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Preform reinforcements help spur growth of SLCM
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PECIAL FEATURE
m Preform r e i n f o r c e m e n t s help spur growth o f SLCM If s t r u c t u r a l l i q u i d c o m p o s i t e m o u l d i n g t e c h n o l o g i e s s u c h as SRIM and RTM are to fulfil their p r o m i s e o f rapid growth further d e v e l o p m e n t must take place in system c o m p o n e n t s such as automated preform manufacture, says PPG Industries. J o h n F. Dockum Jr o f PPG in Pittsburgh, Pennsylvania, USA, and Roger A. Goodwill, w h o is based at the company's Wigan, UK, plant l o o k at the i n f l u e n c e o f roving parameters and p r o c e s s variables on c o m p o s i t e s made w i t h fibre directed preforms.
S
tructural liquid composite moulding (SLCM) encompasses a family of moulding processes: liquid composite compression moulding (LCCM), structural resin t r a n s f e r (or injection) m o u l d i n g (SRTM), and structural reaction injection moulding (SRIM). In all the processes a reinforcement preform is positioned in the mould prior to combining with resin. High production rates, lower costs and consistently high mechanical properties over long periods will spur growth of structural liquid composite moulding, principally SRTM and SRIM. Some estimates place annual growth rates at between 35 and 45% during the next five to ten years. Among all the fabrication methods and material systems available for making polymer m a t r i x composites, SLCM is m o s t p r o m i s i n g for r e p l a c i n g s t a m p e d steel assemblies. The reason: moulders can produce complex, structural, three-dimensional shapes economically and at high production rates using preforms tailored precisely to the shape of the part. Reinforcements are not
REINFORCED PLASTICS APRIL 1991
grossly dislodged when moulded with lowviscosity resin, as they are during flow in compression, injection or transfer moulding. As a result, the finished composite will carry the loads for which it is designed. Glass fibre preforms can be prepared manually or thermoformed from continuous strand mat. They can also be built-up to shape by directing chopped multi-.strand roving onto a rotating screen, a process called directed fibre preforming (DFP). Regardless of method, orientation of fibre is isotropic (random), and, assuming t h a t the fibres are sized properly, initial strengths are similar. Other considerations, however, point to directed fibre preforming as the process of choice for high volume SLCM, including less waste, lower cost of glass fibre, and a broader selection of rovings for preform design and performance. Recent studies by PPG focused on the influence of roving parameters and other process variables on t h e p r o p e r t i e s of composites made with fibre-directed preforms in thermoset polyurethane and vinyl ester resins. Also studied was the performance of bumper beams produced by SRIM a n d reinforced with p r e f o r m s m a d e of continuous strand mat and chopped fibre. A new roving for surface veiling is now available, along with the DFP process for applying it.
Taguchi experiments Experiments to determine the relationships and effects of five preform variables on the mechanical properties of composites were designed using the Taguchi m e t h o d , a technique for designing experiments that generates a m a x i m u m a m o u n t of useful information with a minimum number of experiments. Once the relationships and effects were understood more clearly, composites could be designed to meet specifica-
0034-3617/91/$3.50 © 1991, Elsevier Science Publishers Ltd.
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FIGURE 1: In the first step in making a driected-
FIGURE 2: After cooling, the preform is
fibre preform, preform roving is chopped and conveyed onto a rotating screen. Simultaneously, preform binder is applied.
removed from the screen. This preform includes a layer of SprayVeil roving.
tions for t h e final p a r t m o r e precisely. The v a r i a b l e s i n c l u d e d f o u r different p r e f o r m binders, all s o l i d s - i n - w a t e r e m u l s i o n s : a polyvinyl a c e t a t e h o m o p o l y m e r , t h e r m o s e t polyester, e p o x y novolac, a n d a p o l y u r e t h a n e c o m p o s i t i o n . Two levels of b i n d e r c o n t e n t (inclusive of a b o u t 1.2% f i l a m e n t sizing) w e r e tested: a p p r o x i m a t e l y 8 a n d 14 wt%. Ms() t e s t e d w e r e two f i l a m e n t d i a m e t e r s of 9.9 a n d 13.9 btm, two s t r a n d b u n d l e sizes of 200 a n d 400 f i l a m e n t s p e r s t r a n d , a n d two s t r a n d c h o p l e n g t h s of 5 a n d 12.7 c m (2 a n d 5 in). F i l a m e n t sizing w a s h e l d c o n s t a n t . T h e d i r e c t e d - f i b r e fiat p r e f o r m s w e r e 46 x 71 cm (18 x 28 in) with a d e n s i t y of 2.3 kg/ m e (7.5 oz/fte). P r e f o r m s w e r e m a d e on a f o u r - s t a t i o n m a c h i n e in t h r e e steps: - - m u l t i - e n d p r e l b r m roving w a s c h o p p e d a n d conveyed t h r o u g h a d i r e c t i n g t u b e to a p e r f o r a t e d screen s h a p e d to t h e p a r t to be moulded. Simultaneously, preform binder was applied. Mr drawn through t h e r o t a t i n g screen h e l d t h e glass fibres in place. (Figure 1); -- the p r e f o r m w a s i n d e x e d 9ff ~ on its 'ferris wheel' into t h e c o n v e c t i o n oven. H e a t e d air w a s d r a w n t h r o u g h t h e p r e f o r m to c o m p l e t e t h e drying a n d set t h e binder; - - t h e wheel w a s r o t a t e d a n o t h e r 90 ~ for cooling, and the completed preform remove(t f r o m the screen. (Figure 2).
