Note: Descriptions are shown in the official language in which they were submitted.
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PROC~', FOR PR~-TREATING C~Tl~i3L~ POLY~T~R RL~IN
BAC~GROUND 3F ~,~ VtNI:~
Field of the Invention
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This invention relates to synthe~ic reslns, and more
particularly to processes for casting polyester resin.
tio_ of the Prior Art
Cast unsaturated polyesters using inert fillers is an art
which has been prac~tlced commerclally since the early 1960's.
Products are known variously as cultured marble, cast marble,
precast marble and molded marble. In the 1370's, a synthetlc
onyx was developed using dlfferent fillers. Products in both
marble and onyx include table tops, lamp bases, window sills,
countertops, wall panels and various items of sanitary-ware such
as vanity tops, bathtubs and showers.
Typically, cast marble consists of 20-35~ unsaturated
polyester resin such as, for example, propylene glycol
esterified with adipic and maleic anhydride and filled with an
inert material such as calcium carbonate. Cast onyx consists of
a specially clarified polyester resin with glass frlt and
aluminum trihydrate used either separately or in combination as
the filler. The conventional manufacturing process consists of
the ~ollowing steps:
(1) preparing tne molds by covering the surface with a
suitable mold release agent, typically a wax or organic alcohol.
(2) sprayin~ the mold surface with a clear, unsaturated
polyester resin referred to as a gel coat into which has been
incorporated an ultravlolet stabili~er and a thixatrope. An
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orgallic peroxide i5 used as catalyst.
(3) after the gel coat nas partially cured, a matrix is
troweled or poured into the m~ld. The matrix is prepared by
mixing polyester resin, organic peroxide catalyst, filler and
olorants in the desired proportions.
(4) after the molds are filled, they are vibrated for
several minutes to try to effec~t air release from the matrix,
but especially From the interface with the gel coat.
(5) the material is then allowed to cure at ambien~
temperature to a hard and "rigid" state before removal of the
formed part from the mold.
A typical example -- U.S. 3,562,379 to Duggins --
discloses an improved cast marble polyester resin process. U.S.
3,562,379 and U.S. 3,488,346 are also examples of the advanced
state of the art.
Cast marble has only one real competitor~ In the 1960's,
du Pont developed an aluminum trihydrate filled
methylm thacrylate polymer which it has marketed under the mark
"Corian". In competing with methylmethacrylate polymer, cast
marble has had several significant advantages. It can be
manufacturea at a lower cost, it has considerably greater
versatility as to colors, shapes and sizes, and its physical
properties are better. In addition, it is more satisfac~torily
repairable.
The major disadvantage of cast marble has been its lac~k
o machineability because of the porosity of the matrix
resulting from lncomplete rernoval of entrapped air during the
casting operation. Vibrating, as practiced conventionally, is
inadequate to remove the air, This has effectively ruled out
cast marble in the kitchen countertop market where traditionally
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the count*rtops are fiel~ cut tO speciFlc requirements.
De-gassinq agents such as disclosed in U.~ 4,129,543 ~o
Kaplan have been used, but witn limited success due to the hlqh
viscosity of the resin which inhibits air release. Further,
they are expensive in the large quantities called for in, for
example, the Kaplan patent and which quantities have heretofore
been necessary to achieve even limited success and which,
accordlng to the Kaplan patent, may result in surface charring
Evacuating the blend in the mold tends to devolve styrene
monomer preferential ~o deyassing the air and, further, it is
not economically feasible to put the large molds common to the
industry in vacuum chambers.
There is, therefore, a need for a simple, economical cast
marble process which will remove entrapped air from a polyester
resin to leave a uniformly dense matrix and to accomplish this
end result without deliterious efects on the polymerization
process itself~
SU~MARY OF THE INVENTION
The aforementioned prior art problems are solved by the
process of this invention in which castable polyester resin is
mechanically pre-treated before the casting process in a
critical combination of process steps to rupture and release
entrapped air, leaving thereby a material which, atter
hardening, may be machined and the exposed edges will be
nonporous and uniform, giving a pleasing appearance.
In the process of this invention, an unsaturated
polyester resin such as, for example, propylene glycol
esterified with adipic and maleic anhydride of about 600 to
about 3000 centipoise viscosity, and containing styrene monomer
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as a crc~ss linking agent is first formulated by adding an
organi~ peroxide and solid filler material such as calcium
carbonate in the conventional manner to form a blend of about 20
to about ~0 weight perçent of polyester resln and about 60 to
abo~t ~0 weight petcent iller~ In a batch process, this simply
means mixing these materlals together.
