jacking screw settings.
Fig. 3.1. A typical five sample test result where stiffness and peel resistance are recorded allowing average tack to be calculated. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3.1 Typical extension range of measurement areas. Extension range
Ambient (mm)
With oven (mm)
Total Stiffness Peel resistance
200 20–50 80–180
130 10–35 55–110
effect appeared exaggerated where very low values of tack were recorded overall. In such cases a comparison with experimental error is required. The average experimental batch standard deviation is taken for all levels and then divided by the average value (Eq. (3.4)). If the error value approaches or exceeds the effect then the effect is most likely to be a product of experimental noise. Expression of the effect of variables
Batch deviation was expressed to determine the deviation in average tack between samples and typically appeared higher where individual samples exhibit unsteady peel.
rs
ð3:1Þ
where rs is the standard deviation in a sample; ls the average tack or stiffness value; x the tack or stiffness values; n is the number of values.
rb
ð3:3Þ
Error ¼
Av g:std:dev : 100% Av g:v al
ð3:4Þ
4. Observations and discussion 4.1. Reducing uncertainty
Standard deviation in a batch of samples
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X ¼t ðxi lb Þ2 n i¼1
Max:v al Min:v al 100% Max:v al
Expression of overall experimental error
Standard deviation in a single sample
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X ¼t ðxi ls Þ2 n i¼1
Effect ¼
ð3:2Þ
where rb is the standard deviation in a batch; lb is the average batch tack or stiffness value; x is the average sample tack or stiffness values; n is the number of samples. In order to compare the effect of multiple level experiments directly the effect of the variable under consideration is defined by the maximum change as a percentage of the maximum recorded value for (Eq. (3.3)). This method proved effective for results where large changes of tack were recorded. However, the value of the
Observations during test development highlighted sources of error which were controlled in order to achieve reliable results (Table 4.1). Prepreg tack was found to be sensitive to temperature fluctuations which were limited to ±0.5 °C. A thorough cleaning schedule was followed for rigid substrates where cleaning and handling procedures were found to significantly affect experimental results. Disposable nitrile gloves were worn at all times as sweat residue deposited by finger prints on test could be seen in resin deposition patterns (Fig. 4.1). Any residue left by solvents or cleaning agents was completely removed from the rigid plates. Handling of the prepreg was minimised to prevent the transfer of body heat to the sample or rigid substrate.
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Table 4.1 A summary of observed experimental errors. Cause
Effect
Control
Environmental Temperature fluctuations
Severe
Use a temperature regulated room or environmental chamber
Relative Humidity fluctuations Set up and procedure Temperature fluctuations through excessive Handling Sweat or grease residue on rigid plates Excess resin on test rig Plate oscillations during testing Material Uncontrolled sample roll location Uncontrolled sample face Rips, tears, folds or defects Resin remains on backing film
Low
Severe
Minimise handling, use rubber gloves, allow sample to cool after handling
High
Avoid skin contact with rigid plates, use rubber gloves Clean with acetone Ensure the rigid plate is always positioned only 10 mm from the end of the prepreg
Med Low
High
Only samples cut from the same position along the width and on the same face should be compared
High
Chose only uniform samples in good condition Reject samples where resin remains on the backing paper after peeling
Severe
surface at the start of the test did not significantly affect final tack results. Three time delays of 0 (minimum possible loading time), 2 and 10 min were used. The results showed no noticeable effect on prepreg tack and stiffness exceeding experimental error (Table 4.2). However, a negligible rise in stiffness was attributed to the formation of excess resin squeezed out in front of the compaction roller during the dwell period. A raised pull force was recorded during this initial stiffness stage which was mostly outside the area of analysis. Therefore, although the effects appeared negligible the dwell time was minimised. 4.3. Radius of bending Isolating stiffness from peel resistance requires significant additional calculations and measurements in traditional peel testing [20]. Stiffness is now measured during the first stage of the new test, where a thin film covers the tack surface. However, there is a potential inaccuracy caused by a change in bending radius. The radius of bending has been seen to change between the stiffness and peel sections of the test when the peel force becomes elevated. For the stiffness section the sample typically follows the radius of the roller. During the peeling section the prepreg may be retained on the plate surface extending away from the roller, resulting in a smaller bending radius (Fig. 4.3). The presence of the fibres ensures that the change in radius remains small in all but the highest tack situations, at which point the bending force is considerably lower than tack. Therefore, the effects of a change in radius are considered negligible in the comparison of prepregs with similar stiffness. 4.4. Negative tack
Fig. 4.1. A finger print on the rigid substrate present before testing is revealed by lack of resin adhesion after the test.
