Sheet molding compound

A light weight molding compound comprises an inner layer constituted by a thermosetting resin including a hollow filler, cloth layers constituted by woven or non-woven fabric adhered on the both surfaces of the inner layer, and surface layers constituted by a thermosetting resin including reinforcing fiber and integrally connected to the both surfaces of the inner layer by way of the cloth layers. The light weight sheet molding compound has superior surface quality to be painted after molding.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light weight sheet molding compound. 
2. Brief Description of the Prior Art 
Recently, two-layered sheet molding compounds comprising a layer including 
reinforcing fiber and a layer including no reinforcing fiber have been 
used for an engine hood and outside plates of an automobile, for example, 
a rear spoiler, a roof, a hood and so on. These sheet molding compounds 
include a hollow filler in order to reduce the weight. 
The light weight sheet molding compounds are produced as follows: Resin 
paste for compounds including a hollow filler is applied on two 
polyethylene films respectively at a predetermined thickness. Then, roving 
glass fiber is cut and scattered uniformly on one spread resin paste, 
thereby placing reinforcing fiber. Then, the other resin paste applied 
film is laminated thereon to make the laminated layers into one integral 
sheet. Therefore, as shown in FIG. 5, a conventional sheet molding 
compound 30 comprises an inner layer 31 including reinforcing fiber, and 
resin layers 32 including a hollow filler 33 and disposed on the both 
sides of the inner layer 31. 
The viscosity of the sheet molding compound 30 varies with the heat for 
molding and hardening the compound 30. At that time, the filler 33 which 
has been dispersed in the resin paste and which has a smaller specific 
gravity than the base resin tends to move upward and concentrate on the 
surface. A part of the hollow filler 33 breaks or cracks by shearing force 
applied during molding the compound 30. These broken materials and the gas 
which has been contained in the filler 33 are scattered on the surface of 
the sheet molding compound 30. Therefore, the surface to be painted is not 
always smooth, and a paint film formed on the compound 30 often develops 
coating defects such as blisters. After-treatment such as repair of 
coating defects is required and productivity is decreased. 
As an attempt to remove these disadvantages, Japanese Unexamined Patent 
Publication (KOKAI) No. 193940/1988 proposes a low specific gravity sheet 
molding compound in which one resin sheet layer in contact with the other 
resin sheet layer by way of reinforcing fiber includes no hollow filler, 
and a low specific gravity sheet molding compound in which one resin sheet 
layer includes a smaller amount of a hollow filler than the other resin 
sheet layer. 
In the above sheet molding compounds in which the resin sheet layer on the 
surface side includes no hollow filler or a smaller amount of a hollow 
filler, however, only short fiber exists as reinforcing fiber between the 
one resin sheet layer and the other resin sheet layer including a larger 
amount of the hollow filler, and the filler can easily pass through the 
reinforcing fiber. Therefore, when the resin viscosity is lowered during 
heating for hardening the resin, it is difficult to inhibit the low 
specific gravity filler from passing through the reinforcing fiber and 
transferring to the surface side of the resin sheet layer. Further, it is 
sometimes hard to hold the resin layer including a smaller amount of the 
hollow filler on the upper side. Therefore, the sheet molding compound 
surface to be painted is difficult to be kept smooth, and appearance 
defects such as blisters are often developed in a paint film formed on the 
sheet molding compound surface. The coating defects necessitate repairing 
steps. Accordingly, productivity enhancement by removing steps of 
repairing the paint film is required to be used as panel component parts. 
SUMMARY OF THE INVENTION 
The present invention has been conceived in view of the above 
circumstances. It is an object of the present invention to provide a light 
weight sheet molding compound having superior surface quality to be 
painted after molding. 
The sheet molding compound according to the present invention comprises an 
inner layer constituted by a thermosetting resin including a hollow 
filler, cloth layers constituted by woven fabric or non-woven fabric 
adhered on the both surfaces of the inner layer, and surface layers 
constituted by a thermosetting resin including reinforcing fiber and 
integrally connected to the both surfaces of the inner layer by way of the 
cloth layers. 
The inner layer is formed by mixing a hollow filler in resin paste which 
constitutes the sheet molding compounds. Examples of the hollow filler are 
hollow glass balloons, hollow plastic beads, hollow ceramic balloons, and 
foam plastic beads. The specific gravity of the hollow filler is 
preferably from 0.1 to 1.2 in view of the effects of reducing the weight 
and keeping up the strength of moldings produced from the sheet molding 
compound. The specific gravity is more preferably from 0.3 to 0.5 to make 
the effects remarkable. Further, in general, the amount of the filler is 
preferably 15% by volume or more. The filler amount is more preferably 
from 20% by volume to 60% by volume in view of the superior effects of 
reducing the weight and keeping up the strength of moldings produced from 
the sheet molding compound. 