The blend is then transferred to a vacuum vessel. This
vessel must include means to allow the blend to be subjected to
a shearing action such as, for example, by rocking action or
mechanical agitation such as using a slow speed stirrer, and to
simultaneously subject the blend to vibration, all while a
vacuum is being applied. These three process steps --
evacuating, vibrating and shearing -- are req~ired to be done
concurrently and are criti-al within certain broad parameters.
For example, the vacuum should be between about 15 to about 3Q
inches of mercury and the vibration is preferable at about 2,000
to about 4,000 cycles per minute.
The mechanisms of this process are (1) to invoke
migration of entrapped air to the surface by means of applying a
vacuum and vibration, and (2~ to facilltate release of this air
upon its reaching the surface by shearing of the bubbles by some
mechanical means~ The latter mechanism is assisted by
incorporating into the blend an anti-~oam or air release agent
whose function is to reduce the surface tension of the liquid.
The shearing action is accomplished preferably by rocking the
vessel gently over a total of about a 60 degree arc as measured
rom a line perpendicular to the horizontal plane of the blend.
A preferred embodlment also includes performing the shearing
step by inserting into the blend a baffle comprising a series of
fingers between which the blend passes as it is being rocked.
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rhe just-described eva(uatirlg, vlbrating and shearing are
critical as to co~bination. That is, all are required
simultaneously. !10wever, there ls permissible variation as to
the time and quantum. These steps will effectively remove
entrapped air which heretofore has been ineffectively removed by
other process steps or by the use of degassing or defoaming
chemical agents.
After thls pretreatment step, it is generally necessary
to vibrate the blend agaln. This vibrating step is accomplished
without any accompanying evacuation or shearing and is merely
for the purpose of re-incorporating devolved volatiles such as
styrene monomer, the entrapped alr now being no longer present.
This last process step can be accomplished after the blend has
been poured into a mold, in which case the vibrator is merely
attached to the mold's exterior surface.
The afore~entioned pretreating process produces a blend
which may be utilized in all conventional casting processes and
will produce a cured rigid material, or part, which is
substantially void free in cross section. This means that cast
marble, which has heretofore been limited to uses as bathroom
sinks, table tops, lamps, etc. which are totally precast and
merely installed in the Field, can now be expanded to such
applications as countertops for kitchens, bars, etc. wher~ the
material to be used must be cu. to fit on the job site.
The cast marble produced by the pretreating process of
this invention is pleasing to the eye in cross section and
inherently stronger due to the absence of voids in the material.
BRIEF DESCRIPTION OF THE DRA~ING(S)
Figure l is a flow diagram showing the process steps of
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this lnvention~
Figure 2 illustrates as a s~hematic~ drawing, a preferred
embodiment of the concur~ently performed shearing, vibrating and
evacuating step.
Figure 3, a front vlew taken along lines 3--3 of Eigure
2, shows, in longitudinal cross section, an evacua~ing vessel
containing a baffle suitable for use in the process of this
invention.
DETAILED DESCRIPTION OF TH~ PREFERRED EMBODIMENT(S)
Referring now to the drawings, and more particularly to
Figure 1 and at arrow 10, the words "to mlxer" are intended to
indicate the formulation of a liquid thermosetting polyester
resin in a typical batch mlxture as lndicated in Example I
below:
Exam~le I
,'J 300 grams of polyester resin; 1,200 cp; Silman 585 brand
,,r; ~
700 grams of calclum carbonate filler; 325 mesh
Pfizer FGD brand
grams of 50 percent methyl ethyl ketone peroxide;
Superox 709 Reichold Chemicals brand
are mlxed together. Up to 200 parts per mlllion of poly
dimethyl siloxane, Union Carbide SAG 100 may be added, if
desired~ Any larger amount is not required and the siloxane may
be eliminated altogether. The resulting mixture is blend-ed as
indi~ated at step 12 for about five minutes or as long as needed
to thoroughly blend the mix. The mix is then transferred as
indicated by arrow 14 to a vacuum vessel as indicated at block
16. The vacuum vessel is preferably a rocking vessel capable of
til~lng at least 30 degrees from vertical in each direction for
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a total arc o~ rotation of 60 degrees. The vessel includes
means to induce rocking at a frequenc~y oF one to two cycles per
minute during the evacuation cycle. Addltionally, a vibrator is
attached to the vessel to induce a vibration cycle of From about
2,00~ to about 4,000 cycles per minute.
