Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF MAKING A WATER LAID FIBROUS WEB CONTAINING
ONE OR MORE FINE POWDERS
This invention relates to a method of making a water
laid fibrous web such as a paper web or a web of plastics
material and reinforcing fibres for consolidation or moulding
into a fibre reinforced plastics sheet or article.
In paper webs, it is frequently necessary to incorporate
particulate materials or powders such as pigments and
fillers. In the case of webs of plastics material and
reinforcing fibres one or more kinds of particulate plastics
material may be included, together with materials such as
pigments, fillers and antioxidants in the form of powders of
substantially smaller particle size than the plastics
material.
The process for making such water laid webs requires as
a prerequisite the formation of an agueous dispersion of the
fibres and particulate materials from which the web is to be
formed. Preferably, a foamed dispersion is used as described
in the Applicants' co-pending European Application No.
85300031.3 (European Patent Publication No. 0 148 760). The
dispersion so formed is then drained on a foraminous support
such as the Fourdrinier wire of a paper machine, to form the
web.
Two problems arise in the mechanism of more than one
particulate material in an aqueous or foamed dispersion as
referred to above.
First, the electrochemical conditions within such
dispersions make it difficult to achieve a homogeneous
mixture of the various components within the dispersion,
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and this is reflected as a lack of homogeneity in the web as
laid down on the foraminous support.
Secondly, there will be a tendency for the particulate
material to be lost during the wet laying process depending
on the relative dimensions of the powder particles and the
apertures in the foraminous element, for example the mesh
size of a Fourdrinier wire.
When certain particles or fibres are dispersed in
water, it is thought that an aqueous film forms around each
individual particle or fibre and sets up an electro-chemical
regime such that other particles are repelled. As a resul~,
when fine powders are added individually, they do not
agglomerate either with themselves or with other solid
components of the dispersion. Thus, when the dispersion is
laid down on the Fourdrinier wire, the fine particles pass
through the wire with the water as drainage.
The invention is particularly concerned with a
technique which will improve the homogeneity of the
dispersion and resulting sheet, and which, where larger and
smaller particles need to be incorporated in the sheet, will
increase the retention of the smaller particles where the
relative mesh size is such that they would otherwise be
substantially lost in drainage through the foraminous
element.
Accordingly the present invention is a method of making
a web of reinforcing fibres containing in its interstices
particles of at least one plastics material coated with at
least one kind of powde~ed material, comprising the steps of:
mixing the plastics particles in a solidstate and
powdered material together in a substantially dry condition
to cause the powclered material to coat the plastics
particles,
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dispersing said coated plastics particles, together
with the reinforcing fibres in water to form an aqueous
dispersion, and
draining the aqueous dispersion to form a web.
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Thus, if a fibrous reinforced plastics material web is
being made, the particulate materials can be a mixture of
particulate thermoplastics. Alternatively a thermoplastic or
a thermosetting polymer, can be mixed with a fine powder such
as carbon black or titanium dioxide, the dry mix being
dispersed in an aqueous dispersion of fibres prior to
drainage to form a web. The fibres may, for example, be
glass fibres.
The method of the invention is suitable for use with
the process for forming a fibrous web set forth in the
Applicants' co-pending European Application No. 85300031.3
tEuropean Publication No. 0 148 760).
Again, the invention can be employed in the manufacture
of a web in which the particulate material is a fibre, the
dry mix being used to form an aqueous dispersion of fibres
and drained to form a web.
The method can also be employed where two or more
different fibres are to be incorporated in a web, with one or
more of the fibres acting as the particulate material which
is dry pre-mixed with the dry fine powder before an aqueous
dispersion is formed.
It has been found that when mixed dry the homogeneity
of the web is improved. Also, it has been found that dry
mixing causes fine powder to adhere to substantially coarser
particulate material such as thermoplastic, and that this
adhesion persists when the dry mix becomes part of or forms
the aqueous dispersion. Since the fine powder thus becomes
part of a substantially larger agglomerated component of the
dispersion, it is retained in the web.
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It has also been found that the invention provides
a successful method of incorporating a fine pigment
powder in the web so as to effect a very uniform
coloration of mouldings produced using the fibrous web
produced by the process, although cther fine powders for
other purposes such as antioxidants can of course be
employed a]ternatively or in addition to pigments.
It will be appreciated that the method can also be
employed in the manufacture of paper to enhance the
retention of powdered additives.
The fine powder and particulate materials can
conveniently be pre-mixed in mixers are of the high
shear type.
