 |
Description  |
|
|
The present invention relates to an article for wiping a surface, for
example, the surface of a household or industrial object, or the human
skin, in order either to deliver a liquid active material to that surface
or to pick up liquid from that surface; or for gradually releasing an
active material, such as a bubble bath composition, an air-freshener or a
perfume, without surface contact. The article includes a substrate which
in one embodiment of the invention carries a liquid active material, for
example, a detergent, or a skin treatment material.
Various wet tissues and towelettes are available in the market for various
purposes for example, for personal cleansing or baby hygiene. Artcles of
this type have to have quite a high liquid content if they are to give
adequate cleansing, and thid means that moistrueproof packaging is
essential. One approach to this problem is to package the articles
individually in moistureproof sachets, as is done, for example, with the
moist towelettes provided by airlines. This is, however an expensive
solution. More recently thers have appeared on the retain market packs of
wet tissues for personal cleansing in which a roll of moist tissue
perforated at suitable intervals is contained in a moisture tight
container with a tight closure through which tissues can be drawn out and
torn off. These containers are generally of fairly elaborate design and
are expensive to produce.
The need for moistureproof packaging can be obviated if the liquid is
carried on the substrate in a protected form so that the overall article
is dry up to the point of use. One method of protecting the liquid is to
encapsulate it in microcapsules which can be incorporated into or coated
onto a substrate and which can be ruptured by the application of pressure,
as described, for example, in GB No. 1 304 375 (L'Oreal).
We have now discovered a convenient alternative method by means of which a
substrate article may be produced which has a high liquid content yet
which may be dry up to the point of use. This is achieved by including the
liquid in a porous polymeric material from which it can be released only
by the application of pressure. Furthermore, a substrate carrying such a
porous polymeric material in the dry state, that is to say, without an
included liquid, can be useful as an absorbent wiper for mopping up spilt
liquids.
Accordingly, the present invention provides an article suitable for
delivering or absorbing a liquid, the article comprising a substrate
carrying a pressure-sensitive porous polymeric material capable of
retaining at least 5 times, and preferably at least 10 times, its own
weight, defined in terms of water, of liquid, and of releasing at least
some of that liquid on the application thereto of hand pressure, the
porous polymeric material being dry or containing an aqueous or
non-aqueous liquid.
The present invention thus has two main aspects within this generic
concept. The first is an article suitable for absorbing liquid, for
example from a surface, said article comprising a substrate carrying a dry
porous polymeric material capable of absorbing and retaining at least 10
times its own weight, defined in terms of water, of liquid.
This article according to the invention has an exceptionally high
absorbency for liquids and is thus useful for mopping up liquid spillages.
The second aspect of the invention is an article suitable for delivering a
liquid active material, said article comprising a substrate carrying a
porous polymeric material capable of retaining at least 10 times its own
weight of liquid, defined in terms of water, and of releasing at least
some of that liquid on the application thereto of hand pressure, the
porous polymeric material containing an active liquid material.
The term "active liquid material" is used to indicate a liquid that can
usefully and beneficially be delivered by the article of the invention.
The polymer/liquid composite included in the article of the invention (in
its second aspect) preferably consists to an extent of at least 90%, more
preferably at least 95%, by weight, of liquid.
The polymer is such that the liquid contained in it remains enclosed within
the polymer unless expressed by the application of hand pressure; the
liquid-containing polymer can consist of up to 98% by weight of liquid
while feeling virtually dry to the touch. Thus an article containing a
high proportion by weight of entrapped liquid can be produced. The liquid
can be hydrophobic or hydrophilic depending only on the intended use.
Articles of this general type may be used for many purposes, for example,
hand and face cleaning; skin treatment other than cleaning (for example
anti-acne treatment); baby hygiene; cleaning, polishing, disinfecting or
deodorising industrial and domestic surfaces (for example, windows,
paintwork, machinery, carpets, clothing, shoes); air freshening and
perfume delivery; and hospital hygiene.
The article can remain dry during handling and storage, until the liquid in
the polymer is released at the point of use by the application of
pressure. It is also within the scope of the invention for the article to
be wet, for example, impregnated, either with the liquid contained in the
polymer or with a different liquid. If a second liquid is present, this
may not necessarily be compatible with the first, since mixing will not
occur until the polymer is squeezed in use. As mentioned in more detail
below, an article of the invention may include a plurality of separate
regions of polymeric material containing the same or different liquids,
and any additional liquid present outside the polymer may be the same or
different from any of the polymer-included liquids.