The resin paste may include an ordinary filler such as calcium carbonate. 
The resin paste constituting the sheet molding compound may be any ordinary 
resin to be used for sheet molding compounds. Examples of the resin paste 
are thermosetting resins such as epoxy resin, unsaturated polyester resin, 
vinyl ester resin, urea resin, melamine resin, and phenol resin. 
Examples of the cloth layers to be adhered on the surfaces of the inner 
layer are woven fabric or non-woven fabric produced from organic fiber 
such as aramid fiber, polyester fiber, nylon fiber and so on, and woven 
fabric or non-woven fabric produced from inorganic fiber such as glass 
fiber, ceramic fiber, ceramic whisker and so on. When woven fabric is used 
as cloth layers, a binder is preferably applied on the texture to make the 
texture gaps smaller. More preferably, the area of one texture gap with 
the binder applied is 0.5 mm.sup.2 or less. In the cloth layers having the 
above texture gap area, since the gap area is smaller than the diameter of 
the ordinary hollow filler, the filler is prevented from passing through 
the cloth layers and transferring to the surface of the upper layer. 
Therefore, it is possible to prevent defects on the surface of the sheet 
molding compounds due to the existence of the hollow filler. On the other 
hand, when non-woven fabric is used as cloth layers, the weight of the 
fabric is preferably from 5 to 300 g/m.sup.2 in view of the superior 
strength and superior adhesion to the resin paste. More preferably, the 
fabric weight is from 10 to 100 g/m.sup.2 to make these effects 
remarkable. 
The surface layers are constituted by a resin including reinforcing fiber. 
Examples of the reinforcing fiber are glass fiber, organic fiber, and 
other inorganic fiber. For example, the glass fiber may be produced by 
twisting 50 to 5000 glass filaments of 5 to 30 microns into a strand and 
cutting the strand into a length of about one-eighth inch to 4 inches. 
More preferably, the strand length is from a half inch to 2 inches. The 
amount of the reinforcing fiber to be mixed in the surface layers is 
preferably 20% by volume or more. More preferably, the reinforcing fiber 
amount is from 20 to 50% by weight, in view of the superior strength of 
moldings produced from the sheet molding compounds. 
The resin paste constituting the surface layers can have the same 
composition as the inner layer. 
In producing the sheet of the sheet molding compound, the both surfaces of 
the sheet are preferably covered with films in view of easy production and 
handling. Generally, the films are polyethylene films or the like which 
can be easily peeled off from the molded sheet. 
The sheet molding compound covered with films can be produced, for example, 
as follows. First, paste resin for the surface layers is spread on two 
films, and reinforcing fiber is scattered on the paste resin at a 
predetermined thickness. Next, cloths constituting the cloth layers are 
placed thereon. Then, resin paste including a hollow filler is spread on 
one cloth layer surface, and laminated on the resin paste spread surface 
is the other film on which the resin paste and the reinforcing fiber are 
spread and the other cloth layer are further placed. Then the laminated 
layers are pressed and wound, thereby forming a sheet continuously. The 
continuous sheet is cut into a predetermined length and hardened by 
pressure molding, thereby producing a molding material. The molding 
material is molded under pressure, thereby producing moldings. The 
moldings can be used as panels or the like by painting the surface. 
The sheet molding compound according to the present invention is in the 
form of a sheet comprising three resin paste layers: The two outer layers 
including reinforcing fiber are respectively connected to the both 
surfaces of one inner layer including a hollow filler, and in the 
boundaries of the inner layer and the two outer layers, cloth layers are 
formed by cloths with small texture gaps. Because these cloth layers 
divide the inner layer and the surface layers, the low specific gravity 
hollow filler is inhibited from moving upward due to the low specific 
gravity., even when the viscosity of the resin varies. Therefore, the 
hollow filler is prevented from concentrating on the upper surface, and 
faults caused by damage on the hollow filler during molding are prevented. 
As a result, the surface of the sheet molding compound is kept smooth, and 
a paint film formed on the sheet molding compound is free from blisters 
and other appearance defects. 
In summary, in the sheet molding compound according to the present 
invention, since the filler does not exist on the surface after molding, 
appearance defects such as blisters are prevented when the surface is 
coated with paint. 