The combined steps o~ evacuatlon, vibration and shearing
are performed for about thirty minutes which for a mix of this
volum~ has been found su FFicient to rupture and rel~ase
substantially all the entrapped air.
This shearing action thus des~ribed is facilitated by
placing, in the vessel prior to the sealing and evacuation, a
baffle consisting of a linear series of fingers which extend
into the blend so that the blend must pass between the fingers
during the rocking cycle.
Following completion of the thirty minute evacuating,
shearing and vibrating step, the vessel is opened, a pigment or
colorant such as two ounces of titanium dioxide are added to the
blend, as is conventional in this art, and the blend, or matrix,
is then transferred as indicated at arrow 18 to a mold.
~ fter the blend is poured or trowled into a mold (which
may be prepared with a gel coat, etc. as is well known in the
art~, the mold is vibrated, again at about 2,000 to 4,000 cycles
per minute, for a period o~ about 15 minutes to facilitate
redisolving of styrene monomer. At this step in the process, it
is important to note that vibration is not for the purpose of
releasing air, in as much as little or no air is left, and the
vibration at this point would be ineffective for this purpose
anyway, as the prior art makes clear. That is, vibration as
indicate~ by step 21 in Figure 1 is for the sole purpose of re-
incorporating the devolved volatiles such as styrene monomer
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back into ~he hlend. Devolving styrene can be readily seen by
surface bubble formation and the vibrating step may be
eliminated if no devolving takes place.
Referring now to Figure 2, the critical combination step
of evac~uating, shearing and vlbrating step is shown
schematically. In Flgure 2, vessel 20 is shown diagramatically
to bett*r illustrate the shearing process. In Figure 2, vessel
is shown first at rest as indicated by lead line 22 and
showing blend 24 at rest> Also shown in Figure 2 are two
additional positions of vessel 20, both illustrated in phantom
and each showing a 30 degree arc as indicated by arrow 26.
Thus, the total ar- of rocking of vessel 20 is 60 degrees as
lndicated by arrow 28.
Also shown in cross section in Figure 2 is baffle 30
which is better illustrated by reference now to Figure 3,
In Figure 3, taken along lines 3--3 of Figure 2, vessel
20 is shown in cross section again but with baffle 30 shown in
front elevation, including fingers 32 between which blend 24
passes during the rocking of vessel 20 which gives rise to the
shearing step of the process as previously desc~ribed.
Following the completion of the shearing, evacuating and
vibrating step, vessel 20 is opened. ~t this point, the
colorants or pigments as previously described in Example I are
added. Blend 2¢ is now transferred to a previo~sly prepared
mold as indicated at arrow 18 in Figure 1, and which has
prevlously been describecl including a gel coating applied in the
conventional manner.
There are many variations which may be practiced within
the scope of this invention. For example, while described and
illustrated as a batch process, th* process steps may be
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co~ined Wi.ttl d continlJous coatiny process.
Also, although illustrated as a casting process, since
the process of this invention ls dir~cted to pre-casting, the
part forming step itself is not limited to casting, but is also
suitable for injection ~olding as well as all other part ~orming
steps.
Also, this invention ls not llmlted to a rigid mold. A
nonrigid, or flexible, mold such is common for decorative items
like lamp bases may be used, or parts may be cast ln flexlble
molds using wood fillers such as is common for furniture parts.
It should be noted that clear casting resins without fillers may
be used such as is common for decorative items.
There are many advantages to the pre-treatment process of
this invention. Chiefly is the ability to manufacture cast
marble items or parts which are nonporous and, thus, may be
machined or cut to leave an exposed edge which is pleasing to
the eye. The use of lnexpensive mechanical process steps
eliminates the need for expensive, and largely ineffective,
chemical degassing additives.
Improved physical properties including greater tensile
strength (resistance to thermal and mechanical stresses) and
greater bond strength between the matrix and the gel coat are
additional advantages achieved by the use of the process of this
invention.
Having now illustrated and described my invention, it is
not intended that such description limit this invention, but
rather that this invention be limited only a reasonable
interpretation of the apended Claims.