The finished p r e f o r m s w e r e m o u l d e d into 3.2 m m (0.125 in) t h i c k t e s t p a n e l s u s i n g a B a t t e n f e l d RIM u n i t a n d s t r u c t u r a l RIM u r e t h a n e resin. I n j e c t i o n r a t e w a s a b o u t 0.5 k g / s (1 l b / s ) . The flat p l a q u e steel t e s t p a n e l m o u l d w i t h s h e e r e d g e s for a m e t a l - t o m e t a l seal w a s closed d u r i n g injection. Glass c o n t e n t w a s held c o n s t a n t in t h e region 40% by weight. The t e s t s d e m o n s t r a t e d that: s t r a n d b u n die size, i.e. t h e n u m b e r of f i l a m e n t s p e r s t r a n d , s i g n i f i c a n t l y a f f e c t s Lensile a n d f l e x u r a l p r o p e r t i e s of t h e c o m p o s i t e , b u t d o e s n o t affect i m p a c t p r o p e r t i e s . On t h e o t h e r h a n d , with t h e e x c e p t i o n of n o t c h e d I z o d i m p a c t , t h e r e w e r e no s i g n i f i c a n t differences b e t w e e n t h e a v e r a g e m e c h a n i c a l p r o p e r t i e s o b t a i n e d w i t h t h e two f i l a m e n t diameters. Tests also s h o w e d t h a t , a g a i n with t h e e x c e p t i o n of n o t c h e d Izod, s t r a n d c h o p length (within t h e r a n g e i n v e s t i g a t e d a n d w i d t h l i m i t a t i o n s of t h e t e s t s p e c i m e n s ) d o e s not affect p r o p e r t i e s of t h e c o m p o s i t e . But s t r a n d c h o p length can affecl: p r o c e s s i n g efficiency a n d costs. As c h o p l e n g t h increases, w a s t e g e n e r a t e d d u r i n g p r e f o r m i n g d e c r e a s e s a n d resin f l o w - t h r o u g h d u r i n g m o u l d i n g is i m p r o v e d . Lower p r e f o r m b i n d e r c o n t e n t s i n c r e a s e c o m p o s i t e tensile a n d flexural p r o p e r t i e s .
REINFORCED PLASTICS APRIL 1991
PECIAL FEATURE
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TABLE 1: Results of confirmation experiments.
Property
Tensile (MPa) strength (psi)
Average 1
Predicted 2
Actual 2
% Increase
Predicted 3
Actual 3
% Increase
167 24 160
193 29 920
202 29 290
+21
203 29 470
212 30 800
+27
% retention4
Flexural (MPa) strength (psi)
77
252 36 520
308 44 710
% retention4
318 46 100
84
+26
259 37 5505
72
Flexural (GPa) strength (Mpsi)
9.4 1.364
11.2 1.631
10.5 1.518
% retention4
305 44 260
+21
82
+11
9.7 1.404
79
10.6 1.540
+13
84
(1) Average property number from the eight experiments in the experimental design. (2) Predicted and actual properties using a polyvinyl acetate homopolymer binder, K(13.9 p.m) filament diameter, 200 filaments/strand, 12.7 cm chop length, 8% binder content. (3) Predicted and actual properties using epoxy novolac binder, K(139 I~m) filament diameter, 200 filaments/strand, 12.7 cm chop length, 8% binder content. (4) % of initial property retained after 48 hour water boil. (5) Anomalus data reflecting the low (38.5%) glass content in run No. 5.
