Processing-property relationships in compression molding of sheet molding compounds

Processing-Property Relationships in Compression Molding of Sheet Molding Compounds

SANG YEOL OH andCHANG DAE HAN

Department of Chemical Engineering Polytechnic Institute of New York

Brooklyn, New York 11201

An experimental investigation was conducted into establishing relationships between the processing variables and the mechanical properties of compression-molded parts of sheet molding compounds (SMC). Emphasis was placed on investi- gating the effects on the tensile properties, impact strength, and dynamic mechanical properties of composite specimens, of low-profile additives, and of treating glass fibers (for reinforcement) with sizing chemicals. The processing variables investigated were cure time, mold temperature, and mold pressure. It was found that: (1) An optimum cure time and mold temperature exist for achieving molded SMC composites of the greatest tensile and impact strengths; (2) Of the four different types of low-profile thermoplastic additives employed, the poly(viny1 acetate) modified with acrylic acid gives rise to molded SMC composites having the greatest ’ tensile and impact strengths; (3) An optimum cure time and mold temperature exist for achieving the highest glass-transition (Tg) of the low-profile additive; (4) The values of cure time and mold temperature that have yielded the greatest tensile and impact strengths also yield molded specimens having the highest Tg of the low-profile additive.

INTRODUCTION omposites of polymeric materials and glass fi- C bers are very common in the plastics industry.

The two primary reasons for using glass fibers in polymeric materials are: (1) to improve the mechanical/physical properties of the polymeric materials, e.g., tensile modulus, dimensional stability, fatigue endurance, deformation under load, hard- ness, abrasion resistance, and (2) to reduce the cost of production by replacing expensive resins with inexpensive glass fibers.

In place of metals, the automotive industry substitutes glass-fiber-reinforced polyester composites, and the aerospace industry substitutes graphite fiber-reinforced epoxy composites. One reason for this substitution is that the weight per unit volume of composite materials is quite low compared to that of metals. This has made considerable reduc- tions in the fuel consumption of automobiles and airplanes possible. Another reason is that composite materials are less expensive than metals.

A number of manufacturing processes have been developed to fabricate composite structures, such as injection molding, compression molding, transfer molding, oven/vacuum bag, filament winding, pul- trusion, and others (1, 2). It is a well-known fact that the nature of fabrication processes dictates the end-use properties of composite materials. For instance, depending on the specific applications of

molded parts, a variety of properties might be produced, especially with thermoset molding compounds. Needless to say, molding variables have a profound influence on the mechanical properties and shrinkage of thermoset molding compounds, especially unsaturated polyester. The mechanical properties of glass-fiber-reinforced molding compounds, in turn, depend, to a large extent, on the degree of fiber orientation produced in the molding operation, the ultimate size of fibers in the molded articles, and mold design.

Goettler (3, 4) has pointed out the importance of the degree of fiber orientation and the proper design of the mold’s runner in controlling the mechanical properties of molded parts. He reports that maximum tensile properties and the highest degree of longitudinal alignment of fibers were obtained by properly designing the mold’s runner to yield a converging flow, and that an improvement in the properties in the transverse direction was achieved by having a diverging flow at the mold’s gate. Other researchers (Fj-8) have also reported their investi- gations of fiber orientation in suspension.

In recent years, there have been many studies on fiber orientation during the molding of polyester molding compounds. Burns and Gandhi (9) found that the aspect ratio of the fibers was critical to the degree of alignment obtained in bulk molding compounds (BMC). Owen and Whybrew (7) also stud-

POLYMER COMPOSITES, JANUARY, 1985, Vol. 6, No. 1 13

S. Y. Oh and C. D. Han

ied the problem of fiber orientation in molding operations of BMC. And Owen et al. (8) conducted molding experiments using BMC, and attempted to correlate the degree of fiber orientation to the mechanical properties of the molded articles. Par- ker (10) considered the effects of mold geometry and gate design on anisotropy and fiber damage in BMC. Marker and Ford (11) and Lee et al. (12) studied the rheology and mold flow of sheet molding compound (SMC). Others (13-18) investigated the effects of material and processing variables on the mechanical and thermal properties of SMC composites.