Accordingly, troublesome steps of repairing the surface defects are no 
longer required, decrease in yield due to the detects is inhibited, and 
production costs can be lowered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
(The First Preferred Embodiment) 
Referring first to FIG. 1, a sheet molding compound 1 according to a first 
preferred embodiment of the present invention comprises three resin 
layers: an inner layer 4 constituted by inner resin paste 3 in which a 
hollow filler 2 is dispersed in a thermosetting resin, cloth layers 5 
constituted by woven or non-woven fabric adhered on the both surfaces of 
the inner layer 4, surface layers 7 constituted by a thermosetting resin 
in which reinforcing fiber 6 is dispersed and integrally connected to the 
both surfaces of the inner layer 4 by way of the cloth layers 5. The both 
surfaces of the sheet molding compound 1 are covered and protected by 
polyethylene films 8, 8'. 
FIG. 2 shows a schematic diagram of production steps according to the first 
preferred embodiment of the present invention. Referring now to FIG. 2, 
the production steps and constitution of the sheet molding compound 1 will 
be described. 
Resin paste 12 for one surface layer 7 of the sheet molding compound 1 was 
stored in a resin supply tank 10 and stirred sufficiently. The resin paste 
12 was unsaturated polyester resin including an ordinary filler such as 
calcium carbonate (`Polylite PS-260` manufactured by DAINIPPON INK KAGAKU 
KOGYO CO., LTD.). The stirred resin paste 12 was supplied to a doctor 
blade 13 through a pipe, and a predetermined amount of the resin paste 12 
was continuously spread by the doctor blade 13 on a polyethylene film 8 
which was supplied from a polyethylene film roll 14. 
Next, glass roving 15 as the reinforcing fiber 6 was supplied above the 
polyethylene film 8 on which the resin paste 2 had been spread, and 
continuously cut into a length of about 25 mm by a cutter 16. The cut 
glass roving as short fiber was uniformly scattered on the spread resin 
paste 12 so that the surface layer 7 contained the reinforcing fiber by 
35% by volume. Thus, the polyethylene film 8 on which the surface layer 7 
was formed was produced. 
This polyethylene film 8 was conveyed on a belt 22 which was driven by 
rollers, and a surface mat 17 made of glass fiber non-woven fabric 
(`SM3600E` manufactured by ASAHI FIBER GLASS CO., LTD., the weight: 30 
g/m.sup.2) was continuously supplied on the upper surface of the 
polyethylene film 8 on which the reinforcing fiber 6 was dispersed, and 
overlaid and adhered on the reinforcing fiber 6, thereby forming one cloth 
layer 5. 
In this case, it is preferable to apply the resin paste 12 on the surface 
mat 17 beforehand in order to remove the air between the reinforcing fiber 
6 and the surface mat 17 and enhance the impregnating ability of the 
resin. 
Then, the resin paste 12 (the unsaturated polyester resin including an 
ordinary filler such as calcium carbonate (`Polylite PS260` manufactured 
by DAINIPPON INK KAGAKU KOGYO CO., LTD.)) was stored in a resin tank 21, 
and glass microballoons having a specific gravity of 0.5 were mixed with 
the resin paste 12 to be contained by 45% by volume and sufficiently 
stirred. The mixture 18 was supplied on the surface mat 17 at a certain 
thickness through a pipe to form the inner layer 3. 
Separately, as well as above, the resin paste 12 for the other surface 
layer 7 of the sheet molding compound 1 (the unsaturated polyester resin 
including an ordinary filler such as calcium carbonate (`Polylite PS-260` 
manufactured by DAINIPPON INK KAGAKU KOGYO CO., LTD.)) was stored in a 
resin supply tank 10', and supplied to a doctor blade 13' through a pipe. 
Then, a predetermined amount of the resin paste 12 was continuously spread 
by the doctor blade 13' on a polyethylene film 8' which was supplied from 
a polyethylene film roll 14'. 
Further, the glass roving 15 was supplied and continuously cut into a 
length of about 25 mm by another cutter 16, and the glass roving as short 
fiber was scattered on the resin paste 12 in order to be contained by 35% 
by volume in the surface layer 7. Thereby, the polyethylene film 8' on 
which the surface layer 7 was formed was produced. 
