Mica reinforced plastics: a review

Mica reinforced plastics: a review F. 14/.MAINE and P. D. SHEPHERD

Compared with such materials as glass flakes, clay, aluminium diboride and silicon carbide, mica offers the advantages of a relatively high modulus and low cost: this combination makes it attractive for use as a platelet-type reinforcement material. This paper considers the theoretical and practical aspects of mica-reinforced plastics materials. It is suggested that the most promising area for full utilization of the planar reinforcing properties of mica is in sheet materials, although other fabrication techniques are being investigated.

INTRODUCTION

Composite materials can be divided into three classes according to the-type of reinforcement or Idler used. These are: (1) composite materials in which the principle reinforcement is one-dimensional (eg fibres); (2) that in which the principle reinforcement is two-dimensional (eg flakes or platelets); and (3) that in which the discontinuous phase is three-dimensional (eg spheres or beads). Examples of platelet type materials are glass flakes, clay, aluminium diboride, silicon carbide platelets and mica. SiC and A1B2 platelets, although possessing a very high modulus, are quite expensive. Clay, although quite inexpensive, has platelets of about 50)~ diameter which are impractical if not impossible to utilize individually and as clusters or aggregates do not yield an ideal composite and so physical properties are poor. Of the two remaining, glass flakes have a lower modulus and a higher price when compared with mica. Glass flakes have a modulus of 72.4 GN/m 2 ( I 0.5 x 106 lb f/in 2) and a cost of £0.66/kg (75c/lb) whereas mica has a modulus of 172 GN/m 2 (25 x 106 lbf/in 2) and a cost of£0.033 to £0.13/kg (4 to 15c/lb). The density of the glass flakes is 2.54 and the density of mica is 2.7 to 3.0 g/cm 3. Hence the choice of mica as the candidate reinforcement for two-dimensionally reinforced composites is obvious. THEORETICAL CONSIDERATION

platelet reinforced composites have been examined by Padawer 1 , Shepherd 2 and Piggott 3 ; fibre reinforced composites by Kelly4, Piggott s , Kelly and Tyson 6 ; sphere reinforced composites by Nielsen7 . The results of these calculations for modulus are shown in Fig. 1; for tensile strength when the matrix is brittle in Fig.2; for tensile strength when the matrix is ductile in Fig.3. It is evident from these figures that fibres and platelets are considerably more efficient reinforcements than spheres. Further, fibres generally give higher moduli and strengths than platelets. However, flakes have advantages over fibres when properties in a plane are considered; fibres can only reinforce in one direction while flakes reinforce in a plane. The results of Economy and Wohrers and Kelly 9 illustrate this and are summarized for strength in Fig.4 and for modulus in Fig.5. Note that pseudo-planar fibre com-

..............

125

&" I00 E

:E

o

,//"

75

eel

sot/,, .//" .........

Theoretical strengths and moduli of the three classes of composites have been calculated by a number of authors:

C O M P O S I T E S . S E P T E M B E R 1974

" /

I

The same general principles of reinforcement that apply to fibres also apply to flakes.

The authors are both with Fiberglas Canada Ltd, PO Box 3603, Guelph, Ontario, Canada

vp.t, o;,

[

I0

--~ ....

-'"

,v= -0.2

I

I

30

SO

I

IO0

Aspect Fig.1

Fibres

I

f

300

I000

ratio

Theoretical elastic moduli for mica/thermoplastic composites

193

400

D - . -------"Vf - 0 7

/....__..

/-" if/°

/

300

%

.._.._._.. ---------

. / - "

vt - a S

//ot

Z ..C

200

/ /

..

/ /

P

....

/," ¢s.,-

vp,o2

_ .....

___,,_ G : - " ...... vp-o.7 Vf -0.2

.- --------- . . . . . .

,,

.:::.;"....-'--'f

I-

IOC

vp , /

---......

IO

,,.c,,,°

,.,.,o,...,

Fibres

Plot.lilts • Spheres

• VS: 0'7 • Vs = 0"2

I

t

I

I

I

30

SO

IOO

300

IOOO

Asl~c t ratio Fig2 Theoretical tensile strength o f mica reinforced plastics when the m a t r i x is elastic t o failure

f500

j~;..--

E

. ....--TT'.'~--='=vp, +" ~

sS

mKXX: ;.