The position of the rigid plate in relation to the prepreg was also seen to add a degree of fluctuation to results. Leaving an excessive plate overhang caused the plate to lift and drop within the clearance of the guide rollers. This occasionally led to oscillations in the recorded force and contributed to causing an unsteady peel condition. Therefore, for consistency, the rigid plate was consistently positioned approximately 5–10 mm from the end of the prepreg sample. Samples where resin has remained attached to the backing paper after peeling were excluded. Patches of lost resin were seen as an area where tack was reduced (Fig. 4.2). Defects in the prepreg material were found to have a detrimental effect on results, with lumps bumps, folds or tears showing up as artificial peaks in force as they passed through the rollers. Such defects are likely to be less influential on prepreg tack during actual lay-up where ATL machines typically use compliant rollers. Therefore, every effort was made to test uniform samples with the absence of defects.
Negative tack values were recorded for very low tack prepreg material. A component of the negative tack value is due to the bending resistance of the covering film which is present in the stiffness test but absent from the peel resistance section. Therefore, when stiffness is removed from peel resistance a negative value is found. To account for the bending stiffness films are calibrated by measuring rolling resistance through the rig. The relevant calibration value for the film used (Table 4.3) is then added to the average peel resistance. A small negative value for tack (typically < 2 N) has been observed in some results after the backing film correction has been made. Negative tack values may signify that surfaces are repellent, observed as zero tack in practice. Therefore, all negative tack values are considered negligible and are regarded as zero tack. These additional small negative tack values could be attributed to: Unusually high average stiffness values attributed to imperfect bending or folds, most often seen in stiff samples. Efforts should be made to avoid anomalous peaks in rolling resistance when analysing results. Frictional interaction between the film and the prepreg during bending which is not included in the calibration. Changes in the bend radius between the stiffness and peel section. A difference in the measured film stiffness and the actual film stiffness when applied to a prepreg ply. 5. Results and discussion 5.1. Repeatability study
4.2. Dwell time A dwell time test was conducted to ensure that the length of time that the material rests under gravity on the ridged plate
A blind trial was conducted on nine epoxy glass fibre prepreg batches of the same stock with a tenth tackier control batch were supplied randomly by the manufacturer. A total of 30 samples
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Fig. 4.2. Dry resin patches on the prepreg (top left), lead to reduced tack levels during the test observable by lost resin patches on the test plate (top right), bubbles and folds (bottom) were also responsible for artificially high peaks in tack readings.
Table 4.2 The effect of dwell time before the start of the test where material is allowed to rest under gravity on the rigid substrate surface. Dwell Time (Mins)
0
2
10
Stiffness N/75 mm
34.06 1.77 65.42 24.73
37.2 1.33 50.07 21.14
38.32 1.33 56.9 23.76
r Tack N/75 mm
r
Effect% Error% Effect% Error%
11.1 4 23.5 40.4
Fig. 5.1. Results of a repeatability experiment with nine same stock samples and a higher tack control sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4.3. The deviation in bending radius from the radius of the roller during the peel of high tack prepreg.
Table 4.3 Covering film calibration values.
Five samples were tested for each material on three occasions. The rig was removed from the universal testing machine between each occasion to allow for possible set up deviations. Stable and repeatable values were recorded for all stiffness values with minimal standard deviation (Fig. 5.2). Tack values also appeared stable and repeatable for unidirectional (UD) materials with reasonably low standard deviation. However, multidirectional materials with higher resin content showed inconsistent tack results with significant deviation between samples and batches. The results also show significant deviation from manufacturers specified tack levels. 5.2. Commercial prepreg variability
Backing film
Calibration value
Embossed polythene Red polythene Clear PET
4 N/75 mm 1 N/75 mm 0.3 N/75 mm
were tested in batches of three samples each (Fig. 5.1). The nine similar materials measured an average tack level of 4.8 N with a 0.8 N (16%) standard deviation, which was acceptable considering environmental fluctuations of 20% relative humidity and 1 °C in temperature. The 10th sample, a tackier material, was correctly identified registering a significantly higher reading of 17 N in line with expectations. The repeatability study was then extended to characterise several commercial hand lay-up prepreg materials with epoxy resins of various specified tack levels, fibre weight and architecture (Table 5.1).