Although lower binder contents may produce a preform t h a t is less resistant to fibre wash, longer strand chop lengths of 7.6 to 12.7 cm TABLE 2: Mechanical (3 to 5 in) will c o m p e n s a t e with their properties of fibre g r e a t e r n u m b e r of o v e r l a p p i n g strands. directed preforms and Binder types gave less dramatic results with continuous strand mat polyvinyl a c e t a t e p r o v i d i n g t h e h i g h e s t in a vinyl ester resin 1 at c o m p o s i t e flexural properties, while the 50% glass. epoxy novolac and u r e t h a n e binders were Mechanical property
Chopped fibre preforms PPG 5540 PPG 5542
Continuous strand mat Nonthermoformable Thermoformable
Tensile (MPa) strength (psi) % retention2
187 27 140 71.9
174 25 230 92.4
183 26 560 65.5
221 32 050 70.0
Flexural (MPa) strength (psi) % retention
357 51 810 81.3
293 42 520 82.8
312 45 310 50.1
334 48 470 71.9
Flexural (GPa) modulus(Mpsi) % retention
12.1 1.75 93.9
10.6 1.54 87.5
11.5 1.67 69.7
12.1 1.76 96.9
Notched (J/m) Izod (ft-lb/in) impact
939 17.6
1308 24.5
902 16.9
966 18.1
Instrumented impact
Max load (N) (Ibf)
5111 1149
5026 1130
4070 915
4341 976
Max (J/m) energy (ft-lb/in)
3694 69.2
4030 75.5
3769 70.6
3459 64.8
Total (J/m) energy (ft-lb/in)
11 695 219.1
10 585 198.3
10 831 202.9
11 354 212.7
(1) Dow Derakane 411-C50 (2) Property retention after a 24 hour water boil
I.
REINFORCED PLASTICS APRIL 1991
superior in i nst rum ent ed impact and retention of initial properties after a 48 h o u r water boil. In summary, s t r a n d b u n d l e size a n d binder content influence most strongly the initial tensile and flexural properties of the composite, while strand chop length, filament diameter and binder type tend to affect impact properties. The combination of preform characteristics for opt i m um initial and aged mechanical properties of the composite were predicted to be the polyvinyl acetate and epoxy novolac TABLE 3: Mechanical properties of combination chopped fibre preform and unidirectional reinforcement in a vinyl ester at 45% glass 1' Mechanical property s
Construction A 3
Construction B 4
276 40 040
198 28 710
Flexural (MPa) strength (psi)
276 40 020
256 37 200
Flexural (GPa) modulus (Mpsi)
9.9 1.44
9.4 1.36
Notched (J/M) Izod (ft-lb/in) impact
2557 47.9
1762 33.0
Tensile (MPa) strength (psi)
(1) PPG 5540 preform roving. (2) Dow Derakane 411-C50. (3) 30% unidirectional roving symmetrically sandwiched by chopped roving. (4) 10% unidirectional roving symmetically sandwiched by chopped roving. (5) Properties were measured parallel to the directional reinforcement.
PECIAL
S or polyurethane binders, respectively, at 7-9% by weight. Also predicted as o p t i m u m were K (13.9 ~m) filament diameter, 200 filaments p e r strand, and any strand chop length or combination of lengths over 5 cm (2 in). Tensile and flexural strengths, and flexural moduli of composites m a de with o p t i m u m preforms were predicted and measured both before and after aging by boiling in water. Results, which show improvements to mechanical properties, confirmed predictions very closely ex cep t when a processing anomaly lowered the glass content to about 38.5%. (Table 1).
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forced with continuous strand m at retained only 71%. At 35% glass, the difference widened to 9,1 and 73% respectively. Specific results are depicted in Table 2. Table 3 shows the increased directed properties obtained by including continuous unidirectional roving in chopped fibre preforms.
Comparing chopped and c o n t i n u o u s stand reinforcements
FIGURE 3: Bumper beams are ideal applications for composites reinforced with fibre directed preforms.
B u m p e r beams: an ideal a p p l i c a t i o n
Recent e x p e r i m e n t s c o m p a r e d the mechanical properties of composites made with two types each of c h o p p e d and c o n t i n u o u s strand glass fibre flat preforms or mats in a vinyl ester matrix. The c h o p p e d fibres were fr o m e i t h e r 5540 p r e f o r m roving (200 f i l a m e n t s / s t r a n d ) or 5542 roving (400 f i l a m e n t s / s t r a n d ) . Both are made by PPG and have the same sizing chemistry as t h a t u s e d in t h e T a g u c h i e x p e r i m e n t s . The continuous strand mats were thermoformable and non-thermoformable. Conclusions are t h a t composites reinforced with directed fibre preforms demonstrate mechanical properties a p p r o x i m a t e l y equal to composites reinforced with continuous strand mat, and t h a t after aging in boiling water are greater t ha n composites made with continuous strand mat. Those r e i n f o r c e d with c h o p p e d fibre p r e f o r m s retained an average 85% of their mechanical properties. In contrast, c o m p o s i t e s rein-
A commercial b u m p e r beam (Figure 3) was moulded by Dow Chemical USA using an SRIM polyisocyanurate resin and directedfibre preforms made by PPG. Because of both the thickness and length of the b u m p e r beam (which exceeded the diameter of the preform t urnt abl e), the preform was made in two sets of two pieces each. The right and leJ~ hand sets had t apered overlaps in the middle of some 12.7 cm (5 in). Specifications for glass loadings of up to 40% by weight were met by nesting inside and outside preforms. Bumpers reinforced with 34 to 40% by weight of either continuous strand mat of chopped directed-fibre preforms passed centre and corner impact tests. In both areas, d e f l e c t i o n of t h e b u m p e r s w i t h directed-fibre preforms was slightly greater. At glass contents in the 30% range, those with continuous strand mat cracked during the tests, while bum pers m ade with directed-
Impact at 8kmlh (5 milh)
Impact at 4.8 kmlh (3 milh)
Fibre glass preform
% glass
Centre medium newton (Ib-f)
Deflection cm (inch)
Comments
Corner medium Deflection, newton (Ib-l) cm (inch)
Thermoformable continuous strand mat
33-35
45 283 (10 180)
6.20 (2.44)
No damage
21 885 (4920)
4.39 (1 73)
Chopped strand, directed fibre
38-39
40 123 (9020)
6.40 (2.52)
No damage
18 994 (4270)
4.57 (1 80)
Thermoformable continuous strand mat
31-34
37 721 (8480)
8.41 (331)
20 cm (8 in)
Chopped strand, directed fibre
28-30
39 767 (8940)
6.76 (2.66)
No damage
19 261 (4330)
4.32 (1 70)
R E I N F O R C E D PLASTICS APRIL 1991
TABLE 4: Bumper beam performance test results.
