Note: Descriptions are shown in the official language in which they were submitted.
-1- 2a2~ ~&~~
PROCESS FOR THE SURFACE ENHANCEMENT OF ARTICLES OF FIBRE
REINFORCED PLASTICS MATERIAL
Thls invention relates to a process for the surface
enhancement of articles of fibre reinforced plastics
material.
United States Patent No. 4 734 321 sets forth a
process for making a permeable sheet-like material comprised
of unconsolidated particulate plastics (especially
'chermoplastics material, and relatively short reinforcing
fibres, typically 50 millimetres or lèss in length. After
intermediate processing to cause the plastics component to
be brought to a molten condition and fully wet the
reinforcing fibres, the material is suitable for chill
moulding into fibre reinforced plastics articles. Various
intermediate processing routes can be used.
Thus, for example, and as described in the
aforementioned United States Patent, the permeable material
may be subjected to heat and pressure and then cooled to
form a consolidated sheet. Before use in a moulding
process, the sheet is reheated so that the plastics
component softens and permits the stresses in the fibre
reinforcement to relax and re-expand the sheet, which can
then be chill moulded.
,.
The use of short single fibres in the starting
material has considerable advantages in that they flow
readily with the molten thermoplastic in which they are
entrained when subjected to pressure in the mould. This
results in a very even distribution of the fibres through
the moulding to give consistent reinforcement throughout the
moulded structure, even in the most intricate mouldings.
2 ~
_ --2--
In the chill moulding process, a charge material pre-
heated to above the melt temperature of the plastics
component is charged into a mould in which the mould parts
are controlled at a temperature lower than that at which the
plastics component will solidify. Moulding pressure is then
applied to the hot charge sufficiently fast to cause both
the plastics and fibre components of the charge to flow into
the configuration of the mould before solidification of the
plastics component occurs.
Chill moulding is a well known technique for moulding
unreinforced plastics materials. It has however been found
that its use for moulding plastics materials reinforced with
short fibres can result in surface marring of the final
moulding. For different reasons other kinds of fibre
reinforced thermoplastic material can also suffer from
surface defects due to fibre protrusion.
The production process for the starting material as
set forth in the United States Patent No. 4 734 321 causes
the component fibres to be oriented generally in the plane
of the sheet. It has however been found that where surface
marring occurs during subsequent moulding, it is caused by
the projection of fibres through the surface of the sheet.
Another form of fibre reinforced plastics sheet is
produced by laminating together, in a platen press, layers
of needled glass fibre mat and thermoplastic resin such as
polypropylene. Methods used to produce such products are
described in United States Patents Nos. 3 664 909,
3 684 645, 3 713 962 and 3 850 723. However, it has been
found that the use of such sheets in chill moulding or
stamping operations results in slight protrusion of the
fibres at the surfaces of the resulting moulding so that
apparent visual irregularities in the surface occur.
_ 3 ~ 8 ~
Another system for forming fibre reinforced plastics
sheet is disclosed in United States Patent No. 3,328,383
in which a mixture of glass and thermoplastic fibres is
laid down on a flat surface and then consolidated by
pressure after heating. Here again, subsequent moulding
of the consolidated sheet results in fibre strike through
at the surfaces of the sheet.
It is among the objects of the present invention to
improve the surface finish of mouldings formed from
sheets of the kind heretofore described.
According to the present invention, there is
provided a process for preparing a permeable sheet of
glass fibre reinforced thermoplastic material for
moulding into a shaped product which comprises binding to
a permeable sheet of glass fibre reinforced thermoplastic
material a surface layer of particulate material which is
compatible with or inert to the thermoplastic content of
said sheet and which acts to improve the surface finish
thereof when the sheet is subsequently heated and chill
moulded under pressure.
The particulate material may be a mineral filler,
for example a clay, carbon black or titanium dioxide
calcium carbonate, a thermoplastic, a thermosetting
plastic, an antioxidant material, a pigment or mixtures
thereof. Alternatively, or in addition, inorganic
fillers being for example described in United States
Patent No. 4,800,103 issued on January 24, 1989.