Commercial prepreg rolls of up to 1.5 m wide are typically produced in a continuous process which is believed to result in uniform tack along the length but not necessarily across the width. Other deviations may also occur as a result of changes in surface texture caused by alternate fibre orientation or backing film patterns between faces. Therefore, alternate faces and roll width positions were tested to quantify any variability. 5.2.1. Roll position Samples from the previous repeatability experiment had their location on the roll recorded to allow for the distribution of prepreg tack across the roll width to be investigated (Fig. 5.3). All samples showed differences in average stiffness across the roll with two samples increasing towards the middle (Fig. 5.4). The glass triaxial material showed the greatest increase of up to 10 N stiffness towards the centre of the roll. UD and biaxial glass materials showed the least variation in stiffness across the roll width.
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Table 5.1 Specifications of the existing commercial prepregs tested. Ref.
Fibre weight (g/m2)
Fibre direction
Resin content by weight (%)
Specified tack level
GB600 GT1200 GUD1600 CUD600
600 1200 1600 600
Bi-axial ± 45° Tri-axial 0° ± 45° UD 0° UD 0°
45 38 32 32
Medium Medium High Low
All samples appeared to show some tack variability across the roll width with the exception of the CUD600 carbon samples which did not display any measurable tack (Fig. 5.5). The GT1200 sample showed a significant increase in tack towards the centre of the roll. The variations in tack and stiffness at roll width positions are generally attributed to the prepreg production method. Fibre distribution methods, roller position, pinch and tension settings may result in variations in impregnation across the roll due to variability such as bleed out at the roller ends. 5.2.2. Sample face Experiments were carried out to determine if tack is dependent on the prepreg face tested (Fig. 5.3). Three samples from each face were tested for each of the four commercial prepregs. Stiffness values remained reasonably consistent for all materials (Fig. 5.6). However, tack values were found to vary significantly between faces. The GT1200 triaxial sample showed the greatest variation between faces which is likely to be the result of a difference in fibre pattern and therefore the resin surface layer between faces. CUD600 displayed little variation due to its low tack properties. Overall, experimental standard deviation in stiffness and tack can be reduced by controlling the roll position and face tested. Small variations in tack between faces in other samples could be the result of the roll direction, with the inner face experiencing a difference in strain during production and storage causing differences in surface resin migration.
Fig. 5.2. A tack and stiffness repeatability study with 35 sample batches of four commercial hand lay-up prepregs tested on three occasions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.3. Specimen location for analysis of tack and stiffness variability as a function of roll position and face.
5.3. Commercial prepreg repeatability A total of 21 samples per prepreg were tested. Consistent overall values for tack and stiffness were recorded between the 15 sample roll position study and the six sample face study (Table 5.2). The stiffness values appear reasonable, with CUD600 displaying the highest stiffness. This is most likely a result of the increased stiffness of carbon fibres in comparison to E-glass fibres. It would be reasonable to assume that the stiffness of glass fibre prepreg would be proportional to material weight. However, GT1200 shows increased stiffness in comparison to GUD1600. Therefore, fibre architecture and increased resin content may also contribute to increased stiffness. Repeatable values for tack were found which were inconsistent with specified tack levels (Table 5.2). Manufacturers typically specify tack values based on constituent resin tests with the absence of fibres. Multidirectional fabrics display the highest deviation. Therefore, It is likely that fibre direction and resin content also effect tack. They may change the resin layer thickness and surface pattern, which has also been shown to have an effect on probe tack test results [18]. A suction component may account for increased tack values recorded for multidirectional fabrics. This cavitation and suction effect may be less pronounced in UD prepregs due to long cavities normal to the peel front. The cavities, which follow the fibre pattern, may extend out beyond the peel and application zone. Therefore, they may be mostly open to the atmosphere preventing the suction effect from occurring. Unidirectional material showed the least variability and is used in future investigations
Fig. 5.4. Stiffness distribution across the 1.5 m width of commercial hand lay-up prepreg rolls. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