-/ /,

----~i-:''" .11.>'"

,/2" •

~ E~C)OL

IO

/ / / /

. . . . . Platelets Vp.f IVolum¢ WoctiO. rGInforc¢lltCnt

~,,"

/I o" /

,, ,* //

I

30

._.--~-:_ _ .= ,-=:'aT=V., - O'~

I

50

I

I

IOO 300 Aspect ratio

transfer. The maximum stress that the reinforcing element must withstand occurs where an adjacent reinforcing element ends. The stress diagrams for the reinforcing elements show how the six nearest neighbours in the case of the fibres allow a higher average stress in the fibres whereas the 2 nearest neighbours in the case of the flake must take 3 times the amount of stress transferred in the fibre case; hence the average stress with the platelet is much lower. The equation shows that the average stress in the fibre can be 6/7 of the. fibre strength whereas the average stress in the platelet can be only 2/3 the strength of the platelet. Hence the major advantage of flakes over fibres is with respect to modulus and not strength of planar isotropic composites. It should be noted that other properties such as shrinkage and thermal expansion will be planar isotropic in flake composites but not for fibre composites. M/CA COMPOS/TES

In a real flake composite, considerable difficulty is experienced in defining aspect ratio since the flakes are not regular in shape; a sample of mica flakes is shown in Fig.7. This problem has been considered in some detail by Woodhams 11 who concluded that a workable definition of aspect ratio is average diameter/average thickness; the diameter of a flake being defined as the diameter of a circle of area equal to the flake in question. Woodhamsl 1,,2 has found a correlation between flexural modulus, flexural strength and aspect ratio defined this way as shown in Fig.8. As expected theoretically, the properties of a mica composite are also a linear function with respect to volume fraction, as illustrated in Fig.9. The strength and modulus increase with mica volume fraction up to 60 volume percent (v/o) [83 weight percent (w/o)] and decrease at higher loadings. This decrease is due to incomplete wetting of the flakes with resin resulting in direct flake to flake

I

IOOO

3OC ~

\,+

"--2 O°

/ ......~ o

°

Fig3 Theoretical tensile strength o f mica reinforced plastics when the matrix is ductile Z

o t~

posites can be fabricated but the strength and modulus are reduced to about one-half the unidirectional values (volume fraction constant). For planer isotropic composites, one would expect from Figs.l-3 that flakes would be far superior to fibres. Unfortunately, there is an offsetting disadvantage with flakes due to stress transfer possible in the three-dimensionally packed situation. As discussed in detail by Riley l° for fibres and by Piggott 3 for platelets, Fig.6 shows the nearest neighbours seen by a high aspect ratio reinforcing fibre and flake. In the X - Y plane the two are similar, but in the Y - Z plane, the fibre in thebest packing situation, hexagonal packing, has 6 nearest neighbours which can share the stress when it must be transferred at the end of the reinforcing fibre. The flake only has 2 nearest neighbours to assist in stress

194

o

,o+,.J.IV~/J. /I / 1+o~ o°-,s~-9o" O

50

IOO

~

150 2 0 0 250 Modulus (GNIm 2)

\

300

350

Fig.4 Comparison o f the Young's Modulus o f AIB2 flake composites t o a number of fibre reinforced composites: note the constant value o f modulus with respect to testing angle f o r the flake c o m p o s i t e

COMPOSITES SEPTEMBER 1974

,,%

Fibres X-Y

-

X-Y--

-

0 0 000 0 0

Y-Z o

Plates

Y-Z

-

-

o Glass fibre/epoxy

E

o- - --o A I Bz Flake/epoxy v t-

6, e-

4.e,+ ~.:z ~ 6 6 E I.-

%

Fig.6

~

200 I00

~ o - - - _ _ _ I

0

Stressconcentration effects in flakes and fibres

I0

I

2o

i

3o

I

4o

I

so

o

O

-)o

I

80

O |

90

Testing direction ~ IDegrees) Fig.5

Comparison of the tensile strength of AIB 2 flake composites

to a fibre reinforced composite: note the constant value for strength with respect to testing angle for the flake composite