~ ,,~,
,
~ _ 2 ~
--4--
The formation of a particulate surface coating has
especial advantages in terms of sheet handling when used
with sheet materials of the kind disclosed in United States
Patent No. 4 734 321. Because each particle is directly or
indirectly bound individually to the sheet surface, it
interferes neither with the flexibility of the sheet, nor
significantly with its permeability. As a result, the sheet
can be reeled and also pre-heated prior to moulding by the
transmission of hot air through the sheet, for example as
described in European Patent Application No. 85.300033
(Publication No. 0 148 762).
Such particulate coatings also have advantages for
use with other forms of fibre reinforced plastics sheet.
Thus, the use of a mineral filler in the surfacing layer has
been found to increase the viscosity of the plastic content
of the sheet at the surface so as to resist surface strike
through by the reinforcing fibre. The use of particulate
plastics materials in the surfacing layer produces a surface
enrichment of the plastic content with a similar result.
The particulate surface layer may be applied by a
number of techniques. Thus for example an aqueous coating
of a dispersion of the particulate material may be applied
in a conventional manner on an aqueous coater.
Alternatively, a surfactant may be added and the dispersion
turbulated to form a foam, the foam then being applied as a
coating as described in UK Patent No. 1 039 ~40 or United
States Patent No. 4 263 344.
Still again, the particulate material may be applied
as a dry powder by various techniques, provided that process
conditions are such as to ensure that the powder bonds to
the sheet.
8 6 ~
--5--
An aqueous dispersion may be applied at a consistency
in water of 0.5 to 30% (Wt/Wt) or 5 to 300 grammes per litre
of water. For a typical production rate of 7 metres per
minute of sheet to be coated, the coating would be applied
at a rate of 0.8 to 500 litres per minute per metre of sheet
width.
A foamed coating may be applied at a rate of between
50 to 500 grammes per square metre, with the consistency in
foam being 0.5 to 10% (Wt/Wt) or 5 to lO0 grammes per litre
of water. For an air content of 67~ this is equivalent to
1.6 to 33 grammes per litre of foam.
The sheet is coated at a typical production rate of 5
metres per minute, with the flow rate range for the foam
being from 8 to 4000 litres per minute per metre width of
the sheet.
Dry powders may be applied at a rate of between 50
and 500 grammes per square metre at a typical production
rate of 7 square metres per minute.
In a known process for making glass mat-reinforced
thermoplastic sheet material a mixture of chopped glass
fibres, polypropylene powder and an inorganic filler, such
as kaolin, calcium carbonate, talc, mica, titanium dioxide
or alumina trihydrate, is dispersed in water containing a
surfactant (which may be anionic or nonionic), using a high
speed mixer, to form a stable foam of relatively low solids
content. This foam is spread uniformly over the surface of
a travelling wire mesh, such as that conventionally used in
a Fourdrinier paper making machine, and the aqueous phase is
withdrawn through the wire mesh by gravity and suction. The
-6- 20~ ~ ~5~
mat of solid materials thus formed on the upper surface of
the wire mesh is then dried at a temperature of about 100~C.
The resultant dried mat, usually after being cut into pieces
of suitable size and stacked to form a multi-layer sheet
material, is pressed between steel plates at a temperature
of about 200~C and then allowed to cool in the press to form
a sheet-like component, such as a car body panel.
Alternatively, the stacks of cut pieces may be heated to
about 200~C by means of a current of hot air or by infra red
radiation and subsequentl,v allowed to cool in a press at
about 60~C to form the sheet-like material.
In addition to the glass fibres~; polypropylene and
inorganic filler it is often necessary to include in the
starting mixture for the glass mat-reinforced thermoplastic
material a small amount, normally up to about 4% by weight
of the total solid material, of carbon black. (Carbon black
is the term used widely to describe a range of fine carbon
products which may be made by partial combustion or thermal
decomposition of hydrocarbons in the vapour phase. The
ultimate colloidal units of carbon black may occur as
aggregates, which are fused assemblies of particles.