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FIGURE 4: Preforms can be veiled with SprayVeil roving by PPG, extending the directed fibre preform process.
FIGURE 5: SprayVeil roving is applied using a modified chopper~binder sprayer.
fibre preforms passed. Results of the tests are detailed in Table 4.
for the process of using SprayVeil roving (Figure 5). The chopped, filamentized veiling layer is typically 0.5 mm (20 mils) thick. The surfaced preform is then dried and cooled in the same manner as an ordinary preform (steps 2 and 3). Together, the directed-fibre preform and process for using SprayVeil roving produce surfaces of parts t h a t are as smooth and resin rich as surfaces veiled with conventional veil mat. Production costs are lower and long term quality is higher. The labour and waste material associated with cutting and fitting m a t are avoided, as is the occasional print-through of tucks and folds to the surface of the moulded part. Surface roughness of panels having 5540 preform reinforcement was measured in microinches with the Feinpruf M4-P perthometer; lower numbers indicate smoother surfaces. Details are presented in Table 5.
New veiling and veiling process The directed-fibre process for making preforms can be extended to include a veiling layer of SprayVeil roving by PPG, a single strand of glass fibre coated with a proprietary sizing system (Figure 4). The sizing system and the process for using SprayVeil roving encourage the filaments to separate and disperse evenly on the preform. Once the prescribed a m o u n t of structural glass has been deposited on the preform (step 1 in the process for making directedfibre preforms), SprayVeil roving is applied using a second glass chopper/binder sprayer and directing tube t h a t has been modified
TABLE 5: Surface roughness of panels having 5540 preform reinforcement.
Surface roughness of 5540 DFPISLCM panels, microinches Matrix
Glass fibre content % weight
W/SprayVeil roving
w/o SprayVeil roving
Polyester
35
461
1O0
Polyester low profile SMC resin with equal amount of CaCQ3 filler
30
8
52
Polyurethane
40
15
66
(1) Compares favourably with 41 microinches for veil mat. (2) Matched die compression moulded for comparison.
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REINFORCED PLASTICS APRIL 1991
Continuing developments and applications The family of processes t h a t comprises structural liquid composite moulding has evolved with the development of moulding and preforming processes, machinery, tooling, resins and glass fibre reinforcements specifically designed for the processes. Fulfilling the promise of rapid growth depends on further development of these system components, especially automated
PECIAL FEATURE
S preform manufacture. All must work in concert to reduce costs, raise productivity, and produce parts that meet specifications. Only then can SLCM realize its full potential.
References (1). J.F. Dockum and P.L. Schell 'Fibredirected preform reinforcement: factors that may influence mechanical p r o p e r t i e s in liquid c o m p o s i t e moulding', Proceedings from the Sixth Annual ASM/ESD Advanced Composites Conference and Exposition, 8-11 October, 1990. (2). E.P. Carley et al, 'Preforming for liquid
composite moulding', Society of Automotive Engineers, International Congress and Exposition, 26 February-2 March, 1990. (3) E.P. Carley et al, 'Preforming for liquid composite moulding', 45th Annual Conference, Composites Institute, the Society of the Plastics Industry Inc, 12-15 February, 1990. (4) E.P. Carley et al, 'Preforming for liquid composite moulding', Processing'. from the Fifth Annual ASM/ESD Advanced Composites Conference, 25-28 September, 1989. (5) E.P. Carley et al, 'Preforming for liquid composite moulding', 44th Annual Confer'ence, Composites Institute, the Society of the Plastics Industry Inc, 6-9 Februa .ry, 1989. m
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REINFORCED PLASTICS APRIL 1991
:!