contact and very low load transfer stresses. To avoid this problem we limited our composites to 50 v/o or less. At 50 v/o the only feasible moulding technique was found to be compression moulding of powders. Other techniques, such as injection moulding powders, injection or compression moulding of extrusion mixed composites, resulted in much reduced physical properties as the data in Table 1 illustrates. This decrease in properties with moulding techniques other than compression moulding results from degradation of the mica aspect ratio and from poor flake alignment during injection moulding. Thus the best values obtained for mica reinforced plastics have been with compression moulded samples. Table 2 shows values obtained with thermoplastic matrices and Table 3 shows values obtained with thermosetting matrices. These tables show that moduli of 32 to 48 GN/m 2 (5 to 7 x 106 lbf/in 2) can be obtained with a wide variety of plastics and are the highest values to date for many of these plastics. These composites are quite practical when they consist of a thermoset matrix; but the thermoplastic matrix composites are impractical for high volume compression moulding as the composites need to be cooled prior to mould removal and this causes a very long cycle time. For thermoplastics, the usual fabrication method is injection moulding. Although this technique is successful, it has its disadvantages. First of all, because of the high viscosity of 75 w/o (50 v/o) mica reinforced thermoplastics difficulty has been experienced in injection moulding such composites. An upper limit of reasonable processability is 65 w/o with 50 w/o being equivalent in processing to 20 w/o glass fibre-reinforced thermoplastics. The main disadvantage therefore is that with a lower reinforcement

COMPOSITES . SEPTEMBER 1974

Fig.7 Photomicrograph of mica platelets illustrating their nonuniform shape

250

60

e

200

/

z

/"

.

~

lr

150

t ,l

SO

J

tI

4o

t

~

ioo

3o

//

2O

50

Fig.8

o E ~ x

I

0

o _~ 'V

iI

// J

E

E

z

i

--~ Modulus . . . . . . Strength % - 0.5 1

i t I0

t

I00 20O Asl~ct ratio

300

Experimental effect of mica aspect ratio on flexural

strength and modulus

195

Weight perctmt (w/o) mica 65 7.0 75 80 ! i I i

5s!

Table 2. Compression moulded mica/thermoplastic composites - with comparison to 40 w/o glass compounds

65 u 220

Flexural strength (MN/m 2)

Flexural modulus (GN/m 2)

Matrix

50v/o mica

40w/o glass

50v/o mica

40w/o glass

Poly.styrene

165

120

44.7

10.3

SAN

207

160

53.1

12.7

210 ~" Z vl,

2

5O

200 Z 190 ,4::

""

45

t80

'0 0

~

170

E 4e

~t/

/4 / ~ "

U

ft. 35 '~/

35

I

160

.-4--~ Modulus -L-S-- Strength

I

I

[

I

o. x

150 u. u 140 I

I

40 45 50 55 60 65 70 vlo mica

Nylon 6/6

186

289

44.7

11.0

Polyester

186

234

47.5

11.0

Polypropylene 172

72.3

37.9

6.54

Polyethylene

96.5

31.0

7.57

124

Fig.9 Experimental effect of mica volume fraction on flexural strength and modulus

Table 3. Compression moulded thermoset composites (50 v/o mica) Flexural strength (MN/m 2)

Compound

Fig.10 Macrophoto of a fracture surface from an injection moulded bar of mica reinforced SAN: note the alignment along the mould surfaces and random orientations in the centre of the section

level, the properties are lower. The second problem experienced with injection moulding is mechanical damage done to the mica platelets resulting in a decrease of the aspect ratio and therefore lower values of the reinforced properties. In addition, reinforcement orientation is affected by fabrication processes and is not always controllable. Flow orientation occurs in the injection moulding and extrusion processes resulting in a variety of parts that are influenced by dies and moulds and

Flexural modulus (GN/m 2)

Experimental mica/epoxy

165

44.1

Experimental mica/polyester

158

46.9

Experimental mica/phenolic

145

51.7

Commercial mica/phenolic

55-69

17-34

Commercial bmc*

69-138

10-14

~Bulk moulding compound: 20--30 w/o glass fibre-reinforced

other such part~ of these processes. In injection moulded parts, then, it is difficult to measure the anisotropic properties due to the difficulty of controlling reinforcement orientations; Fig.lO illustrates this problem in an injection moulded bar - note the alignment near the mould surfaces and misorientation in the central portion of the bar. Consequently, physical property values quoted from injection moulded samples of both fibre and flake composites must be considered as relative or average