Typically, carbon blacks may have particlè diameters up to
500 nanometers. Most carbon blacks have an elemental carbon
composition greater than 90%.) The carbon black is
introduced not only to render the final sheet material black
~.
in colour, as is often required in commerce, but also to
provide a screening effect against the harmful action of
ultraviolet radiation on the material. However, when carbon
black is introduced into the starting mixture problems are
experienced because of the well known difficulties of
handling fine carbon black powder without creating a cloud
of black particles which settle out over a wide area. This
problem is especially severe when dry carbon black is mixed
with a dry thermoplastic material. The retention of the
~ 7
fibre carbon black in the mat during the forming
operation on the wire mesh has also been found to be
poor, and it is generally found that the carbon black is
unevenly dispersed in the final dried and pressed
material, with the result that the sheet material has a
veined or mottled appearance with dark areas having a
high concentration of carbon black alternating with areas
which are light in colour and almost translucent.
It is also often advantageous to include in the
starting mixture for the glass mat-reinforced
thermoplastic material from 0.1% to 10% by weight, based
on the weight of inorganic filler, of an antioxidant.
GB-A-2 179 665 described a process for preparing an
inorganic filler which process comprises treating a
particulate inorganic filler with a natural or synthetic
latex composition and dewatering and drying the resultant
mixture. The surface-treated inorganic filler can be
used with advantage in a polymeric resin composition.
it has now been found that an improved glass mat-
reinforced thermoplastics material may be obtained if themat of solid materials formed on the upper surface of the
wire mesh of the Fourdrinier paper making machine is
coated with a composition comprising an aqueous
suspension of a mixture of polypropylene powder,
inorganic filler granules, a foaming agent and,
optionally, additional natural or synthetic latex solids.
It has been found that when uncoated glass mat-reinforced
thermoplastic material in the form of a stack of cut
pieces is hot pressed as described above, there is a
tendency for the reinforcing glass fibres to be pressed
through the surface of the sheet material giving a round
"hairy" texture to the surface. However, if at least one
side of each mat is coated with a composition of the type
described above, and a stack of cut pieces of mat is
~.,
-8-
prepared so that a coated surface is outermost on each
side of the stack, the surface of the pressed, sheet-like
component is effectively sealed so that the glass fibres
are prevented from projecting through the surface.
The invention will now be further described with
reference to the accompanying drawings in which:
Figure 1 is a sectional elevation of a permeable
fibre reinforced plastics sheet material having a
particulate surface layer according to the invention;
Figure 2 is a sectional elevation of the sheet of
Figure 1 after consolidation under heat and pressure;
~,~,.
2 1 ~
_9_
Figure 3 is a sectional elevation of another
permeable fibre reinforced plastic sheet according to
the invention,
Figure 4 is a sectional elevation of the sheet of
Figure 3 after consolidation under heat and pressure,
Figure 5 is a diagrammatic representation of the
sequence of operations in one process for
manufacturing a fibre reinforced plastics material
and in which a particulate surface layer according to
the invention may be applied,
Figures 6 to 9 are four semi-diagrammatical sectional
elevatiGns showing four different kinds of apparatus
for forming permeable fibre reinforced plastic sheet
material according to the invention,
Figures iO to 12 are semi-diagrammatic views showing
various techniques for distribution of dry particles,
on a web of sheet material,
Figure 13 is a diagrammatic illustration of
continuous laminating process for producing fibre
reinforced plastics sheets.
Referring first to Figure 1, this shows a permeable
sheet material comprised of a substrate 2 made according to
the process described in United States Patent No. 4 734 321,
to which a particulate surface layer 3 has been bonded.