PECIAL FEATURE
m Preform r e i n f o r c e m e n t s help spur growth o f SLCM If s t r u c t u r a l l i q u i d c o m p o s i t e m o u l d i n g t e c h n o l o g i e s s u c h as SRIM and RTM are to fulfil their p r o m i s e o f rapid growth further d e v e l o p m e n t must take place in system c o m p o n e n t s such as automated preform manufacture, says PPG Industries. J o h n F. Dockum Jr o f PPG in Pittsburgh, Pennsylvania, USA, and Roger A. Goodwill, w h o is based at the company's Wigan, UK, plant l o o k at the i n f l u e n c e o f roving parameters and p r o c e s s variables on c o m p o s i t e s made w i t h fibre directed preforms.
S
tructural liquid composite moulding (SLCM) encompasses a family of moulding processes: liquid composite compression moulding (LCCM), structural resin t r a n s f e r (or injection) m o u l d i n g (SRTM), and structural reaction injection moulding (SRIM). In all the processes a reinforcement preform is positioned in the mould prior to combining with resin. High production rates, lower costs and consistently high mechanical properties over long periods will spur growth of structural liquid composite moulding, principally SRTM and SRIM. Some estimates place annual growth rates at between 35 and 45% during the next five to ten years. Among all the fabrication methods and material systems available for making polymer m a t r i x composites, SLCM is m o s t p r o m i s i n g for r e p l a c i n g s t a m p e d steel assemblies. The reason: moulders can produce complex, structural, three-dimensional shapes economically and at high production rates using preforms tailored precisely to the shape of the part. Reinforcements are not
REINFORCED PLASTICS APRIL 1991
grossly dislodged when moulded with lowviscosity resin, as they are during flow in compression, injection or transfer moulding. As a result, the finished composite will carry the loads for which it is designed. Glass fibre preforms can be prepared manually or thermoformed from continuous strand mat. They can also be built-up to shape by directing chopped multi-.strand roving onto a rotating screen, a process called directed fibre preforming (DFP). Regardless of method, orientation of fibre is isotropic (random), and, assuming t h a t the fibres are sized properly, initial strengths are similar. Other considerations, however, point to directed fibre preforming as the process of choice for high volume SLCM, including less waste, lower cost of glass fibre, and a broader selection of rovings for preform design and performance. Recent studies by PPG focused on the influence of roving parameters and other process variables on t h e p r o p e r t i e s of composites made with fibre-directed preforms in thermoset polyurethane and vinyl ester resins. Also studied was the performance of bumper beams produced by SRIM a n d reinforced with p r e f o r m s m a d e of continuous strand mat and chopped fibre. A new roving for surface veiling is now available, along with the DFP process for applying it.
Taguchi experiments Experiments to determine the relationships and effects of five preform variables on the mechanical properties of composites were designed using the Taguchi m e t h o d , a technique for designing experiments that generates a m a x i m u m a m o u n t of useful information with a minimum number of experiments. Once the relationships and effects were understood more clearly, composites could be designed to meet specifica-
0034-3617/91/$3.50 © 1991, Elsevier Science Publishers Ltd.
S
FIGURE 1: In the first step in making a driected-
FIGURE 2: After cooling, the preform is
fibre preform, preform roving is chopped and conveyed onto a rotating screen. Simultaneously, preform binder is applied.
removed from the screen. This preform includes a layer of SprayVeil roving.
tions for t h e final p a r t m o r e precisely. The v a r i a b l e s i n c l u d e d f o u r different p r e f o r m binders, all s o l i d s - i n - w a t e r e m u l s i o n s : a polyvinyl a c e t a t e h o m o p o l y m e r , t h e r m o s e t polyester, e p o x y novolac, a n d a p o l y u r e t h a n e c o m p o s i t i o n . Two levels of b i n d e r c o n t e n t (inclusive of a b o u t 1.2% f i l a m e n t sizing) w e r e tested: a p p r o x i m a t e l y 8 a n d 14 wt%. Ms() t e s t e d w e r e two f i l a m e n t d i a m e t e r s of 9.9 a n d 13.9 btm, two s t r a n d b u n d l e sizes of 200 a n d 400 f i l a m e n t s p e r s t r a n d , a n d two s t r a n d c h o p l e n g t h s of 5 a n d 12.7 c m (2 a n d 5 in). F i l a m e n t sizing w a s h e l d c o n s t a n t . T h e d i r e c t e d - f i b r e fiat p r e f o r m s w e r e 46 x 71 cm (18 x 28 in) with a d e n s i t y of 2.3 kg/ m e (7.5 oz/fte). P r e f o r m s w e r e m a d e on a f o u r - s t a t i o n m a c h i n e in t h r e e steps: - - m u l t i - e n d p r e l b r m roving w a s c h o p p e d a n d conveyed t h r o u g h a d i r e c t i n g t u b e to a p e r f o r a t e d screen s h a p e d to t h e p a r t to be moulded. Simultaneously, preform binder was applied. Mr drawn through t h e r o t a t i n g screen h e l d t h e glass fibres in place. (Figure 1); -- the p r e f o r m w a s i n d e x e d 9ff ~ on its 'ferris wheel' into t h e c o n v e c t i o n oven. H e a t e d air w a s d r a w n t h r o u g h t h e p r e f o r m to c o m p l e t e t h e drying a n d set t h e binder; - - t h e wheel w a s r o t a t e d a n o t h e r 90 ~ for cooling, and the completed preform remove(t f r o m the screen. (Figure 2).
