Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR l\LAKING A MINERAL FTT,T,F.O ACRYLIC ~
~IAVING A MATTE FINISH WITH IMPROVED STAIN RESISTANCE
Technical Field
This invention relates to the m~mlf~ctllring of slab or sheets of poly (methyl
methacrylate~ (PMMA) which contain mineral fillers and which can be thermoformed into such
articles as vanity tops or sinks. In particular, this invention describes how to produce sheets or
slabs with a matte finish surface and appearance that does not require additional finiching steps
and provides good stain recict~n~e7 as measured by ~xi.cting industry standards.
Background of the Invention
Sheets and slabs of synthetic mineral appearing m~teri~l are now commonly used as
kitchen counter tops and interior and exterior decorative coverings of all kinds for buildings such
as banks, air termin~l~, stores, and the like. Such applications frequently require that the material
be fabricated to fit custom designed areas.
The fabrication process requires extensive time and specially trained craftsmen to be
completed succçssfully, since special tools and procedures are necessary. Until recently, if a
shaped, one piece part of continuous monolithic m~t~ri~l is desired, such a part could only be
produced by casting it in a closed mold cavity under special conditions.
the silicone forms a release layer.
In Oshima et al., U. S. Patent No. 4,041,120 discloses an abrasion resistant surface layer
for synthetic resin articles. However, that layer is firmly adherent to the base resin. Similarly,
Toyooka et al., U. S. Patent No. 4,254,074 discloses synthetic resin sheets having a film,
possibly bearing printing, ~lh~ring thereto.
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Daff et al., in U. S. Patent No. 5,308,425 discloses a protective film placed on acrylic
sheet. However, the present differs in that the film here is non-colnr~tihle with a mineral filled
acrylic sheet. The non-compatibility must be to a degree such that the surface characteristics of
the mineral filled sheet are not subst~nt~lly ch~n~ç~l Additionally, the r~sllltinf~ sheet will have
a matte finish. Neither of these cha~ ;stics are present in Daff et al.
In U.S. Patent No. ~,521,243 ~inghetti et al., disclose the composition of a mineral
appearing material which may be cast or otherwise formed into the shape of a sheet. These
sheets are capable of being thermoformed into a vanity top or sink. As with prior art mineral
appearing sheets, the post-polymerized sheet will have a glossy appearance. However, this
glossy appearance becomes matte in areas of deep draw upon thermoforming. Thus, the final
product will have a non-uniform appearance.
The same problem is experienced with non-thermoformed acrylic compositions such as
those described by Ross et al., in U. S. Patent Nos. 4,961,995 and 5,032,625. In addition to the
high costs associated with installation of the acrylic parts (placing the sheets into place, fitting,
glueing, and the like) the intersection of adjacent sheets will necessarily be sanded to give a
smooth fit. This s~n(1ing process will similarly remove the glossy appearance in that area, giving
a non-uniform appearance.
To combat the difficulties listed above, mineral appearing sheets are often sanded in their
entirety. This gives a uniform matte finish before and after the above applications are
accomplished. However, the uniform ~nrlin~ process has its own distinct drawbacks.
Vi~n~li7in~ the contents of a mineral appearing acrylic sheet, mineral filler particles such
as alumina trihydrate (ATH) will be dispersed throughout the acrylic matrix. Some of these
particles will be very near the surface of the sheet. These particles near the surface will have a
very thin film of acrylic covering them. When the surface of the sheet is subsequently sanded
this thin film of acrylic is nnint~ntionally removed. The filler particles near the surface are then
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exposed and some of them will have been cut in half (or more-or-less) by the s~n~ling
One of the qualities possessed by acrylic sheets is stain reei~t~n~e. The acrylic surface
resists absorption of foreign m~tçri~le commonly associated with the above-mentioned uses for
mineral appearing products. When the mineral filler material such as ATH is exposed by
s~n~ling, staining sllbst~n-.es will more easily penetrate the surface, by being absorbed directly by
the ATH or by penetrating at the interf~ce of the ATH particles with the polymer matrix. This
results in a poor aesthetic quality in the product.
The present invention overcomes this dra~,vback in the prior art. The mineral appearing
sheets and products thermoformed therefrom of the present invention have a matte appearance.
There is no need for subsequent sanding procedures. The mineral filler particles near the surface
are, for the most part, not exposed. Most, if not all, retain the acrylic film sepalaLirlg the particles
from the outside enviromnent. The products of the present invention also have improved stain
resistance when compared to typical matte surfaces in the prior art.
Summary of the Invention
Acrylic sheets such as those described in U.S. Patent No. 5,521,243 can be produced with
dif~~ l surface fini.eh~s which will change after lhermoforming and the appearance being
unacceptable. When the hot sheet is sketched into a mold, there is a significant loss of gloss,
which is variable and proportional to amount of stretching of any given unit of surface. If, for
example, a bathroom vanity top is thermoformed fTom a sheet with a smooth finish, the resulting
part will show:
- a minor change in gloss (slight decrease) in the flat area surround the bowl area,
- a more noticeable change of gloss (moderate decrease) in the rim area around the
bowl,
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- a very noticeable change of gloss (~ign;ffc~nt decrease to a dull finish) in
the bottom or drain area of the bowl.
All this occurs because the polymer matrix stretches out and away from the solid particles
of the mineral filler. This is a well known l~h~;nomc~on in the thermoforming of high gloss
sheets where, at times, a small amount of pigm~ntc or solid powders (1-2% by weight) may be
sufficient to cause the problem. In compositions cont~nnplzlted by the present învention, the
amount of mineral filler may be more than an order of magnitude (30-50% by weight) and the
loss of gloss problem is unavoidable and very noticeable.