Table 1. Comparison of compression moulding to injection molding

Composite

Compression moulded

Injection moulded

Extruded and injection moulded

Flexural modulus (GN/m 2)

Flexural modulus (GN/m 2)

Flexural modulus (GN/m 2)

Flexural strength (MN/m 2)

Flexural strength (MN/m 2)

Flexural strength (MN/m 2)

50 v/o mica in ABS

41.6

154

36.9

105

32.5

99.9

50 v/o mica in polystyrene

41.3

123

36.9

114

25.4

67.7

50 v/o mica in polypropylene

37.9

172

26.9

196

86.1

--

-

COMPOSITES . SEPTEMBER 1974

properties. However, some arrisotropy hasbeen noted in large parts where differential shrinkage has resulted in warpage. One of the inherent disadvantages found with mica composites is their low impact strength. Since impact strength is primarily a function of reinforcement length, ternary injection mouldable composites of glass fibre/mica/polymer were investigated. The range of interest was from 0-60 w/o reinforcements. The response surfaces for flexural modulus, flexural strength, and notched izod impact strength with respect to composition were determined by fitting a third degree polynomial (10 coefficients) to ten experimental points. 13 An eleventh test point was used to confirm that the response surfaces generated were reasonable. SAN, ABS, nylon 6/6 and thermoplastic polyester have been investigated; the response surfaces generated are illustrated in Figs.11-!6 for nylon 6[6 and thermoplastic polyester. As anticipated, the addition of glass fibre increased impact strength while decreasing modulus. More important, however, is the fact that the maxima in flexural strength, flexural modulus, and notched impact strength do not occur at the same composition. It was felt that a suitable

Table 4.

Properties of nylon 6 / 6 compounds

Property Specific gravity

20% GF 1.24

RFM 2030

RM 5000

1.60

1.55

(mm/mm)

Flexural strength (MN/m 2) Flexural modulus (GN/m 2) Shear strength (MN/m 2) Compressive strength (MN/m 2) Izod impact strength notched (J/m)

Property

20% GF

Specific gravity

1.19

Mould shrinkage - 6.2 mm section (mm/mm)

.005 107 5.30 161 6.47 71.1

.0038 124 16.1 180

.0054 94.8

AFM 2030 AM 5000 1.52

.0006

Tensile strength (MN/m 2)

73.7

Tensile modulus (GN/m 2)

5.58

Flexural strength (MN/m 2)

102

Flexural modu,lus (GN/m 2) Shear strength (MN/m 2)

42.9

Compressive strength (MN/m 2)

80.6

Izod impact strength notched (J/m)

1.52

.0008

.0014

80.0

65.2

14.8

14.6

123

5.58

96.5

14.0

14.7

57.9

51.1

120

113

69

43

32

230

150

130

Heat distortion temperature at 0.45 MN/m 2 (°C) 100

110

109

96

106

103

Composition w/o glass fibre

20

20

0

Composition w/o mica

0

30

50

at 1.88 MN/m 2 (°C)

Mould shrinkage

Tensile modulus (GN/m 2)

Properties of ABS compounds

unnotched (J/m)

- 6.2 mm section

Tensile strength (MN/m 2)

Table 5.

17.7 124

14.1

13.3

76.1

65.5

6 0 weight pcrccrtt 19 3 IO

130

131

~9

110

17 15

•

13

48

75

48

unnotched (J/m) 410

400

220

Heat distortion temperature at 0.45 MN/m 2 (°C)

251

288

253

at 1.88 MN/m 2 (°C)

233

245

229

Composition w/o glass fibre

20

20

0

Composition w/o mica

0

30

50

It

5

7

9

3

COMPOSITES . SEPTEMBER 1974

Fig.11 Flexural modulus responsesurface for mica, glass-fibre, nylon 6/6 composites: modulus in GN/m2

197

Table 6.