When cooled after subjection to heat and pressure, in a
chill moulding process, a consolidated sheet or moulding 4
is formed as shown in Figure 2, the moulding 4 having a
fibre reinforced base layer 5 and an unreinforced surface
layer 6, the layers 5 and 6 being derived respectively from
-10-
the layers 2 and 3 in Figure 1.
If desired, the substrate 2 may alternatively be
formed by dry laying a mixture of reinforcing fibres and
thermoplastics material generally as described in United
States Patent No. 3,328,383j a particulate layer then
being applied to the resulting substrate prior to heat
fusion into a permeable structure.
Referring now to Figure 3, this shows a sheet
structure comprising two glass fibre mats 7 between which
a layer 8 of a thermoplastics material has been extruded
so as to bond them together. A surfacing layer 9 of
particulate material is also bonded to one of the glass
fibre mats.
Figure 4 shows the construction of Figure 3 when
cooled after subjection to heat and pressure in a chill
moulding process. The thermoplastic layer 8 has filled
the interstices in the glass fibre mats 7 to form an
integrated base structure 10, a continuous surface layer
11 being simultaneously formed from the particulate layer
9.
The character of the particulate layers 3 and 9 in
Figures 1 and 3 can vary in dependence upon the character
of the substrate to which they are being applied and the
end result required in the moulded article. Thus,
particles of a mineral filler mixed with a sufficient
quantity of binder to adhere them to the surface of the
substrate may be appropriate in some cases. During the
moulding process, the filler acts to increase the
viscosity of the thermoplastic near the surface of the
base layer and resist the strike through of fibres.
Alternatively, particles of thermoplastic may be used,
which, when melted, have the effect of enriching the
surface portions of the laminate
.,,..~
-11- 2~2~ 8~
with thermoplastic and preventing the strike through of
fibres. Again, thermoplastic coated mineral particles may
be used alone or in combination with thermoplastic particles
to form the surface layer. Suitable particulate materials
are disclosed in International (PCT) Publication No. WO
88/00608 and in United Kingdom Patent Application No.
2 179 7665 A. The particulate material added as a surface
layer may also include particulate antioxidant material.
Referring now to Figure 5, this shows a sequence of
operations for manufacturing a fibre reinforced plastics
moulding. As will be subsequently expiained, the
particulate surfacing layer can be appIied at various points
in this sequence.
Figure 5 shows a Fourdrinier wire 12 on which a
flexible and permeable open sheet-like structure is formed
from reinforcing fibres and particulate plastics material,
generally as described in United States Patent No. 4 734 321
This sheet structure is passed through a heater/dryer 13 as
described in this patent and is then treated in one of three
different ways. It may be consolidated in a double belt
press 14, again as described in United States Patent No.
4 734 321. Alternatively, the permeable sheet may be pre-
heated in a heater 15 and then consolidated and re-expanded
in a nip roller system 16 of the kind described in Eu,opean
Patent Application No. 87306602 (Publication No.
0 255 316). This system produces a rigid but permeable
sheet. As a second alternative, the permeable sheet
material produced on the Fourdrinier wire 12 may be reeled
as shown at 17 for further use without any intermediate
treatment.
It will be seen that the alternative processes
proposed will result in an impermeable consolidated sheet,
2 ~
-12-
an impermeable semi-expanded sheet or a permeable flexible
(reeled) sheet. By the use of a guillotine 18 these three
forms of material may be converted into sheets as shown at
19 .
The sheets 19 are then fed sequentially through a
pre-heater Z0 prior to chill moulding in a mould 21 so as to
form a fibre reinforced plastics moulding 22. The type of
pre-heater 20 used will depend upon the nature of the sheets
being heated. For impermeable sheets, the use of infra red
heating is appropriate, but where the sheets are permeable,
flat bed or rotary through air heaters may be used.
Microwave heating may also be used for--all forms of sheet.
A surface layer of particulate material may be
applied at various positions in the sequence of operations
above described, the optimum position would depend upon both
the nature of the particulate material being applied and of
the sheet to which it is being applied.