One or more further liquids may if desired be present in microencapsulated
form. This is especially advantageous in the case of mutually incompatible
liquids.
Preferably the porous polymeric material is capable of retaining at least
15 times, more preferably at least 25 times, and especially at least 40
times, its own weight of liquid, defined in terms of water. It will not
necessarily be capable of absorbing these quantities of all types of
liquid spontaneously; in some cases suction may be necessary to assist the
introduction of liquid. It is, however, essential that once the liquid is
inside the pore system of the polymer it remains there unless the polymer
is squeezed, apart of course from the unavoidable slow loss of liquid by
evaporation.
It is thus essential that the porous polymeric material combines a high
capacity for liquid with an ability to retain the liquid unless subjected
to pressure. There must be little or no tendency for the liquid to run out
of the polymer under the influence of gravity; the liquid should remain
distributed throughout the material until expressed by pressing or
squeezing the material at hand pressure. After the liquid has been
expressed, the material may not regain its original shape or pore
structure.
This combination of properties may be found in a material having relatively
large voids interconnected only by relatively narrow passageways. On
squeezing there is a partial collapse of the structure allowing escape of
the liquid.
The polymer preferably has a pore volume greater than 9 cc/g, more
preferably greater than 15 cc/g.
The dry density of the polymer is preferably less than 0.1 g/cc, more
preferably within the range of from 0.03 to 0.08 g/cc. This is the density
of the material when its pore system contains air. Some polymers that can
be used in the article of the invention, however, cannot exist in the dry
state; they are prepared by methods which leave the pore system full of
liquid, and this liquid can if desired be exchanged for another liquid,
but if dried their pore system collapses. Such materials are useful in the
second aspect of the invention although inherently unsuitable for use in
the first aspect of the invention.
Various polymers suitable for use in the present invention have been
described in the literature. Techniques for the production of porous
polymers range from what is termed, in the art, classical phase inversion,
to nuclear bombardment, the incorporation of microporous solid particles
in a matrix material followed by the leaching out of the particles, and
the sintering together of microporous particles.
The porous polymeric material used in the article of the invention may if
desired be in the form of beads, either discrete or coalesced. Such porous
polymer beads are disclosed, for example, in GB No. 1 513 939
(Ceskoslovenska Akademie Ved); they are prepared by dissolving the polymer
to be used in a solvent and then dispersing the solution into a compatible
carrier liquid, and adding this mixture to a coagulating liquid such as
water to precipitate the porous beads of polymer. If desired, the beads
may be subsequently coalesced to form a moulding.
Preferably, however, the porous polymeric material that forms part of the
article of the invention is, at least initially, in the form of
homogeneous block or sheet material. Such material has the advantage that
it will be substantially homogeneous or uniform in its porosity, and will
then deliver or absorb liquid in a uniform and predictable manner. The
polymeric material in block or sheet form may of course be cut down into
smaller pieces, or even ground into powder, before use. In the case of a
liquid-containing polymer, this will entail some loss of liquid but this
can generally be tolerated.
One homogeneous porous polymeric material suitable for use in the article
of the invention is described and claimed in GB No. 1 576 228 (Akzo). This
patent specification discloses thermoplastic microporous cellular
structures comprising microcells (pores) having an average diameter of
0.5-100 .mu.m connected by smaller-diameter passageways, the size
drstribution of the latter being a defined function of the average cell
diameter. The structures are composed of a synthetic thermoplastic
homopolymer or copolymer of an ethylenically unsaturated monomer, or of a
condensation polymer, or of a polyphenylene oxide, or of any blend of
these. The structures are prepared by dissolving the thermoplastic polymer
in a suitable solvent at elevated termperature, cooling the solution to
solidify the polymer, and then removing the liquid from the resulting
solid structure. These materials generally have void volumes of 70-80%,
and can retain about 5 times their own weight of liqiid (defined in terms
of water). The Akzo process is of course limited in its application to
thermoplastic polymers, and to polymers that can readily be dissolved in
appropriate solvents, but within these limits yields materials highly
suitable for use in the article of the invention.