The finished p r e f o r m s w e r e m o u l d e d into 3.2 m m (0.125 in) t h i c k t e s t p a n e l s u s i n g a B a t t e n f e l d RIM u n i t a n d s t r u c t u r a l RIM u r e t h a n e resin. I n j e c t i o n r a t e w a s a b o u t 0.5 k g / s (1 l b / s ) . The flat p l a q u e steel t e s t p a n e l m o u l d w i t h s h e e r e d g e s for a m e t a l - t o m e t a l seal w a s closed d u r i n g injection. Glass c o n t e n t w a s held c o n s t a n t in t h e region 40% by weight. The t e s t s d e m o n s t r a t e d that: s t r a n d b u n die size, i.e. t h e n u m b e r of f i l a m e n t s p e r s t r a n d , s i g n i f i c a n t l y a f f e c t s Lensile a n d f l e x u r a l p r o p e r t i e s of t h e c o m p o s i t e , b u t d o e s n o t affect i m p a c t p r o p e r t i e s . On t h e o t h e r h a n d , with t h e e x c e p t i o n of n o t c h e d I z o d i m p a c t , t h e r e w e r e no s i g n i f i c a n t differences b e t w e e n t h e a v e r a g e m e c h a n i c a l p r o p e r t i e s o b t a i n e d w i t h t h e two f i l a m e n t diameters. Tests also s h o w e d t h a t , a g a i n with t h e e x c e p t i o n of n o t c h e d Izod, s t r a n d c h o p length (within t h e r a n g e i n v e s t i g a t e d a n d w i d t h l i m i t a t i o n s of t h e t e s t s p e c i m e n s ) d o e s not affect p r o p e r t i e s of t h e c o m p o s i t e . But s t r a n d c h o p length can affecl: p r o c e s s i n g efficiency a n d costs. As c h o p l e n g t h increases, w a s t e g e n e r a t e d d u r i n g p r e f o r m i n g d e c r e a s e s a n d resin f l o w - t h r o u g h d u r i n g m o u l d i n g is i m p r o v e d . Lower p r e f o r m b i n d e r c o n t e n t s i n c r e a s e c o m p o s i t e tensile a n d flexural p r o p e r t i e s .
REINFORCED PLASTICS APRIL 1991
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TABLE 1: Results of confirmation experiments.
Property
Tensile (MPa) strength (psi)
Average 1
Predicted 2
Actual 2
% Increase
Predicted 3
Actual 3
% Increase
167 24 160
193 29 920
202 29 290
+21
203 29 470
212 30 800
+27
% retention4
Flexural (MPa) strength (psi)
77
252 36 520
308 44 710
% retention4
318 46 100
84
+26
259 37 5505
72
Flexural (GPa) strength (Mpsi)
9.4 1.364
11.2 1.631
10.5 1.518
% retention4
305 44 260
+21
82
+11
9.7 1.404
79
10.6 1.540
+13
84
(1) Average property number from the eight experiments in the experimental design. (2) Predicted and actual properties using a polyvinyl acetate homopolymer binder, K(13.9 p.m) filament diameter, 200 filaments/strand, 12.7 cm chop length, 8% binder content. (3) Predicted and actual properties using epoxy novolac binder, K(139 I~m) filament diameter, 200 filaments/strand, 12.7 cm chop length, 8% binder content. (4) % of initial property retained after 48 hour water boil. (5) Anomalus data reflecting the low (38.5%) glass content in run No. 5.