Figure 6 shows a Fourdrinier wire 30 having a
multiple flow box 31 associated therewith. The flow box 31
feeds a mixture of foamed fibres and particulate plastics
material onto the wire 30 through a section 32 and a foamed
particulate material onto the upper surface of the web thus
laid down through a second section 33. The base and surface
layers 34 and 35 thus formed are simultaneously drained
through the wire.
Figure 7 shows a configuration similar to that of
Figure 6. In this case however separate flow boxes 36 and
37 are provided for laying down respectively the base and
surface layer from foamed dispersions.
Figure 8 shows a configuration for use in applying a
-13-
surface layer from an aqueous dispersion. Here, the base
sheet 34 is laid down from a foamed dispersion through a
flow box 36 and the surface layer is applied from an
aqueous dispersion by a curtain coater 37.
Figure 9 shows a configuration in which the layers
34 and 35 are laid down from,a multiple layer head box of
the kind sold by KMW or by Beloit under the trade-mark
"Converflo". The base layer 34 is laid down from a
foamed stock fed through the lower slide 38 of the head
box and the surface layer of particulate material is laid
down from the upper slide 39 of the head box.
Figure 10 shows a system which is usable at the
entrance to the dryer or the pre-heater shown in Figure
5. The system comprises a loss-in-weight screw or belt
feeder 40 which may be of the kind sold by K-Tron Soder
AG of Switzerland. The screw feeder supplies particulate
material to a shaker tray 41 which in turn deposits the
material on a distribution shute 42 from which it falls
onto the web 43. Subsequent heating in the dryer or
heater causes the particulate material to bond to the
web.
Figure 11 shows a similar system in which materials
such as thermoplastic fibres 45 are deposited by a
vibratory loss in weight feeder 46 onto a shute 47,
similar to the shute 42 and thence onto the web 48.
Again, the fibres are bonded to the web during passage
through the dryer or heater.
The systems disclosed in Figures 10 and 11 may also
be used at the exits to the dryer or heater provided that
the temperature of the emerging base material is
sufficient to cause the particulate material to fuse
thereto.
Figure 12 shows a system for applying particulate
r,~l
2 ~
-14-
material to a porous web within a dryer or heater. Air from
a fan 50 is circulated through a ducking circuit 51 which
includes a heating bank 52 and a heating chamber 53 through
which the web 54 passes. A screw feeder 55 feeds
particulate material 56 into the heating chamber above the
web, the material being distributed across the web by air
turbulence and simultaneously bonded thereto to form a
surface layer 57.
As shown in Figure 13, a double belt laminating
machine is employed to produce a continuous sheet 72 which
is composed of a resin and fibre glass mat. In the process
depicted fibre glass mats 71 and 71' are fed between two
laminating belts 73 and 74. Molten resin 75 is fed between
the mats 71 and 71' from an adjustable slot 76 located along
the length of an extrusion die 77. Particulate material 78
is also distributed across the surface of the mat 71' by a
powder feeder system 79, which may be of the kind shown in
Figure 10 ~bove.
Belt 74 as it passes around the roller 81 is pre-
heated by a heater 83 prior to its engagement with the
heating, press roll 85. Similarly, belt 73 is pre-heated by
heater 86 prior to its engagement with press roll 85. The
heating, press roll is equipped with heaters 88. Pressure
is applied to the laminate 72 by applying tension to the
belts 73 and 74. Tension applied to the belts 73 and 74
results in the application of radial forces on the resin mat
composite. The radial forces and the resulting pressures
assist in saturating the mats 71 and 71' with resin when
coupled with the heat applied as the tension is applied.