weight. percent mzca 19 60

Properties of SAN compounds

Property Specific gravity

20% GF 1.24

Mould shrinkage 6.2 mm section (mm/mm) Tensile strength (MN/m 2) Tensile modulus (GN/m 2) Flexural strength (MN/m 2) Flexural modulus (GN/m 2) Shear strength (MN/m 2)

BF'M 2030 BM 5000 1.53

.0005

1.55

.0014

~OO

102 9.09

82.7

68.2

16.1

17.6

132

00

220

Nylon • \ v 6/6 103 IO0

8.81

16.7

Fig.12

61.1

59.3

/v IO0

v

i v 120

i\/ 140

240 v _ L _~r_= =v;. ~ 60r weight 2OU 2ZU 24U zt,U p,crc?nt

i.J._l 160 I ~

18.5 48.9 107

Izod impact strength notched (J/m)

48

43

27

180

120

120

Heat distortion temperatu re at 0.45MN/m 2 (°C)

99

113

109

at 1.88 MN/m 2 (°C)

97

110

108

20

20

0

0

30

50

Flexural strength response surface for mica, glass-fibre,

nylon 6/6 composites: strength in MN/m2 60 weight percent mica 40

compromise was to develop two grades of compound which mould with the same ease as 20 w/o glass fibre. These were a compound of 50 w/o mica, 0 w/o glass, giving the highest modulus composites, and a compound of 20 w/o glass, 30 w/o mica, giving impact strength equivalent to 20 w/o glass while increasing the modulus. Tables 4, 5, and 6 compare these compounds in nylon 6/6, in ABS and in SAN respectively. Additionally, 40 w/o mica compounds in nylon 6 and nylon 6/6 were prepared for an automotive application; Table 7 compares these composites. "Unlike results with glass fibres it was found that uncoupled and coupled polypropylene did not give radically different mica composites, as the data in Table 8 illustrate. Since it was anticipated that mica would have a surface similar to glass, this was entirely unexpected and has not yet been resolved. From Tables 4 - 8 it can be seen that the principal gain resulting from using mica is an increase in modulus. The biggest loss is the decrease in impact strength which can be partially compensated for by the addition of some glass fibres. Strengths are roughly comparable with the glass fibre-reinforced thermoplastics as are the coefficient of expansion and the heat distortion temperature. The most promising area to utilize fully the planar reinforcing properties of mica is in sheet materials. Sheet is made using the extrusion process and the effects of reinforcement orientation are encountered. With glass fibres,

198

8 9

99.9

140

Composition w/o mica

180

9

139

131

Composition w/o glass fibre

60

.0015

Compressive strength (MN/m 2)

unnotched (J/m)

40

4

Nylon ~

~

43

°

°

60cW~Gn~ht

Fig.13 Notched Izod impact strength responsesurface for mica, glass fibre, nylon 6/6 composites: impact strength in J/m

Table 7. Comparison of nylon 6/6 to nylon 6 in 40 w/o mica composites

Composites Matrix composition [w/o nylon 6/6)/ (w/o nylon 6)] Property

0/100

25/75

50/50

75/25

100/0

Flexural strength (MN/m 2)

139

134

136

136

148

Flexural modulus (GN/m 2)

13.9

11.7

11.9

12.1

14.1

Heat distortion temperature (°C at 1.88

MN/m 2)

186

191

200

208

223

COMPOSITES . SEPTEMBER 1974

Table 8. Comparison of uncoupled a and coupled b polypropylene" 30 w/o mica and 30 w/o glass fibre compounds

6 0 weight perccRt mica 2. 21

30 w/o mica composites

30 w/o glassfibre composites

1719

I

IS

Tensile strength (MN/m 2)

[a]

[b]

[a]

[b]

33.9

43.4

50.4

79.9

Tensile modulus (GN/m 2)

6.89

Flexural strength (MN/m 2)

7.16

52.4

Flexural modulus (GN/m 2)

65.3

6.89

Izod impact strength (J/m) notched

7.57 75.9

6.40

6.47

6.26 98.5 8.75

~leYr 2"6

3

5

7

9

II

"3

IS

r7

percelt gloss

Fig.14 Flexuralmodulus responsesurface for mica, ~lassfibre, thermoplastic polyester composites: modulus in G~N/m

unnotched

48

48

80

120

160

210

190

530

NB: a = uncoupled, b = coupled

6 0 weight p e rclc~t m i c a

I0o

Table 9. Effect of fibre orientation in the surface skins of reinforced sheet: 20 w/o glass content Sheet thickness (mm) 9