Depending upon the application and the manufacturing process selected, a number of additives are employed to provide specific products or end- use properties. For instance, in the preparation of polyester molding compounds, such as BMC and SMC, suitable for hot-press matched molding (e.g., compression and transfer moldings), the additives commonly used include inert fillers, low-profile thermoplastic additives, viscosity thickener, and mold release agents. Each additive in a molding compound has a specific purpose or purposes. In- organic fillers are used for aiding the dimensional stability of the molded parts and serving as a heat sink to achieve better temperature control across a molded part during cure. They are also used to reduce the amount of resin to be used (hence the cost) and the overall shrinkage. Low-profile thermoplastic additives are used for achieving low Shrinkage in molded parts during cure and provid- ing a good surface appearance. Viscosity thickeners are used in order to increase the viscosity (say, up to one million poises) of fiber-reinforced polyester molding compounds suitable for hot-press matched molding operations.

Very recently we also carried out an experimental investigation on establishing processing-property relationships in SMC molding. In this paper, we shall report the highlights of our findings.

EXPERIMENTAL Materials

Eight different formulations of sheet molding compound (SMC-R2S) were supplied to us by Ow- ens-Corning Fiberglas Corporation. The SMC pastes were prepared specifically for our study, using different glass-fiber “types” and different thermoplastic low-profile additives. All eight SMC pastes were made from the same polyester resin, OCF P-340, and were made with the same basic formulation, as follows: 100 parts of polyester resin and thermoplastic additive; 180 parts of calcium carbonate as filler; 1.5 parts of t-butyl perbenzoate as initiator; 5.0 parts of magnesium hydroxide as viscosity thickener; 4.0 parts of zinc stearate as mold release. The OCF P-340 resin is a 1 to 1 propylene-maleate unsaturated polyester containing 35 weight-percent styrene monomer.

Since the primary interest of our study was to investigate the effect of low-profile additive “types” on the end-use properties, we investigated four

different types of low-profile additive with the following compositions: (1) 60 weight-percent of resin (OCF P-340) and 40 weight-percent of polymethyl methacrylate modified with acrylic acid (OCF P- 701); (2) 60 weight-percent of resin (OCF P-340) and 40 weight-percent of impact-grade polystyrene modified with acrylic acid (OCF E-573); (3) 60 weight-percent of resin (OCF P-340) and 40 weight-percent of polyvinyl acetate copolymer modified with acrylic acid (Union Carbide LP 40A); (4) 65 weight-percent of resin (OCF P-340), 22 weight-percent styrene monomer, and 13 weight- percent of polyethylene powder (Arco Super Dylan 640).

To each of the four resin/low-profile additive mixtures, 2.5 weight-percent of one-inch chopped E-glass fiber roving was added. In order to investigate the effect of the nature of sizing chemicals on the mechanical properties of molded parts, two different types of fiber glass were used in the present study: (1) OCF-9Sl treated with sizing chemicals which are relatively insoluble in styrene; (2) OCF-433 treated with sizing chemicals which have good solubility in styrene. The sizing chemicals in OCF-9Sl are expected to maintain the integrity of fiber bundles in the SMC, whereas the sizing chemicals in OCF-433 are expected to filamentize the fiber bundles in the SMC. Table 1 gives sample codes for the eight SMC-R2S formulations employed in the present investigation.

Molding Experiment We used a 250 ton compression/transfer molding

machine (Stokes Press Machine Company) to pre- pare specimens for the measurement of mechanical properties. Flat panels 2 mm by 88 mm by 228 mm were compression molded. In the molding operations, we varied the mold temperature, mold pressure, and cure time. Table 2 summarizes the molding conditions employed in the preparation of specimens. The cure time is defined as the time during which pressure is actually applied on the material. The mold temperature was measured by thermo- couples at the mold surface, and the mold pressure was calculated from measurements of the press ram force applied on the panel surface. Several specimens were prepared at each molding condition, and they were used later for measuring mechanical properties.

Table 1. Sample Codes of the SMC-R25 Formulations Employed.