This polyethylene film 8' was conveyed on a belt 22 which was driven by 
rollers, and a surface mat 17 made of a glass fiber non-woven fabric 
(SM3600E` manufactured by ASAHI FIBER GLASS CO., LTD., the weight: 30 
g/m.sup.2) was continuously supplied on the surface of the polyethylene 
film 8' on which the reinforcing fiber 6 was scattered, and overlaid and 
adhered on the reinforcing fiber 6, thereby forming the other cloth layer 
5. The resin paste side of the polyethylene film 8' was reversed by a 
roller 23 and overlaid on the polyethylene film 8. 
The surface layers 7 overlaid on the inner layer 3 were compressed by 
compression rollers 19, whereby the resin paste 12 was sufficiently 
impregnated into the reinforcing fiber 6 and the surface mats 17. Then the 
sheet molding compound 1 the both surfaces which were covered with the 
polyethylene films 8, 8' was wound around a take-up roller. 
The wound sheet molding compound 1 with the both surfaces covered with the 
polyethylene films 8, 8' has a cross section shown by FIG. 1. 
The sheet molding compound 1 was thermoset by pressure molding under a 
pressure of 70 kg/cm.sup.2 at a temperature of 145.degree. C. for three 
minutes into a flat plate sheet. The cross section of the flat plate sheet 
remained almost the same as that of the sheet molding compound before 
molding shown in FIG. 1. The flat plate sheet was cut into a flat plate of 
300.times.300.times.2.5 mm in dimensions, and one surface of the flat 
plate was coated with urethane paint. Then, the number of blisters was 
examined on the paint film immediately after painting and after being left 
in water at 40.degree. C. for 240 hours. The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Compar- 
Example 
Example Example ative 
No. 1 No. 2 No. 3 Example 
______________________________________ 
Number of blisters 
0 0 0 2 
soon after painting 
Number of blisters 
0 0 0 7 
after being left 
at 40.degree. C. for 240 hours 
______________________________________ 
No blisters were observed on the paint film both immediately after painting 
and after being left in water at 40.degree. C. for 240 hours. 
(The Second Preferred Embodiment) 
A sheet molding compound 1' was produced by using the same materials by the 
same production method as in the first preferred embodiment, except that 
polyester fiber non-woven fabric (`20207WTD` manufactured by UNICHIKA 
KABUSHIKI KAISHA) was used instead of the surface mats 17 constituting the 
cloth layers 5. 
The sheet molding compound 1' was press molded into a flat plate sheet in 
the same way as in the first preferred embodiment, and coated with the 
paint and the number of blisters was examined immediately after painting 
and after being left in water at 40.degree. C. for 240 hours in the same 
way as the first preferred embodiment. As shown in Table 1, no blisters 
were observed. 
(The Third Preferred Embodiment) 
A sheet molding compound 1" was produced by using the same materials by the 
same producing method as in the first preferred embodiment, except that 
glass cloth (`WF110D100BS6` manufactured by NITTO BOUSEKI KABUSHIKI KAISHA 
(the weight: 97 g/m.sup.2, the density: 19 warps/25 mm, 18 weft/25 mm)) 
was used instead of the surface mats 17 constituting the cloth layers 5. 
The sheet molding compound 1" was press molded into a flat plate sheet and 
coated with the paint in the same way as in the first preferred 
embodiment. Then, the number of blisters was examined immediately after 
painting and after being left in water at 40.degree. C. for 240 hours. No 
blisters were observed. 
(Comparative Example) 
A sheet molding compound was produced in a production line where the 
surface mats 17, and the resin paste 12 and glass chopped fiber for one 
surface were not used in the production steps of FIG. 2. 
The cross section of the obtained sheet molding compound is shown in FIG. 
3. The sheet molding compound comprises two layers: a resin paste layer 
including a hollow filler 2, and a resin paste layer including glass fiber 
6. Two sheet molding compounds thus produced were overlaid to face the 
resin paste layers including the hollow filler 2 each other, and the thus 
produced comparative sheet molding compound having a three layer structure 
was molded in the same way as in the first preferred embodiment. Referring 
now to FIG. 4, since there was no cloth layers 5 between the surface 
layers 7 and the inner layer 4, the low specific gravity filler passed 
through the reinforcing fiber 6 and moved to the surface layers 7. 
On account of this, the painted comparative sheet molding compound had two 
blisters immediately after painting and seven blisters after being left in 
water at 40.degree. C. for 240 hours, as shown in Table 1. 
This examination showed that the painted sheet molding compounds having 
constitutions according to the present invention were suppressed from 
blistering. 
Obviously, many modifications and variations of the present invention are 
possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described.