An alternative route to porous polymeric materials having the requisite
pore and passageway structure involves solution or emulsion polymerisation
of an organic film- c forming polymer under controlled conditions. In
particular, according to a highly preferred embodiment of the invention,
the polymer may be prepared by polymerisation of a high internal phase
emulsion in which the internal phase is aqueous and the continuous phase
comprises one or more polymerisable monomers. This method can give
polymers capable of retaining at least 10 times their own weight of liquid
(defined in terms of water).
The higher the proportion of the aqueous internal phase in the starting
emulsion, the higher will be the void volume in the final polymer. Thus
the aqueous phase preferably constitutes at least 90%, more preferably at
least 95%, by weight of the emulsion.
It has been observed from microscopic inspection of samples of the porous
polymer made by this method that it essentially comprises a series of
substantially spherical, thin-walled cavities having a plurality of very
small holes in the walls interconnecting the adjacent cavities. Frequently
six or more holes can be seen in the cavities on inspection of electron
micrographs of polymer samples. It has been determined that the liquid
absorbency and retention capacity is related to the size of the cavities,
expressed in terms of void diameter, and the number and size of the holes
in the cavity walls, expressed in terms of pinholes. In general it is
desirable that the average pinhole diameter should not be less than 0.5
.mu.m and the void diameter should be at least 20% greater than that
figure.
The polymeric material is advantageously crosslinked. Crosslinking
apparently improves the capacity for absorption and retention of liquids
and also gives greater dimensional stability.
In the high internal phase emulsion, the continuous phase comprises the
monomer(s), and a surfactant (as emulsifier) and a polymerisation catalyst
are also present. Preferably the amount of surfactant present is from 5 to
30% by weight, based on the total monomer, and the amount of catalyst
present is from 0.005 to 10% by weight, based on the total monomer.
The mechanism by which the holes form in the thin-walled cavities is not
fully understood. However, experimental work suggests that it is related
to the quantity of surfactant present and its compatibility with the
cross-linked polymer and, hence, also, to the degree of cross-linking in
the polymer. It is thought that prior to polymerisation the high internal
phase emulsion consists of three main elements monomer and surfactants in
the continuous phase and water in the internal phase. The continuous
phase, which consists of a homogeneous solution of surfactant and the
monomer and cross-linking agent and, in this situation, the surfactant is
compatible with the monomer mixture. It is thought that at this stage
there are no interconnecting holes present in the external phase. During
polymerisation chain propagation takes place and as the surfactant is not
polymerisable and has no reactive sites in its structure, it cannot take
part in the polymerisation reaction. As a result, the surfactant molecules
separate because the surfactant is no longer compatible with the growing
polymeric structure and is also insoluble in the water phase. Due to the
nature of a surfactant, the aggregated molecules of surfactant remain part
of the polymer phase and probably cause the production of weak spots and
subsequent pinhole formation in the cross-linked polymer film.
Another factor affecting the structure of the porous cross-linked polymer
is the structure of the high internal phase emulsion from which it is
formed. This can most readily be defined in concepts of viscosity and
Table I and II indicate the effect of stirrer speed on two typical
emulsions and show the viscosity of emulsions produced at different
stirrer speeds and the detailed structure of the cross-linked porous
polymers produced from the emulsions at the different stirring rates.
The basic emulsion used in the work shown in Table I contained 10 ml
styrene, 1 ml divinyl benzene and 2 grams of Span (Trade Mark) 80 and 200
ml water containing 0.2% sodium persulphate. The emulsion used for the
work in Table II was the same except that 300 ml water were used and in
each case the preparation was carried out generally as described in
Example I below.
The emulsions were prepared at stirrer speed of 200 rpm and after all the
components had been mixed the samples of the emulsion were stirred at the
speeds shown in the Tables for 30 minutes prior to cross-linking to yield
the porous cross-linked polymer samples.
Viscosity measurements were made used using Brookfield Viscometer fitted
with a `C` spindle at, as shown in the Tables, 10 and 20 rpm.