Material

Sample Thermoplastic Code Resin Additive Glass Fiber

1 OCF P-340 2 OCF P-340 3 OCF P-340 4 OCF P-340

6 OCF P-340 7 OCF P-340 8 OCF P-340

5 OCF P-340

DYLAN 640 DYLAN 640 OCF P-701 OCF P-701 OCF E-573 OCF E-573 LP 40A LP 40A

OCF-433 OCF-951 OCF-433 OCF-951 OCF-433 OCF-951 OCF-433 OCF-951

14 POLYMER COMPOSITES, JANUARY, 7985, Vol. 6, No. 7

Processing-Property Relationships in Conzpression Molding of Sheet Molding Corn pounds

Table 2. Molding Conditions Employed in the Preparation of SMC-R25 Composite Specimens.

Mold Pressure Mold Temperature Cure Time ( M W (OF) (min)

(a) Effect of Mold Pressure 2.50 290 3 4.37 290 3 6.24 290 3 8.43 290 3 9.36 290 3

(b) Effect of Mold Temperature 6.24 270 3 6.24 290 3 6.24 31 0 3 6.24 330 3 6.24 350 3

(c) Effect of Cure Time 6.24 290 2 6.24 290 5 6.24 290 8

Mechanical-Properties Measurement Both tensile and impact properties were mea-

sured. The ultimate tensile strength, the tensile modulus, and the percent elongation at break of molded specimens were measured at room temperature, by following the procedure described in ASTM D638. The Izod impact strength was deter- mined on notched specimens, by following the procedure described in ASTM D2.56. Five sets of measurements were made on molded specimens obtained under identical processing conditions, and the average values of the five sets were used as the final result. Specimens were prepared first by slic- ing the panels to the proper size with a diamond blade cutter (Model 41AR, Felker Operations, Dresser Industries, Inc.) and, then, by grinding the sliced panels to dogbone shape with a grinder (Vacu-Router, Shyodu Instrument Co.).

Dynamic Mechanical Properties Measurement A Du Pont 1090 thermal analyzer equipped with

a dynamic mechanical analyzer (DMA) module was employed to measure the dynamic moduli (E’ and E”) and loss tangent (tan 6) as functions of temperature. The values of Young’s modulus E’ and tan 6 were calculated from the values of resonance fre- quency, following the procedure described in the Operating Manual of the instrument.

RESULTS AND DISCUSSION Tensile Properties

Figure 1 displays the tensile properties of eight SMC composites, molded under identical processing conditions, namely at the same values of mold temperature, mold pressure, and cure time. The following observations are worth noting: (1) Of the eight samples, the composite containing LP 40A and OCF-433 (Sample #7) has the highest tensile strength and tensile modulus; ( 2 ) The composites containing OCF-433 have higher values of tensile strength and tensile modulus than those containing

100 T

90 c 4

60 I I 1 I I I I 1 I

-- I I g 4.5

3.5

3.0 u) c

P P P P P P

P P

2.5 I I I I 1 I I I I 1 - 71 1 Z I I

P 4

P Q P P P P

3 L 1 I I I I I I I I 1 2 3 4 5 6 7 8

Sample Code

Fig. I . Tensile properties of the compression-molded specimens of SMC-R25. Formulations of the samples are given in Table 1 . Molding conditions employed are: mold temperature, 290 O F ; cure time, 3 min; mold pressure, 6.24 MPa (905 psi).

OCF-9.51. This seems to indicate that the sizing chemicals that are more soluble in styrene are more effective in transferring stress from the resin matrix to the glass fibers than those that are relatively insoluble. This is understandable, because the sizing chemicals that are readily soluble in styrene would filamentize the fiber bundles in the SMC and, thus, disperse the individual fibers uniformly in the resin matrix. Consequently, better interfacial adhesion properties between the resin and the fiber can be achieved, giving rise to higher tensile properties; ( 3 ) Of the composites containing the glass fiber OCF-433, the three different types of low- profile additive, Dylan 640, OCF P-701, and OCF E-.573, give rise to almost the same value of tensile strength and tensile modulus; (4) Of the eight samples, the composite containing Dylan 640 (Sample #1) has the highest value of percent elongation at break; (S) In all cases, the five sets of measurements made on specimens molded under the same conditions exhibited relatively large variations. This may be attributable, among many factors, to variations in glass fiber content and fiber orientation. Earlier, Smith and Jutte (16) discussed the origin of such large variations in the mechanical properties of SMC molded parts.

Figure 2 describes the effect of cure time, Fig. 3 the effect of mold temperature, and Fig. 4 the effect of mold pressure, on the tensile properties of the SMC composite containing LP 40A (as low-profile additive) and OCF-95 1 (as reinforcement).