Property

2.5

3.8

Flexural strength (MN/m 2)

L W

46.4 25.0

52.6 35.7

Tensile strength (MN/m 2)

L W

38.4 35.2

38.2 36.4

Flexural modulus (GN/m 2)

L W

3.79 2.07

3.65 2.20

Tensile modulus (GN/m 2)

L W

3.44 2.76

3.38 3.31

/N/K//vZ:\ Po~,-

ester *S 70 eo~o =30 no 120

130

percent

140

147 gloss

Fig.15 Flexuralstrength responsesurface for mica, glassfibre, thermoplastic polyester composites: strength in MN/m

Note: L - -- extrustion direction; W - acrossthe sheet 6 0 weight perc e~t m l c o

Table 10. Directional properties of sheet with various levels of reinforcement

o

Reinforcement

Glass fibre

Glass fibre

Glass fibre

Mica platelets

w/o solids

10

20

30

10

Sheet thickness (mm) Flexural strength (MN/m 2)

3.8

3.8

3.8

3.8

L

31.1

52.6

61.2

37.4

W

28.0

35.7

33.9

37.1

Flexural modulus

L

2.00

3.65

5.03

2.48

(GN/m 2)

W

1.93

2.20

2.27

2.48

Note: L - along sheet; W -- acrosssheet

COMPOSITES . SEPTEMBER 1974

x \W

ester 3~40 SO

60

70

80

8s

--

0

~o

8S

BOgloss

Fig.16 Notched Izod impact strength for mica, glass fibre, thermoplastic polyester composites: impact strength in J/m

199

the drag flow experienced in the area adjacent to the die wall orients the fibres in t h e machine direction. This effect is only on the surface though and the area in the middle o f the sheet is unaffected and has random o r i e n t a t i o n J 4 The effect o f fibre orientation, therefore, is more pronounced in thin sheet than in thicker sheet as is shown in Table 9 with reinforced polyethylene sheet. The effect is also more pronounced at high glass fibre concentrations as is seen in Table 10. The data in Table 10 show, as theoretically expected, that mica acts as a planar reinforcement since the L- and W- direction properties are the same. Other fabrication techniques, such as pultrusion and sprayup techniques, are currently being evaluated and will be reported on in the future.

3 4 5 6 7 8 9 10 11

12

REFERENCES 13 Padawer, G. E. and Beecher, N. Polymer and Engineering Science 10 No 3 (1970) p 185 Shepherd,P. D. 'The solidification of off-eutectic AI-Cu alloys and their mechanical properties', PhD Thesis University of Toronto, Canada (1969) p B-26

200

14

Piggott, M. R. JMatSci 8 (1973) p 1373 Kelly, A. Strong Solids Clarendon Press, Oxford (1968) pp 121-123 Piggott, M. R. Acta Metallurgica 14 (1966) p 1429 Kelly, A. and Tyson, W. R. JMech PhysSolids 13 (1969) p 329 Nielsen, L. E. JApplPolymer Sci 82 (1966) p 97 Economy, J. and Wohrer, L. C. SAMPE Journal (Dec/Jan 1969) Kelly, A. Strong Solids p 153 (see Ref. 4) Riley, V. R. JCompMat2(1968)p436 Kauffman, S. H. et al. 'The preparation and classification of high aspect ratio mica flakes for use in polymer reinforcement', ACS Division o f Organic Coatings and Plastics Chemistry, 166th Meeting 33 No 2 (1973) p 41; also Powder Technology (in press) Lusis, J., Woodhams, R. T. and Xanthos, M. 'The effect of flake aspect ratio on the flexural properties of mica reinforced plastics', Polymer Engng and Sci 13 No 2 (1973) p 139 Maine, F. W. et al. 'A new family of reinforced thermoplastics', SPI 28th Annual Conference Proceedings (1973) paper 5 - A Osborne, A. D. and Maine, F. W. 'Reinforced thermoplastic sheet for thermoforming', SPI 29th Annual Conference Proceedings (1974) paper 24-E

COMPOStTES . SEPTEMBER 1974