Roll 94 applies tension to belt 73 and roll 95 applies
tension to belt 74. The material as a compact sheet of
resin and fibre glass resulting from the passage of the
materials around the press roll 85 is then passed to a
-15~
cooling roll 90 between the belts 73 and 74 and during
its passage over this roll is partially cooled but not
completely solidified. The roll 90 is equipped with a
cooler 1 to reduce the temperature of the belts and the
resin. The sheet after leaving roll 90 in between the
belts 73 and 74 is then passed through another elongated
cooling zone 92 to further reduce the temperature of the
belts 73 and 74 to further cool and solidify the mats and
resin into sheet 72. Belt 73 is then reflexed over roll
93 for return to the tension roll 94 and belt 74 is
returned over roller 95 to the roll 81 with the product
72 being removed at the point where belts 73 and 74
separate.
The particulate material 78 may be either a mineral,
a thermoplastic, a thermosetting plastic or an
antioxidant or mixtures or compounds of such materials.
When embodied in the sheet product it has the effect, for
the reasons stated above, of preventing fibre strike
through at the surface.
A granular filler suitable for incorporating in a
composition for use in coating the surface of a glass
mat-reinforced thermoplastic material, comprises (a) a
particulate inorganic material, (b) from 0.1% to 10% by
weight, based on the weight of the inorganic material, of
fine carbon black, and (c) sufficient of a natural or
synthetic latex composition to provide from 1% to 10% by
weight of elastomer solids, based on the weight of the
inorganic material, and wherein the granules have
diameters in the range of 0.01 to 0.1 mm.
A composition, for use in coating the surface of a
glass mat-reinforced thermoplastic material according to
the invention comprises an aqueous suspension of a
polypropylene powder, a granular filler according to said
one aspect of
, -
~- 2~2~ g$'i~
-16-
the invention, and a foaming agent. Advantageously, the
composition also includes natural or synthetic latex solids
additional to that present in the granular filler.
The size of the granules which make up the filler is
important because it is necessary for the granules to be
relatively fine if the inorganic material, latex solids and
carbon black constituting the granules are to be
substantially uniformly dispersed in the surface layer of
the glass mat-reinforced thermoplastic material. Also, if
the granules are relatively fine there is less likelihood of
the carbon black being in an aggregated state and a more
intense and more uniform black pigmentation is therefore
obtained.
The filler granules may also contain from 0.1% to 10~
by weight, based on the weight of the inorganic material, of
an antioxidant for the polypropylene. The antioxidant may
conveniently be of the non-staining hindered ph_nol type.
The particulate inorganic material may be a
kaolinitic clay (e.g. kaolin or ball clay), a calcined
kaolinitic clay, a calcium carbonate/ a silicate of
aluminium and calcium (e.g. the natural calcium silicate
known as wollastonite), bauxite, talc, mica, alumina
trihydrate, silica, carbonates and hydroxides of magnesia
(e.g. natural hydrotalcite), dolomite (i.e. the natural
double carbonate of calcium and magnesium), calcium sulphate
(e.g. gypsum) and titanium dioxide. The inorganic material
may be natural or synthetic and, in particular, both natural
and synthetic forms of calcium carbonate, silicates of
aluminium and calcium, silica, carbonates and hydroxide of
magnesium, alumina trihydrate, calcium sulphate and titanium
dioxide are within the scope of this invention. Where the
material is synthetic it may be precipitated (as with
2 ~
-17-
calcium carbonate, silica and titanium dioxide). More
generally, it has been found that the inorganic materials
which are most suitable for use in the invention are those
which may be regarded as white inorganic materials. (It is
to be noted in this connection that the term "white" used in
relation to "inorganic material" does not mean that the
mineral necessarily has a pure-white colour, but that it is
substantially free of any strong non-white hue.) Many of
the inorganic materials which may be employed in the present
invention are crystalline. Preferably, the particles should
be no larger ~han about 50 microns, and most preferably no
larger than 20 microns in diameter.
The filler granules are preferably prepared in the
form of hollow microspheres and can most suitably be
produced by spray drying an aqueous suspension. Most
preferably the suspension is dewatered in a spray dryer of
the fountain nozzle type, as this type of spray dryer
enables goGd control to be coated over the size of the
microspheres produced in the desired size range of 0.01 mm
to 0.1 mm. A suitable aqueous suspension may be prepared by
dispersing together in water, which preferably contains a
dispersing agent, the inorganic material, carbon black, a
latex composition and, optionally, an antioxidant.