TABLE I
______________________________________
Structure of
Viscosity of emulsion cross-linked
for polymerisation polymer (average)
10 RPM 20 RPM Intercon-
Motor Visco- Visco- Sphere
necting
Speed meter .times. 10.sup.3
meter .times. 10.sup.3
size pore
(RPM) Reading poise Reading
poise (um) size (um)
______________________________________
200 12.3 12.3 14.5 7.3 38.4 5.3
300 21.8 21.8 24.5 12.3 25.1 4.1
500 23.2 23.2 26.5 13.3 15.4 2.8
800 50.8 50.8 55.0 27.5 9.1 1.6
1000 60.8 60.8 69.9 35.0 8.1 1.4
2000 100+ 100+ 7.1 1.0
______________________________________
TABLE II
______________________________________
Structure of
Viscosity of emulsion cross-linked
for polymerisation polymer (average)
10 RPM 20 RPM Intercon-
Motor Visco- Visco- Sphere
necting
Speed meter .times. 10.sup.3
meter .times. 10.sup.3
size pore
(RPM) Reading poise Reading
poise (um) size (um)
______________________________________
200 7.1 7.1 8.0 4.0 45.8 5.4
300 13.5 13.5 15.0 7.5 20.0 4.0
500 18.8 18.8 21.5 10.8 17.1 2.4
800 34.9 34.9 42.4 21.2 11.7 1.5
1000 39.7 39.7 46.6 23.3 8.4 1.5
1500 43.4 43.4 54.1 27.1 9.0 1.3
2000 55.6 55.6 61.8 30.9 7.7 0.95
______________________________________
It will be seen from the tables that the emulsion viscosity has a clear
relationship with the pore or cavity size of the cross-linked polymer and
with the size of the holes or interconnecting passages between the
cavities. Clearly by selecting the appropriate stirrer speed and hence
viscosity of the emulsion the size of the cavities in the cross-linked
polymer can be quite closely controlled.
In general it will be noted that the ratio of sphere or cavity size to the
size of the interconnecting pore or pinhole is of the order of 71.
Various monomers may be used in the preparation of those porous polymers by
the emulsion method. Vinyl monomers are preferably used, styrene being
especially preferred. If the polymer is to be lightly cross-linked, a
cross-linking monomer is included in the starting emulsion. A preferred
polymer is a lightly cross-linked polystyrene containing a small
proportion of divinylbenzene. Polymeric materials may also be made using
various acrylate polymers, for example, polymethyl or polybutyl
methacrylate, cross-linked with, for example, allyl methacrylate.
Preferably, the polymerisation catalyst is in the water Phase and
polymerisation occurs after transfer of the catalyst into the oil phase.
Alternatively, the polymerisation catalyst may be introduced directly into
the oil phase. Suitable water-soluble catalysts include potassium
persulphate and various redox systems such as ammonium persulphate
together with sodium metabisulphite. Monomer soluble catalysts include
azodibisisobutyronitrile (AIBN), benzoyl peroxide and
di-2-ethyl-hexyl-peroxydicarbonate. The temperature at which the
polymerisation is carried out can be varied fairly widely between about
30.degree. and 90.degree. C., but is clearly related to the particular
catalyst initiator employed.
The surfactant used in making the high internal phase emulsion which is to
be polymerised is fairly critical, although the long-term stability of the
high internal phase emulsion is not an important factor provided that it
is long enough to maintain stability during polymerisation. Using the well
known HLB terminology in relation to the surfactants, it is desirable that
the surfactant has an HLB value of less than 6 and more than 2, preferably
about 4. Providing the HLB criterion is met, many surfactants can be used
in the preparation of the porous polymers. Amongst those suitable may be
included:
______________________________________
HLB
______________________________________
Nonionic
Sorbitan monoleate ("Span"(Trade Mark) 80)
4.3
Glycerol monoleate 3.8
Glycerol monoricinoleate 4.0
PEG 200 dioleate 4.6
Partial fatty acid esters of polyglycerol
(Admul (Trade Mark) Wol 1403 ex Food
Industries Limited of Bromborough, England)
Castor oil 5-10 EO 3-6
Cationic
Distearyl dimethyl ammonium chloride
5-6
Dioleyl dimethyl ammonium chloride
5-6
Anionic
Bis-tridecyl sulphosuccinic acid (Na salt)
5-6
Amphoteric
Alkylbenzene sulphonate/C.sub.18 amine oxide complex
______________________________________
Experimental work has shown that the amount of surfactant in the system is
critical and htat if insufficient surfactant is employed the cavities have
insufficient holes to generate the desired absorbency. the optimum
concentration of surfactant by weight of monomers is of the order of 20%,
but useful results can be obtained in the range of 5 to 30% and
preferably, 15-25%.