to reduce the relative experimental noise when comparing the effect of variables. 5.4. Effect of resin type Bespoke prepreg samples were prepared using unidirectional Eglass 200 g/m2 impregnated with 32%, by weight, resin content. Three resin types, formulated for their tack properties (‘High’, ‘medium’ and ‘low’), were impregnated using a consistent method and equipment. Other process variables were held constant with feed rate and compaction force set at 500 mm/min and 100 N. Five samples were tested for each resin type; error bars represent the batch deviation. A low measurable effect on stiffness is observed
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Fig. 5.5. Tack distribution across the 1.5 m width of commercial hand lay-up prepreg rolls. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
tack CUD600 specimen was tested for temperature effects. The CUD600 prepreg showed a similar stiffness decrease response to temperature. However, a contradictory tack response was found with a significant (97%) increase in tack compared to 23% error (Fig. 5.9). Newly developed wind energy ATL prepreg tape consisting of 400 g/m2 FAW E-glass fibre impregnated with 28% wt. low tack BPA epoxy resin (GUD400ATL) was also tested. This material showed increased uniformity in results allowing a single sample at each temperature to provide reasonable results at an increased number of temperature intervals. Testing of the new material revealed a peak in tack and continuously reducing stiffness with increased temperature (Fig. 5.10). The three differing responses to temperature in commercial and ATL prepreg cannot be rationalised easily without considering the failure mode. 5.6. Failure mode
Fig. 5.6. Tack and stiffness values between alternate faces of the prepreg roll. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(19.4%) with 8% uncertainty. The effects on tack show good agreement with manufacturer’s specified tack levels (Fig. 5.7). A 98.2% effect with 25.6% uncertainty shows that resin formulation has a significant effect on tack. This result appears to indicate that the resin properties have a dominant effect on tack provided a consistent fibre surface pattern and resin content is maintained. 5.5. Effect of temperature GUD1600 and CUD600 hand lay-up prepregs were tested in batches of three samples at five temperature intervals. An oven was used to heat the sample and rigid substrate. Temperature was measured using an IR thermometer focused on the tack area immediately after the test. Results for GUD1600 single level experiments appear to show a reduction in stiffness and tack with increased temperature (Fig. 5.8). Values for standard deviation reveal a significant level of certainty in the results (Table 5.3). The decreasing tack levels with increased temperature contradicted manufacturers expectations. Typically, when production becomes problematic due to low tack customers are advised to warm the prepreg, often resolving the issue. Therefore, the low
Two types of failure mode were observed during temperature tests. These failure types can be compared with those observed in PSA peeling [6,22]. Some PSA failure modes were not observed during prepreg testing. These failure modes are attributed to failure at the flexible PSA substrate which is not possible in prepreg due to the fibres being clamped. Interfacial failure was observed with the CUD600 low tack prepreg sample and resulted in minimal resin deposition at the surface. Cohesive failure is observed with GUD1600 samples and results in significant resin deposition and the possible formation of fibrils (Fig. 5.11). Both failure modes are observed in GUD400ATL prepreg over the temperature range. The peak in measured tack (Fig. 5.1) corresponds to the temperature at which transition in failure is observed (Fig. 5.12). Interfacial and cohesive failure modes appear to have opposing responses to increasing temperature. Tack during interfacial failure is increased by improved surface wetting and continues to develop into cohesive failure, where failure now appears to occur within the resin bulk. The bulk then reduces in strength as temperature is further increased. Failure between fibres within the bulk of the prepreg appears to be avoided most probably due to the continuous unidirectional fibres being clamped within the grip. A third failure mode is possible at the interface between fibres and resin but was not observed, possibly due to good impregnation which results in a non uniform resin–fibre interface boundary. Further rationalisation of failure modes and temperature response may be found by considering the viscoelastic response of the resin to increasing temperature. 5.7. Viscoelastic properties The suitability of a PSA resin for adhesive applications is typically determined by considering the viscoelastic properties. The most basic definition is that of ‘contact efficiency’ where the Dahlquist criterion states that a PSA must have a shear storage modulus of 3 105 Pa or below to be considered ‘contact efficient’ and a useful PSA [6]. Materials which fall above the criterion are considered too stiff to make adequate contact with the surface. The viscoelastic windows principle is used to further categorise PSAs according to their suitability for particular applications [23].
Table 5.2 Commercial stiffness and tack results compared to manufacturers specified values. Matl. ref.
GB600 GT1200 GUD1600 CUD600
Fibre type
E-glass E-glass E-glass Carbon
Resin content (%)wt.