The invention can be performed in various ways and
one embodiment will now be described by way of example
and with reference tc the accompany flow chart, which
shows the benefits of the present invention by
comparison with known technology.
The flow chart relates to the production of a
foamed aqueous dispersion for use in the manufacture of
c! pigmented web of reinforcing fibre and particulate
plastics material as described clnd claimed in European
Patent App]i_ation 85300031.3 (Publication No.
0 148 760).
The technique shown may be used for theproduction
of dispersions of two or more particulate plastics
materials so as to achieve a homogeneous mix-ture or of
one or more particulate materials with much finer
powders of, fc,r example, pigments or antioxidants so as
to achieve both homogeneity and retention of the powder
ciuring the wet web laylng process.
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However to illustrate the invention in a
comprehensive manner the flow chart shows the technique
of the invention as used for mixing two particulate
polymers of relatively coarse particle size, for
example, 100 to 500 microns, with a pigment having a
particle si~e of, for example, 10 to 100 microns.
As shown in solid lines in the flow chart, the two
particulate plastics materials and the pigment are first
charged into a high shear powder blender 1, for example
a Winkworth Ribbon Refiner Batch Mixer, a Gloucester
Materials Handling High Shear Batch Blender or a
Continuous Gerick E Blender (Powteck Type GAL 351).
Microdiagram 2 shows that after blending the two
particulate plastics materials 3a and 3b are evenly
mixed and that they are both coated with the pigment 4.
The pigment coated particles 3a and 3b are
transferred to a hydropulper 5 in which a foamed aqueous
dispersion of glass fibres 6 has previously been formed.
Because of the homogeneous mixing which has previously
taken place in the mixer 1, the homogeneity of the
dispersion of fibres and particles thus formed in the
hydropulper 5 is assured. Furthermore, substantially
all of the pwdered pigment 4 continues to adhere to the
larger polymeric particles 3a and 3b, as best seen in
microdiagram 7.
The foamed aqueous dispersion formed in the
hydropulper 5 is then used in the formation of a wet
laid web in the process 8 which is carried out according
to the process described in the aforementioned European
Patent Application. The resulting web comprises well
distributed glass fibres and polymeric particles, with
the polymeric particles still retaining adherent
pigments as best seen in microdiagram 9.
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The formed web is then passed to drying, pressing
and moulding stages 10 as required, which do not form
part of this invention.
If the two particulate polymers 3a and 3b and the
pigment 4 are added directly to the hydropulper 5 as
indicated by the broken lines 11 and not premixed in a
substantially dry state in the mixer 1, it is more
difficult to achieve homogeneous mixing with the fibres
6 in the dispersion. Furthermore, the powdered pigment
becomes dispersed as individual particles in the foamed
dispersion, as best seen in microdiagram 12. As a
result they tend to be lost in drainage during the wet
laying process, so that few or none remain in the formed
web, as shown in microdiagram 14.
15 It will be appreciated that the materials whicn can
be premixed as described are not limited to particulate
polymers or pigments. Wood or glass fibres, clays and
other fillers are understood as falling within the scope
of the term "particulate materials" which can be so
premixed.
Where plastics materials are to be included, they
may comprise thermoplastics or thermosetting plastics
particles of various kinds alone or as blends with other
~lastics for example as follows:
Acrylonitrile-butadiene-styrene with Polyvincylchloride
Acrylonitrile-butadiene-styrene with Polypropylene
Polyphenylene ether with Polypropylene
Polyphenylene ether with Polyamide
Polycarbonate with Polyalkylenetere-
phthalate
Polycarbonate with Polyestercarbonate
PolyVinylchloride with PhenolforD1aldehyde
Polypropylene ~ with Lignin
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Where such polymers are incorporated alone or as
blends, finely powdered antioxidants may be adhered to
them by premixiny as above described, together with, if
desired, plgments such as carbon black or titanium
dioxide or fillers such as calcium carbonate.
Alternatively, or in addition, one of two particulate
polymeric plastics to be included may be first ground to
a smaller dimension and then adhered to the other
plastic by premixing as above described.
As referred to earlier the particulate material can
be the fibres themselves and thus the invention can be
used in a paper making process by mixing the fine
powder, again, for example a colouring pigment, with the
fibres, for example, cellulose fibres in dry form and
then using the mix to form an aqueous dispersion, laying
this on a wire and forming a paper web in the usual
manner.
The process can also be used if more than one kind
of fibre is employed in a web, the fine powders being
dry mixed with one or more of the fibres to be used
prior to forming the aqueous dispersion.