A preferred process for preparing the filler granules
comprises the following steps :
a) fine carbon black is mixed with water containing a
dispersing agent t~ form a suspension, preferably
containing from about 5% to about 25% by weight of
carbon black;
b) a suspension, preferably containing from 50% to 65%
on a dry weight basis, of a white inorganic material
-~ 21~21
-18-
and a dispersing agent for the inorganic material, is
mixed with the carbon black suspension formed in step
a) in proportions such that the quantity of carbon
black is preferably in the range of from 0.1% to 10%
on a dry weight basis, based on the weight of the
inorganic material;
c) there is mixed with the inorganic material/carbon
black suspension formed in step b) sufficient of a
natural or synthetic latex composition to provide
from 1% to 10% by weight of latex solids, based on
the weight of the inorganic material; and,
:
d) the mixed suspension formed in step c) is spray dried
to form hollow microspheres of substantially dry
material having diameters in the range of from 0.01
mm to 0.1 mm.
In step a) the dispersing agent may be, for example,
a water-soluble salt of poly(acrylic acid), poly(methacrylic
acid) or a copolymer containing from 10% to 80% by weight of
acrylonitrile or methacrylonitrile monomer units and from
90% to 20% by weight of acrylic acid or methacrylic acid
monomer units. The number average molecular weight of the
water-soluble polymer is preferably not more than about
-
10,000. The amount of the dispersing agent used is
preferably in the range of from 0.1% to 5% by weight, based
on the weight of dry carbon black.
In step b) the dispersing agent for the inorganic
material is advantageously a water-soluble salt of
poly(acrylic acid) or poly(methacrylic acid) having a number
average molecular weight of not more than 10,000.
Preferably, the dispersing agent used for the inorganic
-19- 2~21~
material is the same as that used for the carbon black. The
amount of the dispersing agent used is preferably in the
range of from 0.05%. to 0.5% by weight, based on the weight
of the dry inorganic material. The inorganic material does
not require any chemical pretreatment other than dispersion
with a dispersing agent.
In step c) the latex may be one of a natural rubber,
or of a natural rubber which has been substituted with
functional groups, or of a synthetic rubber such as a
styrene butadiene rubber (SBR). Other suitable lattices
include those formed from acrylic copolymers (either
elastomeric or non-elastomeric, although elastomeric is
preferred) and non-elastomeric materials such as poly(vinyl
acetate) and copolymers of vinyl acetate.
The acrylic copolymer may be a copolymer of a lower
alkyl ester of acrylic acid with a lower alkyl ester of
methacrylic acid. Particularly preferred are copolymers of
ethyl acrylate and methy methacrylate. Also suitable are
copolymers of one or other or both of a lower alkyl ester or
acrylic acid and a lower alkyl ester of methacrylic acid
with a further monomer chosen from vinyl acetate, styrene,
acrylonitrile and mixtures thereof. The alkyl chains of the
lower alkyl esters of acrylic acid and methacrylic acid
preferably have from one to four carbon atoms.
The copolymers of vinyl acetate may be those formed
by copolymerising vinyl acetate with a copolymerising
monomer chosen from a lower alkyl ester of acrylic acid, a
lower alkyl ester of methacrylic acid, styrene,
acrylonitrile and mixtures thereof.
The latex composition, which is a stabilised
suspension of polymer particles in water, typically contains
l S ~ ~
-20-
about 40% to 60% by weight of solids. The latex may be
stabilised with the aid of a surfactant or a water-soluble
colloid, although a surfactant would normally be
employed as it usually gives a latex of lower viscosity.