POLYMER COMPOSITES, JANUARY, 1985, Vol. 6, No. 7 15

S. Y. Oh and C. D. Hun

- 1001 1

$ 6r I

: ' 0 3 m 4 p 7 7

; 2 0 1 2 3 4 5 6 7 8 9

Cure Tima(rnin1

Fig. 2. Effect of cure time on the tensile properties of Sample *8. Other molding conditions employed are: mold temperature, 290°F; mold pressure, 6.24 MPa.

901 T

70 8ob T& 60 1 " so Illlrllllllll

61 I

5 I- .t - P - 3

2 I I I I l I I I I I I 250 270 290 310 330 350 370

Mold Temperotura [OF)

Fig. 3. Effect of mold temperature on the tensile properties of Sample #8. Other molding conditions employed are: cure time, 3 min; mold pressure, 6.24 MPa.

It is seen in Fig. 2 that the tensile strength goes through a maximum at a cure time of 3 min. The observed low values of tensile strength at cure times shorter than 3 min. may be atrributable to under- curing, and at cure times longer than 3 min. to overcuring of the material. An excessively long curing time may bring about thermal degradation of the crosslinked material, yielding a decrease in

Mold Prersure(MFu)

Fig. 4 . Effect of mold pressure on the tensile properties of Sample #8. Other molding conditions employed are: mold temperature, 290°F; cure time, 3 min.

mechanical properties. Similar comments may be made about the tensile modulus observed in Fig. 2.

It is seen in Fig. 3 that a maximum value of tensile strength (and, also, tensile modulus) is obtained at mold temperatures i n the vicinity of 300°F. The observed low values in tensile strength and tensile modulus at mold temperatures higher than 300°F may be attributable to the thermal degradation of the crosslinked matrix resin. It is of interest to note that the effect of mold temperature on tensile strength and tensile modulus is very similar to that of cure time.

An increase in tensile strength is observed with increasing mold pressure, as shown in Fig. 4. This is attributable to an increase in bonding between the glass fibers and the matrix resin, as the mold pressure is increased. The increased bonding, especially in the presence of effective sizing chemicals, is expected to transfer stress effectively from the weak matrix resin to the strong glass fibers. It is of interest to note in Fig. 4 that the tensile properties go through a maximum as the mold pressure is increased.

At this juncture it should be mentioned that earlier, Tung (18) reported the effects of cure time, mold temperature, and mold pressure on the tensile properties of SMC composites. Our results, as reported above, are in general agreement with those reported by Tung.

Impact Strength Figure 5 displays the impact strength of eight

SMC composites, molded under identical processing conditions. It is seen that, of the eight samples, the composite containing LP 40A and OCF-433 (Sample #7) has the highest impact strength. It

16 POLYMER COMPOSITES, JANUARY, 1985, Vol. 6, No. 1


- c 0 C 14 e

?: I% c e 10 12

- 8 1 2 3 4 5 6 7 8 9

Sample Code Fig. 5. Zmpact strength of the compression-molded specimens of SMC-R25. Formulations of the samples are given in Table 1. Molding conditions employed are: mold temperature, 290 O F ; cure time, 3 min, mold pressure, 6.24 MPa.

::y-+- I 1 I I I I I I

the highest tensile strength and tensile modulus (see Fig. 1) of the eight SMC composites investigated.

Figure 6 describes the effect of cure time, Fig. 7 the effect of mold temperature, and Fig. 8 the effect of mold pressure, on the impact strength of the SMC composite containing LP 40A (as low-profile additive) and OCF-951 (as reinforcement). It is seen that a maximum impact strength is achieved at a cure time of 3 min. and at a mold temperature of about 320°F. On the other hand, the impact strength is seen to increase with mold pressure, going through a maximum at the mold pressure, 8.5 MPa.