Although lower binder contents may produce a preform t h a t is less resistant to fibre wash, longer strand chop lengths of 7.6 to 12.7 cm TABLE 2: Mechanical (3 to 5 in) will c o m p e n s a t e with their properties of fibre g r e a t e r n u m b e r of o v e r l a p p i n g strands. directed preforms and Binder types gave less dramatic results with continuous strand mat polyvinyl a c e t a t e p r o v i d i n g t h e h i g h e s t in a vinyl ester resin 1 at c o m p o s i t e flexural properties, while the 50% glass. epoxy novolac and u r e t h a n e binders were Mechanical property
Chopped fibre preforms PPG 5540 PPG 5542
Continuous strand mat Nonthermoformable Thermoformable
Tensile (MPa) strength (psi) % retention2
187 27 140 71.9
174 25 230 92.4
183 26 560 65.5
221 32 050 70.0
Flexural (MPa) strength (psi) % retention
357 51 810 81.3
293 42 520 82.8
312 45 310 50.1
334 48 470 71.9
Flexural (GPa) modulus(Mpsi) % retention
12.1 1.75 93.9
10.6 1.54 87.5
11.5 1.67 69.7
12.1 1.76 96.9
Notched (J/m) Izod (ft-lb/in) impact
939 17.6
1308 24.5
902 16.9
966 18.1
Instrumented impact
Max load (N) (Ibf)
5111 1149
5026 1130
4070 915
4341 976
Max (J/m) energy (ft-lb/in)
3694 69.2
4030 75.5
3769 70.6
3459 64.8
Total (J/m) energy (ft-lb/in)
11 695 219.1
10 585 198.3
10 831 202.9
11 354 212.7
(1) Dow Derakane 411-C50 (2) Property retention after a 24 hour water boil
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REINFORCED PLASTICS APRIL 1991
superior in i nst rum ent ed impact and retention of initial properties after a 48 h o u r water boil. In summary, s t r a n d b u n d l e size a n d binder content influence most strongly the initial tensile and flexural properties of the composite, while strand chop length, filament diameter and binder type tend to affect impact properties. The combination of preform characteristics for opt i m um initial and aged mechanical properties of the composite were predicted to be the polyvinyl acetate and epoxy novolac TABLE 3: Mechanical properties of combination chopped fibre preform and unidirectional reinforcement in a vinyl ester at 45% glass 1' Mechanical property s
Construction A 3
Construction B 4
276 40 040
198 28 710
Flexural (MPa) strength (psi)
276 40 020
256 37 200
Flexural (GPa) modulus (Mpsi)
9.9 1.44
9.4 1.36
Notched (J/M) Izod (ft-lb/in) impact
2557 47.9
1762 33.0
Tensile (MPa) strength (psi)
(1) PPG 5540 preform roving. (2) Dow Derakane 411-C50. (3) 30% unidirectional roving symmetrically sandwiched by chopped roving. (4) 10% unidirectional roving symmetically sandwiched by chopped roving. (5) Properties were measured parallel to the directional reinforcement.
PECIAL
S or polyurethane binders, respectively, at 7-9% by weight. Also predicted as o p t i m u m were K (13.9 ~m) filament diameter, 200 filaments p e r strand, and any strand chop length or combination of lengths over 5 cm (2 in). Tensile and flexural strengths, and flexural moduli of composites m a de with o p t i m u m preforms were predicted and measured both before and after aging by boiling in water. Results, which show improvements to mechanical properties, confirmed predictions very closely ex cep t when a processing anomaly lowered the glass content to about 38.5%. (Table 1).
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forced with continuous strand m at retained only 71%. At 35% glass, the difference widened to 9,1 and 73% respectively. Specific results are depicted in Table 2. Table 3 shows the increased directed properties obtained by including continuous unidirectional roving in chopped fibre preforms.
Comparing chopped and c o n t i n u o u s stand reinforcements
FIGURE 3: Bumper beams are ideal applications for composites reinforced with fibre directed preforms.
B u m p e r beams: an ideal a p p l i c a t i o n
Recent e x p e r i m e n t s c o m p a r e d the mechanical properties of composites made with two types each of c h o p p e d and c o n t i n u o u s strand glass fibre flat preforms or mats in a vinyl ester matrix. The c h o p p e d fibres were fr o m e i t h e r 5540 p r e f o r m roving (200 f i l a m e n t s / s t r a n d ) or 5542 roving (400 f i l a m e n t s / s t r a n d ) . Both are made by PPG and have the same sizing chemistry as t h a t u s e d in t h e T a g u c h i e x p e r i m e n t s . The continuous strand mats were thermoformable and non-thermoformable. Conclusions are t h a t composites reinforced with directed fibre preforms demonstrate mechanical properties a p p r o x i m a t e l y equal to composites reinforced with continuous strand mat, and t h a t after aging in boiling water are greater t ha n composites made with continuous strand mat. Those r e i n f o r c e d with c h o p p e d fibre p r e f o r m s retained an average 85% of their mechanical properties. In contrast, c o m p o s i t e s rein-
A commercial b u m p e r beam (Figure 3) was moulded by Dow Chemical USA using an SRIM polyisocyanurate resin and directedfibre preforms made by PPG. Because of both the thickness and length of the b u m p e r beam (which exceeded the diameter of the preform t urnt abl e), the preform was made in two sets of two pieces each. The right and leJ~ hand sets had t apered overlaps in the middle of some 12.7 cm (5 in). Specifications for glass loadings of up to 40% by weight were met by nesting inside and outside preforms. Bumpers reinforced with 34 to 40% by weight of either continuous strand mat of chopped directed-fibre preforms passed centre and corner impact tests. In both areas, d e f l e c t i o n of t h e b u m p e r s w i t h directed-fibre preforms was slightly greater. At glass contents in the 30% range, those with continuous strand mat cracked during the tests, while bum pers m ade with directed-
Impact at 8kmlh (5 milh)
Impact at 4.8 kmlh (3 milh)
Fibre glass preform
% glass
Centre medium newton (Ib-f)
Deflection cm (inch)
Comments
Corner medium Deflection, newton (Ib-l) cm (inch)
Thermoformable continuous strand mat
33-35
45 283 (10 180)
6.20 (2.44)
No damage
21 885 (4920)
4.39 (1 73)
Chopped strand, directed fibre
38-39
40 123 (9020)
6.40 (2.52)
No damage
18 994 (4270)
4.57 (1 80)
Thermoformable continuous strand mat
31-34
37 721 (8480)
8.41 (331)
20 cm (8 in)
Chopped strand, directed fibre
28-30
39 767 (8940)
6.76 (2.66)
No damage
19 261 (4330)
4.32 (1 70)
R E I N F O R C E D PLASTICS APRIL 1991
TABLE 4: Bumper beam performance test results.