Sanding the surface of these sheets before or after the thermoforming step could provide a
solution to the problem of loss of gloss, since the sanded finish does not change, or more exactly,
the eye of an observer does not appreciate a visible change of the surface finish among the
different areas of the fini~h~rl part. However, the sanding process cuts grooves into the surface,
and this allows a st~ining agent to penetrate more easily below the surface. These stains may be
resistant to a rinsing liquid, such as a soapy solution or similar clç~n~ing products.
This performance is measured, for example, by the ANSI Z. 124.3 :American National
Standard for Plastic Lavatories. In it, st~ining agents are used to create stains which are then
cleaned with specific procedures. An overall rating of the stain resict~n~e of the surface is
obtained and a rating of 50 is required to pass the test.
When the solid surface sheet is produced against an ay~ uliate matte finish, like a plastic
film or paper, the geometry of the surface of the sheet will have the same thin and llniforrn film
of plastic matrix, but it is not flat and it has peaks and valleys which provide a matte look. The
stain rçsi~t~nce will remain about the same as that of a smooth finish.
Another hll~ol L~ll factor affects the performance of the fini~h~-rl part, and that is the type
of mold used. Our work has shown that the stain resistance dccreases after the sheet is formed
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into a female mold, while it may increase or stay the same if a male mold is used. In a female
mold forrnin~, the finiche~l surface is stretched, and it is reasonable to assume that the pores at its
surface will grow larger and/or new ones may be formed, thus favoring the penetration of
st~ining agents. In a male mold configuration, the finieh~cl surface is not stretched to any
~:ipnifir~nt degree and it is pressed against the mold surface.
The male type mold is also preferable for achieving a more uniform and thicker wall in
the finiehecl part. In a female mold, the thermoformed sheet significantly decrease in thickness in
some areas. In an average vanity top bowl, the thickness in the drain area may drop to 1/3 or 1/4
of its original value. If the same shape is produced in a male mold, the thickness in the drain area
may be only 10-20% less than the original thickness. Consequently, if the original thickness is
high enough, the thermoformed part from a male mold may be strong enough to be used as such,
without the need of any type of back side reinforcement, such as polyester-fiberglass.
Detailed Description of the Invention
Several types of film were tested. They are listed in Table 1. These films or papers were
used to cover a casting s~ ce, so that the liquid composition would contact them and
polymerize against them.
Cell casting experiments were performed in which the film or paper covered the surface
of one of the two cell cast plates. The space between them was then filled with the liquid
composition of choice, which would then be polymeri7~1 in a water bath and/or oven, following
the known art methods.
Although the present invention is not limited to any one particular sheet m~n~lf~cturing
process (e.g. continuous cast, injection molding, cell cast, end the like), in the ple~.,~d
embodiment of the invention the continuous casting process, such as is described in U.S. Patent
No. 3,376,371 by Opel et al., and Hellsund's U. S. Patent No. 52,271,383 are followed using an
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alumina trihydrate filled acrylic composition such as those described in U. S. Patent No.
5,521,243. A roll of the selectecl m~t~n~l was mounted on an air brake support and ~ ekin~
system, to provide some tension to elim;n~te wrinkles. The film would then go over a curved
Mt. Hope type roller, also useful to elimin~te wrinkles. The leading edge of the film or paper
was taped across the bottom belt, which would then pull the film from the feed roll.
The polymer- 7:ing composition was dropped onto the film which covered the bottom belt.
The film or paper in question could be covering the top belt (or both belts) depending on
operational choices and the available equipment configuration.
If the sheet is made from a thermoformable composition, the film or paper can be left on
the sheet until the thermoforming operation is initiated. The film may have to be removed if it
does restrain or negatively affects the surface ~uality of the finished part.
Examples
The types of film used for mineral filled thermoformable acrylic sheets and
thermoformed bowls therefrom are given in Table 1. The sheets were prepared as is set forth in
U. S. Patent 5,521,243 (in turn referencing '371 and '383 patents). The evaluations ofthe Stain
i.ct~n~.e per ANSI Z. 124.3 are given in Table. 2.
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TABT F I
Melinex 377: 1 mil thick, a polyethylene terepht ~l~te film sold by ICI Corporation.
M-1000: 2.5 mil, a polyvinyl alcohol film sold by Chris Craft Industrial Products.
~tipkote Papers: 5 mil, paper base with clays and surface treatment coatings sold by S.D.
Warren Company.
Type of Stripkote Tested Results
TK-ETL-Velvet ~ood release - good ~pea~ ce
SK-ETL-Matte Good release - good a~pe~a~ce
,
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T~iBT,F,2
(Stain R~ t~nc~e ratings per ANSI Z.124.3 - 1986)
Flat Surface Thermoforlned Bowl
Film orPaper S~n~le ID ~ Sanded Flat Surface ~Q~l
Melinex 377 85-2484 52 54 44 61(F)
Melinex 377 85-2485 57 54 59 68(F~
Melinex 377 85-6548 48 25(M)
M-1000 85-3341 33 34 39(F)
85-4480 39 56 28(M)
Stripkote 85-6236 57
TK-ETL
Stripkote Cell cast 39
SK-ETL
Notes:
- S~nfle~l used 60 micron s~n~lr~rer.
- Thermoformed bowl: "bowl" results were obtained in the drain area of the bowl.
- (F) and (M) = F denotes a female mold configuration; M denotes a male mold
configuration; both molds produced a part with the same dimensions.
The best stain resistance ratings were obtained with M-1000 film, 33 and 39 for the surface "as
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is." The same film provided the best results when using a female mold (39). With Melinex 377,
the perform~nre of ~e surface "as is" was borderline acceptable (52, 57, 48), and it becarne
unacceptable after ~er nofcl,.,illg in a female mold (61). However, ~e use of a male mold
significantly irnproved the stain resistance (25).