Dynamic Mechanical Properties Figure 9 displays the effects of various low-profile

thermoplastic additives on the loss modulus (E") of SMC composites, molded under identical processing conditions. Curve 1 in Fig. 9 represents the composite containing LP 40A as low-profile additive, which exhibits three transitions, namely -79"C, 72"C, and 122°C. Here the weak broad peak at 122°C represents the glass transition (T,) of the resin matrix; the peak at -79°C is the P-transition that is ascribable to the motion of the polyester segments between crosslinks. The strong peak at 72°C is attributable to the glass transition of the low-profile additive, polyvinyl acetate modified with acrylic acid. The large shift of Tg (72°C) of LP 40A from that (46°C) of polyvinyl acetate (PVAc) itself may be attributable to the partial crosslinking of the modified PVAc to the matrix resin (i.e., hydrogen abstraction and grafting of the styrene- polyester).

Curve 2 in Fig. 9 represents the composite containing OCF P-701 as low-profile additive. This exhibits three transitions, namely -73°C (the p- transition of the matrix resin), 108°C (the glass transition of the matrix resin), and 136°C (the glass transition of the modified polymethyl methacrylate). It is seen that the Tg of the matrix resin is somewhat lowered from 122°C to 108"C, whereas the &transition is slightly increased from -79°C to - 7.3 c .

Curve 3 in Fig. 9 represents the composite containing OCF E-S73 as low-profile additive. Here the peak at -72°C represents the 0-transition and

22

b 18

? I% 14

8 12 -

10

i2::& +--i-= -

- I I 1 I I I I

Cure Time I minl

Fig. 6. Effect of cure time on the impact strength of Sample #8. Other molding conditions employed are: mold temperature, 290°F; mold pressure, 6.24 MPa.

10 I I I I I I I I I I 260 280 300 320 340 3E

Mold Temperoture (OF)

Fig. 7. Effect of mold temperature on the impact strength of Sample #8. Other molding conditions employed are: cure time, 3 min, mold pressure, 6.24 MPa.

122°C the Tg of the matrix resin. Several transition peaks are seen in the temperature range between 0°C and 100°C, which seems attributable to the nature of the particular low-profile thermoplastic additive, acrylic acid modified with impact-grade polystyrene.

Curve 4 in Fig. 9 represents the composite containing Dylan 640 as low-profile additive. Note that the ,&transition and Tg of the matrix resin occur at -71°C and 118"C, respectively, and that the T, of the low-profile additive, polyethylene, is not observed.

Figure 10 describes the effect of mold temperature on the loss modulus (E") of the composite containing LP 40A and OCF-9.51. It is seen that the specimen molded at 310°F (Curve 3) has the highest value of T,. This may be because the crosslinking reaction between the PVAc and the resin matrix is

POLYMER COMPOSITES, JANUARY, 7985, Vol. 6, No. I 17

S. Y. Oh and C. D. Han

*’ 0.31

n3

-120 -80 -40 0 40 80 120 160 200 240 280 320

Temperature (F)

Fig 9 Efiect oftxriozts lowprofile additrves on the loss motlulus of the coi~ipressio~~-i,loltletl speciinens of SMC-R2,5 ( 1 ) Sumple #7, (2) Suinpk #3, (3) Sainpk #5, (4) Sample #I. Formulations of the suniples are given i n Tuhle 1 Molding conditions employed urc. mold ternporature, 290°F, cure time, 3 min; mold prvssure, 6 24 MPa

0 . 7 t

0.1 t- -20 0 20 40 60 80 100 120 140 160 180 200

Temperature (Y)

Fig. 10. Loss modulus of the compression-molded specimens of Sample #8, prepared under various mold temperatures ( O F ) : ( 1 ) 270; (2) 290; (3) 310; (4) 330; (5) 350. Other molding conditions employed are: cure time, 3 min, mold pressure, 6.24 MPa.

most effective at 310”F, whereas higher mold temperatures might have caused thermal degradation, yielding a lower degree of crosslinking. It is of interest to note that the effect of mold temperature on the Tg of the SMC composite is very similar to that on the tensile strength of the molded parts (see Fig. 3).

Figure 11 describes the effect of cure time, and Fig. 12 the effect of mold pressure, on the E“ of the composite containing LP 40A and OCF-951. It is seen in Fig. 11 that the highest value of Tg of the PVAc phase is obtained at a cure time of 3 min. On the other hand, it is seen in Fig. 12 that the Tg of the PVAc phase is little affected by mold pressure.

It can be concluded from the observations made above that dynamic mechanical analysis is a useful tool for indirectly evaluating the extent of crosslinking between low-profile additives and matrix resins.