The polymers used in the article of the invention may be prepared by first
forming a water-in-oil high internal phase emulsion system where the oil
phase is constituted by the monomer or mixture of monomers, together with
a small amount of a cross-linking agent. The polymerisation initiator or
catalyst can be dissolved in either the water phase or in the oil
(monomer) phase. The high internal phase emulsion system is prepared by
the slow addition of the aqueous internal phase to the oil (monomer) phase
in which the emulsifying agent (surfactant) is preferably dissolved, using
a moderate shear stirring. Conveniently the container in which the
polymerisation is carried out is enclosed to minimise the loss of volatile
monomers and the emulsions are thermally polymerised in the container.
This process gives a polymer in which the void system contains an aqueous
liquid--the internal phase of the original emulsion. If desired, this
liquid can be readily removed by subjecting the polymer to a vacuum or
leaving the material to dry in a dry atmosphere at between about
30.degree. and 60.degree. C. The dry polymer thus obtained may be used to
form a dry article according to the first aspect of the invention, which
as indicated previously, is very useful for mopping up spillages of
hydrophobic liquids. One polymer which is described and claimed in British
Patent Application No. 81 07658 and European Patent Application No.
82301199.4, the disclosures of which are hereby incorporated by reference,
is exceptionally useful for absorbing hydrophobic liquids and has an
absorbency for such liquids, defined in terms of oleic acid, of at least 7
cc/g.
In articles according to the second aspect of the invention, the void
system of the polymer contains a liquid. Starting from a high internal
phase emulsion, the liquid-containing polymer may be prepared in three
ways:
(a) a dry polymer may be prepared as described above, and the desired
liquid subsequently introduced;
(b) the liquid initially present in the polymerised high internal phase
emulsion may be exchanged for the desired liquid;
(c) the desired liquid may itself be used as the internal phase of the
emulsion.
When method (a) is used, the polymer may spontaneously take up the desired
liquid if the polymer is of a type which has a high absorbency for the
liquid in question. Otherwise, introduction of the liquid may be
vacuum-assisted.
Method (b) is a direct substitution of the desired liquid for the original
internal phase of the emulsion without an intermediate drying step. The
polymer is preferably washed before the introduction of the desired
liquid, in order to remove traces of the materials present in the original
internal phase, notably the surfactant. Washing with a solvent such as a
lower alcohol is highly effective. Where the desired liquid is a detergent
composition, the desired liquid may itself be used for the preliminary
washing step, although it may then be necessary to wash at a higher than
ambient temperature, for example, 50.degree. C. Liquid exchange may be
carried out as a continuous, vacuum-assisted operation.
Method (c) is of course suitable only for certain aqueous liquids that will
not destabilise the high internal phase emulsion. In particular, it is not
suitable for liquids containing high-HLB surfactants, as do most detergent
compositions. One class of liquids that is suitable for inclusion by
method (c) is comprised of aqueous solutions of oxygen bleaches,
especially hydrogen peroxide-based bleaches.
In the article of the invention the polymer is carried by a substrate. The
substrate may be any suitable carrier material that gives integrity to,
and provides protection for, the polymer. For convenience of handling, it
advantageously comprises one or more layers of flexible sheet material, or
a sponge or pad. The substrate is advantageously porous to allow liquid to
pass through, and may advantageously.be absorbent. In the first aspect of
the invention an absorbent substrate adds to the overall absorbency of the
article; and in the second aspect of the invention an absorbent substrate
will become impregnated with the liquid as the latter is expessed from the
polymer and can assist in its distribution, for example, on a hard surface
being wiped. Alternatively, as previously mentioned, an absorbent
substrate may be impregnated with further liquid which may be the same as,
or different from, that included in the polymer. A preferred substrate
according to the invention includes one or more sheets of fibrous
material, especially wet-strength paper or woven, knitted or nonwoven
fabric.