The mixed suspension formed in step c) may also
contain an antioxidant, suitably of the non-staining
hindered phenol type. Conventionally, such an antioxidant
is mixed with water to form a suspension containing from
about 40% by weight to about 60% by weight of the dry
antioxidant, and the suspension mixed with the suspension
formed in step c) in proportions such-as to provide from
0.1% to 10% by weight of the dry antioxidant based on the
weight of the dry inorganic material.
In step d) the inlet temperature of the spray dryer
is preferably less than 400~C, and most preferably less than
350~C, in order to avoid thermal degradation of the
elastomer material or combustion of the carbon black.
Filler granules in accordance with the invention may
be incorporated into a coating composition for a glass mat-
reinforced thermoplastic material by adding to water the
granules and the polypropylene powder in proportions ranging
from 5 parts by weight of granules to 95 parts by weight of
polypropylene to 95 parts by weight of granules to 5 parts
by weight of polypropylene. Preferably, sufficient of a
latex composition of one of the types described above is
then added to provide from 0.5 to 10 parts by weight of
latex solids per 100 parts by weight of granules and
polypropylene. Finally, a foaming agent is added: this may
be anionic, for example sodium dodecyl sulphonate, or
nonionic. The ingredients are then vigorously mixed
together to form a foam which is conveniently applied by
means of a curtain coater to the glass mat-reinforced
-21-
thermoplastic composition on the Fourdrinier paper making
machine.
The invention is further illustrated by the following
example.
Example
A carbon black powder having an average particle
diameter of 0.02 micrometer was dispersed in a quantity of
water so as to form a suspension containing 20% by weight of
the carbon black, there being dissolved in the water 2% by
weight, based on the weight of the dry--carbon black, of a
sodium polyacrylate dispersing agent having a number average
molecular weight of 1680. The carbon black suspension was
then added to a suspension containing 60% by weight of a
paper filler grade kaolin clay and 0.2% by weight, based on
the weight of dry kaolin, of the same dispersing agent as
was used for the carbon black. The kaolin clay had a
particle size distribution such that 20% by weight consisted
of particles having an equivalent spherical diameter larger
than 10 microns and 35% by weight consisted of particles
having an equivalent spherical diameter smaller than 2
microns. The carbon black suspension was mixed with the
kaolin suspension in proportions such that the quantity of
dry carbon black was 5.6% by weight, based on the weight of
dry kaolin clay.
To the kaolin/carbon black suspension thus formed
there was then added, firstly, a latex containing 50% by
weight of an elastomeric acrylic copolymer of ethyl acrylate
and methyl methacrylate in proportions such that the
quantity of the dry elastomeric copolymer was 5.6% by
weight, based on the weight of dry kaolin clay; and
secondly, a suspension containing 50% by weight of a non-
-22-
staining antioxidant of the hindered phenol type in
proportions such that the quantity of the dry antioxidant
was 1.1% by weight, based on the weight of the dry kaolin
clay.
The resultant mixed suspension was spray dried in a
fountain nozzle spray dryer the inlet temperature of which
was 300~C and the nozzle settings were such that the solid
components of the mixture were recovered in the form of dry
hollow microspheres of diameters in the range of from 0.01
to 0.1 mm.
Two coating compositions were prepared: in each case
quantities of the granules and polypropylene powder was
stirred into water followed by sodium dodecyl sulphonate as
a foaming agent and a quantity of poly(vinyl acetate) latex.
In each case the ingredients were vigorously mixed to
produce a foam which was applied to the upper surface of a
glass mat-reinforced polypropylene material on the
travelling wire mesh belt of a Fourdrinier paper making
machine.
The two coating compositions contained, respectively,
30 parts by weight of filler granules to 70 parts by weight
of polypropylene powder and 50 parts by weight of filler
granules to 50 parts by weight of polypropylene powder. In
each case, after the coated glass mat reinforced
thermoplastic material had been cut into pieces and the
pieces stacked and pressed at 200~C between steel plates and
allowed to cool in the press, the resultant sheet material
had a smooth surface with a uniform black colour and no
protruding glass fibres.