ACKNOWLEDGMENT We wish to acknowledge that the SMC-R2,5 ma-

terials used in the present study were supplied to

0. I

0 .o I I l I I l 1 1 1 1 1 -20 0 20 40 60 80 100 I20 140 I60 180 20(

Temperature (‘c) Fig. 11. Loss inoilulus (if the conipressioit-nitlde~l speciinens of S n n i p I ( ~ #8, prv~iurod under oarious cure times (min): ( 1 ) 2; (2) 3; (3) 5; ( 4 ) 8. Other molding conditions employed are: mold tein~ierattoa, 290 O F ; i n o l d pressure, 6.24 MPa.

0.6

0.5

‘ 0.4

0.3

0.2

2 ’W

0. I

0.0 1 1 1 1 1 1 1 1 1 1 1 -20 0 20 40 60 80 100 120 I40 160 180 20C

Temperature (E)

Fig. 12. Loss modulus of the compression-molded specimens of Sample #8, prepared under various mold pressures (MPa): ( 1 ) 2.50; (2) 4.37; (3) 6.24; (4) 7.49; (5) 8.43; (6) 9.36. Othermolding conditions employed are: mold temperature, 290°F; cure time, 3 min.

us by Owens-Corning Fiberglas Corporation. With- out the supply of the materials, this study would never have been possible. We are especially in- debted to Dr. Douglas L. Denton at Owens-Corning Fiberglas Corporation, who not only helped us in obtaining the test materials, but also guided us to obtain meaningful experimental data.

REFERENCES 1. J. G. Mohr, S. S. Oleesky, G. D. Shook, and L. S . Meyer,

“SPI Handbook of Technology and Engineering of Rein- forced Plastics/Composites,” 2nd Edition, Van Nostrand Reinhold, New York, (1973).

2. G. Lubin (ed.), “Handbook of Fiberglass and Advanced Plastics Composites,” Van Nostrand Reinhold, New York (1969).

3. L. A. Goettler, Mod. Plast., 47, 140 (1970). 4. L. A. Goettler, 25th Annu. Tech. Conf. SPI, Reinforced

Plast. Composites, Section 14-A, 1970. 5. J. P. Bell, J. Compos. Muter., 3, 244 (1969). 6. W-K. Lee and H. H. George, Polym. Eng. Sci., 18, 146

(1978). 7. M. J. Owen and K. Whybrew, Plastics and Rubber, 1, 231

(1976). 8 . M. J. Owen, D. M. Thomas, and M. S. Found, 33rd Annu.

Tech. Conf. SPI, Reinforced Plast. Composites, Section 20- B, 1978.

18 POLYMER COMPOSITES, JANUARY, 7985, Yo/. 6, No. 7


9. R. Burns and S. Gandhi, 32nd Annu. Tech. Conf. SPI,

10. F. J. Parker, 32nd Annu. Tech. Conf. SPI, Reinforced Plast.

11. L. Marker and B. Ford, 32nd Annu. Tech. Conf. SPI, Reinf.

12. L. I. Lee, L. Marker, and R. M. Griffith, Polqm. Ccompos.,

Reinforced Plast. Composites, Section 7-C, 1977.

Composites, Section 6-F, 1977.

Plast. Composites, Section 16-E, 1977.

2, iO9 (1981).

Reinf. Plast. Composites, Section 2-A, 1977. 13. G . S. Kobayashi and E. R. Pelton, 32nd Annu. Conf. SPI,

POLYMER COMPOSITES, JANUARY, 1985, Yo/. 6, No. 1

14. D. L. Denton, 36th Annu. Tech. Conf. SPI, Reinforced Plast.

15. D. L. Denton, 34th Annu. Tech. Conf. SPI, Reinforced Plast.

16. R. J. Smith and R. B. Jutte, 32nd Annu. Tech. Conf. SPI,

17. H. J. Boyd, 31st Annu. Tech. Conf. SPI, Reinforced Plast.

18. R. W. Tung, paper presented at the ASTM/SAE/ASCE

Composites, Section 16-A, 198 1.

Composites, Section 11-F, 1979.

Reinforced Plast. Composites, Section 7-E, 1977.

Composites, Section 2-C, 1976.

Symposium, Minneapolis, April 1980.

19

Documents

Processing-property relationships in compression molding of sheet molding compounds