According to a preferred embodiment of the invention (in both aspects) the
polymer is completely surrounded by the substrate. Thus the polymer, in
the form of beads or a solid block, sheet or film, may be inside a sachet.
At least one wall of the sachet must be permeable to liquid in order to
allow the passage of liquid into or out of the polymer; thus at least one
wall is of inherently permeable material and/or contains openings.
Advantageously the sachet walls (substrate) may be formed of a nonwoven
fabric/plastic film laminate, at least one of the walls being provided
with one or more perforations to allow the passage of liquid.
Advantageously the article of the invention may consist of a plurality of
cells or compartments each of which is in effect a sachet as described
above. This type of article may comprise a first substrate layer and a
second substrate layer so bonded together as to create a plurality of
compartments therebetween, at least some of said compartments containing
the porous polymer and at least some of said compartments being
liquid-permeable.
Advantageously, at least some of the compartments are provided with one or
more perforations.
In use, the polymer itself remains within the compartments but liquid can
pass out of or into it through the substrate walls or by way of the
perforations.
Advantageously different compartments of the article are provided with
different numbers of perforations to allow differing rates of passage of
liquid. This is especially advantageous in the case of articles according
to the second aspect of the invention, in that it allows for controlled
release of the liquid over a relatively long period. This embodiment also
allows for the use of polymers containing different liquids in different
compartments for release at different rates.
Advantageously the substrate layers include heat-sealable material. The two
layers can then be bonded together by welding, for example, by heat
sealing or ultrasonic sealing, around the porous polymer. Nonwoven fabric
including some thermoplastic fibres, and nonwoven fabric laminated with
thermoplastic film, may advantageously be used.
If the porous polymer is in discrete form, such as beads, these may be
sprinkled onto the first layer and the second layer subsequently
heat-sealed to the first. This process may be carried out continuously,
for example, using hot rollers.
The porous polymer may, however, be in continuous (block, sheet or film)
form. A block should first be cut into sheets. If the polymer is itself
heat-sealable, a sheet or film may be interposed between two layers of
substrate laminate and the whole heat-sealed together, in a continuous
operation, for example, using hot rollers.
If the porous polymer is not heat-sealable, it may first be cut into
compartment-sized pieces, arranged on one substrate layer using a
grid-patterned mask to aid positioning, and the second substrate layer
then heatsealed to the first between the polymer pieces.
The perforations may be made at any suitable stage in the proceedings.
Pre-perforated substrates may if desired be used; this of course requires
matching of the perforation pattern to the pattern of bonding between the
substrates. Alternatively, the compartments may be perforated after the
active material/substrate composite has been made up. In a batch process,
perforation may be carried out using a syringe needle.
In a continuous process as mentioned above, the perforations may be made on
one or both sides of the article, after the two substrates have been
bonded together, by passing the composite article over a roller carrying
appropriately spaced pins.
The perforations may be as small as desired, but will generally be at least
0.01 mm in diameter, preferably at least 0.1 mm. Perforations of from 0.2
to 1.2 mm are preferred, especially from 0.5 to 1.0 mm. Of course
relatively large perforations are suitable only when the active material
is not very mobile or is protected as indicated previously.
The distribution of perforations depends on the size of the compartments as
well as on the desired rate of release of the active material. The
compartments preferably have areas ranging from 0.5 to 5 cm.sup.2, more
preferably from 1 to 3 cm.sup.2 and especially from 1.5 to 2.5 cm.sup.2.
The compartments may be of any convenient shape; for ease of manufacture
the bonding lines separating them are preferably straight and hence
parallelogram shapes, such as square, rectangular, rhomboidal (diamond)
and the like, are especially preferred.
The average distribution of perforations is advantageously less than
5/cm.sup.2, and preferably lies between 0.5 and 3/cm.sup.2. Practicable
rates of release of most liquids can be obtained with average perforation
levels within this range. Of course the distribution of perforations among
the compartments may be either regular or irregular as desired.
In articles according to the second aspect of the invention, where the
polymer contains a liquid, the liquid can be any that will deliver a
benefit, as previously indicated; it may be hydrophobic or hydrophilic.
Examples of such liquids include soap and detergent compositions, bleach,
disinfectant, bubble bath and shower preparations, air fresheners, skin
treatment agents, dry cleaning solvents, perfumes, and many more.