45 38 32 32
Fibre weight (g/m2)
600 1200 1600 600
Stiffness (N/75 mm)
Tack (N/75 mm)
Position study
Face study
Position study
Face study
Manufacturers specified tack level
14.4 28.4 21.8 32.2
12 26.7 21.9 33.7
60 44.4 17.22 0
52.2 43.9 18.6 0
Medium Medium High Low
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Fig. 5.7. The effect of resin type on tack and stiffness of experimental prepreg at ambient temperature, 100 N compaction force and 500 mm/min feed rate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.10. Tack and stiffness of GUD400ATL glass prepreg tape in response to increasing temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.11. Interfacial (left) and cohesive (right) peel failure modes seen in CUD600 and GUD1600 prepreg at 500 mm/min, ambient temperature (20 °C). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5.8. Tack and stiffness response of GUD1600 prepreg to temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 5.3 Standard deviation and effect of temperature on the tack and stiffness of GUD1600 prepreg. Temperature (°C)
16
18.3
22.3
27
30.1
Stiffness (N/ 75 mm)
39.37
25.97
19.18
16.36
15.64
r
5.59 55.73 3.26
3.26 35 2.98
0.48 14.49 1
0.75 2.68 1.12
1.53 3.93 0.16
Tack (N/75 mm)
r
[%] Effect
60.27
r
9.964 95.19 7.619
Effect
r
The viscoelastic properties of the prepreg were obtained by oscillatory shear rheology of resin only samples taken before impregnation using ø25 mm parallel plate system with 500 lm gap at 3 Hz. The results appear to support these PSA principles (Fig. 5.13). The low tack resin systems have a higher stiffness at ambient conditions resulting in interfacial failure. The high tack resin system displays cohesive failure at ambient conditions. On heating the low tack resin the viscosity is reduced and surface contact improves with transition to cohesive failure occurring around the point where viscosity drops below the Dahlquist criterion. However, the discrepancies found in the tack testing of commercial prepregs indicate that the actual modulus value for contact efficiency of prepregs is also likely to be a function of fibre surface patterns and impregnation conditions.
6. Conclusions
Fig. 5.9. Tack and stiffness response of CUD600 prepreg to temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
At present, a standardised definition and procedure for the determination of tack and stiffness is undefined and considered low importance, mostly because experienced hand laminators are able to cope with a range of prepreg properties. However, automated methods such as ATL have demonstrated increased sensitivity to these properties. Therefore, a new test has been developed allowing manufacturers to compare prepreg on two important characteristics. A value for stiffness and tack is obtained signifying ease of forming and retention of the prepreg to the mould surface. The new test is an extension of an existing British standard peel method. The new method now includes a pressure controlled application stage where contact time is inversely proportional to peel rate, simulating the ATL process. Values for both stiffness and tack in a continuous two stage test are obtained. ATL production variables such as feed rate, compaction force and mould
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Fig. 5.12. The observed change in failure mode in GUD400 ATL prepreg tape from interfacial (left) through transition at 27 °C to cohesive (right) showing increasing resin deposition with increasing temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
were likened to those found in PSA peeling and the shear storage modulus of the prepreg resin was compared to the PSAs Dahlquist criterion. The prepreg appears to follow the principles of the criterion where higher modulus (stiffer) resins display poor contact. As the stiffness is reduced contact is improved, failure eventually switches to the bulk which becomes progressively weaker as temperature increases further. Although the criterion principle is supported it is likely to be a function of prepreg and mould surface conditions rather than a single value such as that set by Dahlquist for PSA applications. References
Fig. 5.13. Shear storage modulus of the prepreg resins in comparison to the Dahlquist criterion for PSA contact efficiencys. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
surfaces can be adjusted to allow for the future investigation of their effects. A repeatability study revealed consistent results with 16% standard deviation and the successful identification of a tackier control sample. Repeatability in the measurement of tack and stiffness was found in the testing of commercial hand lay-up prepregs. Tack and stiffness variability was observed across the roll width and between faces in commercial hand lay-up prepregs. Variability in further experiments may be reduced by controlling the roll position and face tested. The overall tack and stiffness values for commercial hand lay-up prepregs did not match manufacturers specified tack levels of ‘low’, ‘medium’ and ‘high’. Multidirectional fabrics with medium tack resin systems were found to have higher tack than unidirectional fabrics with high tack resin systems. Although, tack results for equal unidirectional fibre prepregs impregnated with ‘high’, ‘medium’ and ‘low’ tack resin systems did agree well with manufacturers tack expectations. Therefore, results support the existing evidence that tack could also be affected by the surface resin pattern of the fibres and resin volume fraction. Therefore, tack should be specified based on prepreg tack properties, which include fibre surface and impregnation effects, rather than resin only tack properties. The effects of a temperature increase revealed inconsistent results between materials. Low tack prepregs showed increased tack while high tack prepregs showed a reduction in tack. Newly developed ATL prepreg exhibited a peak in tack. The contradictory results were rationalised by observing the failure mode. Interfacial failure was observed in low tack samples with cohesive failure in high tack samples. Interfacial failure was observed to transition to cohesive failure in the ATL prepreg with the transition point corresponding to a peak in measured tack. The two failure modes
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