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FIGURE 4: Preforms can be veiled with SprayVeil roving by PPG, extending the directed fibre preform process.
FIGURE 5: SprayVeil roving is applied using a modified chopper~binder sprayer.
fibre preforms passed. Results of the tests are detailed in Table 4.
for the process of using SprayVeil roving (Figure 5). The chopped, filamentized veiling layer is typically 0.5 mm (20 mils) thick. The surfaced preform is then dried and cooled in the same manner as an ordinary preform (steps 2 and 3). Together, the directed-fibre preform and process for using SprayVeil roving produce surfaces of parts t h a t are as smooth and resin rich as surfaces veiled with conventional veil mat. Production costs are lower and long term quality is higher. The labour and waste material associated with cutting and fitting m a t are avoided, as is the occasional print-through of tucks and folds to the surface of the moulded part. Surface roughness of panels having 5540 preform reinforcement was measured in microinches with the Feinpruf M4-P perthometer; lower numbers indicate smoother surfaces. Details are presented in Table 5.
New veiling and veiling process The directed-fibre process for making preforms can be extended to include a veiling layer of SprayVeil roving by PPG, a single strand of glass fibre coated with a proprietary sizing system (Figure 4). The sizing system and the process for using SprayVeil roving encourage the filaments to separate and disperse evenly on the preform. Once the prescribed a m o u n t of structural glass has been deposited on the preform (step 1 in the process for making directedfibre preforms), SprayVeil roving is applied using a second glass chopper/binder sprayer and directing tube t h a t has been modified
TABLE 5: Surface roughness of panels having 5540 preform reinforcement.
Surface roughness of 5540 DFPISLCM panels, microinches Matrix
Glass fibre content % weight
W/SprayVeil roving
w/o SprayVeil roving
Polyester
35
461
1O0
Polyester low profile SMC resin with equal amount of CaCQ3 filler
30
8
52
Polyurethane
40
15
66
(1) Compares favourably with 41 microinches for veil mat. (2) Matched die compression moulded for comparison.
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REINFORCED PLASTICS APRIL 1991
Continuing developments and applications The family of processes t h a t comprises structural liquid composite moulding has evolved with the development of moulding and preforming processes, machinery, tooling, resins and glass fibre reinforcements specifically designed for the processes. Fulfilling the promise of rapid growth depends on further development of these system components, especially automated
PECIAL FEATURE
S preform manufacture. All must work in concert to reduce costs, raise productivity, and produce parts that meet specifications. Only then can SLCM realize its full potential.
References (1). J.F. Dockum and P.L. Schell 'Fibredirected preform reinforcement: factors that may influence mechanical p r o p e r t i e s in liquid c o m p o s i t e moulding', Proceedings from the Sixth Annual ASM/ESD Advanced Composites Conference and Exposition, 8-11 October, 1990. (2). E.P. Carley et al, 'Preforming for liquid
composite moulding', Society of Automotive Engineers, International Congress and Exposition, 26 February-2 March, 1990. (3) E.P. Carley et al, 'Preforming for liquid composite moulding', 45th Annual Conference, Composites Institute, the Society of the Plastics Industry Inc, 12-15 February, 1990. (4) E.P. Carley et al, 'Preforming for liquid composite moulding', Processing'. from the Fifth Annual ASM/ESD Advanced Composites Conference, 25-28 September, 1989. (5) E.P. Carley et al, 'Preforming for liquid composite moulding', 44th Annual Confer'ence, Composites Institute, the Society of the Plastics Industry Inc, 6-9 Februa .ry, 1989. m
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