In one particular embodiment of the second aspect of the invention, the
liquid is a cleaning composition that will give substantially streak-free
cleaning of reflective household surfaces such as mirror, tiles, paintwork
and furniture.
Such an article has the advantage that it can be applied directly to the
surface to be cleaned; the surface need only wiped over and then allowed
to dry. No additional liquid and no cloths or tissues are required; thus
contamination by streak-forming impurities is eliminated.
In this embodiment the liquid in the void system of the porous polymer is a
homogeneous aqueous liquid composition having a surface tension of less
than 45 mNm.sup.-1, preferably less than 35 mNm.sup.-1, which composition,
when applied to a surface and allowed to dry, dries substantially without
forming discrete droplets or Particles larger than 0.25 .mu.m.
The formation of discrete droplets or particles larger than 0.25 .mu.m on
drying causes scattering of visible light (wavelength 0.4-0.7 .mu.m),
which is perceived by the eye as streaking. Preferably the liquid
composition dries substantially without forming discrete droplets or
particles larger than 0.1 .mu.m.
In this embodiment it is essential that both the substrate and the polymer
be substantially free of streak-forming impurities which might be leached
out by the liquid composition and deposited on the wiped surface as
streaks. The porous polymers themselves have been found to give no
streaking problems provided that they are thoroughly washed (see
previously) before introduction of the streak-free liquid composition.
Some substrates may inherently be free of such impurities; many papers or
nonwoven fabrics, however, contain binders and some of these can cause
streaking problems. Traces of bonding agent, size, clays, fluorescers,
fibre lubricants, emulsifiers or other processing materials may also be
present in papers and nonwoven fabrics and these can also cause streaking.
Accordingly the substrate is preferably pretreated to remove any materials
associated therewith that might cause, or contribute to, streaking. The
treatment may conveniently comprise prewashing the substrate with a
solvent capable of removing the impurities, before the application of the
liquid composition. In some cases washing with hot to boiling
demineralised water may be necessary, while in others a pre-soaking in an
excess of the liquid composition itself may suffice. Some binders used in
paper and nonwoven fabrics, notably crosslinked katpolyalkylimine, do not
appear to cause streaking problems, and substrates in which only this type
of binder is present may not require a prewashing treatment.
The homogeneous aqueous liquid composition for streak-free cleaning may
contain, as well as water, one or more water-miscible solvents, but the
amount of non-aqueous solvent generally should not exceed 35% by weight,
and is preferably within the range of from 0.1 to 15% by weight. Larger
amounts of solvent can cause safety problems and may damage certain
surfaces such as plastics or paintwork; the presence of limited amounts of
solvent is however advantageous in decreasing the drying time of the
composition and in facilitating the removal of oily soil.
Typical examples of suitable solvents are the lower aliphatic
water-miscible alcohols such as ethanol, propanol, isopropanol, butanol
and so on. Other alcohols, such as tetrahydrofurfurol, may also be used.
Glycols such as ethylene- and propylene glycol and glycol ethers, such as
the mono- and dimethyl-, -propyl, -isopropyl, -butyl, -isobutyl ethers of
di- and triethylene glycol and of analogous propylene glycols may also be
used. The preferred solvents are C.sub.2 and C.sub.3 aliphatic alcohols,
especially ethanol and isopropanol. The cellosolves and carbitols are also
useful solvents in the context of the invention.
It will be recalled that the liquid composition for streak-free cleaning
has a surface tension of less than 45 mNm.sup.-1, and preferably less than
35 mNm.sup.-1, in order adequately to wet the surface being wiped. The
lowering of surface tension (the value for water is above 70 mNm.sup.-1)
is conveniently achieved by including in the liquid a surface-active
agent, preferably at a concentration not exceeding 1.5% by weight. Higher
concentrations are unnecessary from the point of view of surface tension
lowering and may cause streaking or excessive sudsing. A concentration
within the range of from 0.009 to 1% by weight is preferred, and one
within the range of from 0.02 to 0.2% by weight is especially preferred.
Although in principle any anionic, nonionic, cationic, zwitterionic or
amphoteric surface-active agent may be used, nonionic surface-active
a | | |
