Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
8~1.
PROCESS FOR THE PREPARATION OF
1,1-DICHLORO-1-FLUOROETHANE
BACKGROUND OF THE INVENTION
The present invention relates to a p~ocess for
the preparation of l,l-dichloro-l-fluoroethane, and
more particularly, to a process for the preparation of
l,1-dichloro-1-fluoroethane wherein a titanium,
hafnium, or zirconium based catalyst is used.
Trichlorofluoromethane (known in the art as CFC-
11) is currently available in commercial quanti~ies and
is used as a blowing agent for rigid urethane thermo-
insulation foam. Currently, l,l-dichloro-l-fluoro-
ethane (known in the art as HCFC-141b) is considered to
be a replacement for CFC-ll because HCFC-141b does not
deplete ozone in the stratosphere to the same extent as
CFC-ll. See K.T. Dishart et al., POLYURETHANES WORLD
20 CONGRESS 1987, 59 (1987) and G. Mouton et al.,
POLYURETHANES WORLD CONGRESS 1987, 67 ~1987). Because
the demand for HCFC-141b will increase dramatically in
the future, commercially viable processes for the
preparation of HCFC-141b in high yield are needed.
HCFC-141b may be produced by reacting vinylidene
chloride with anhydrous hydrogen fluoride. The
reaction is expressed by the following equation:
CH2=CC12 + HF --~ CH3CFC12
The reaction proceeds slowly at room temperature but
speeds up at temperatures above 40C. At 60C, this
non-catalytic reaction gives the highest selectivity
for HCFC-141b.
~0a~`~83~l.
As examples of the non-ca~alyzed reaction, A.L.
Henne et al., J. Am. Chem.Soc. 65, 1271 (1943) report
tha~ after three hours at 65C with four moles of
hydrogen fluoride. vinylidene chloride gave only 50%
HCFC-141b, 15~ tar, 10% recovered vinylidene chloride,
5% l,l,l-trichloroethane and traces of l-chloro-l,l-
difluoroethane tknown in the art as HCFC-142b). U.S.
Patent 3,833,676 teaches the reaction of 1,1,1-
trichloroethane with hydrogen fluoride in the absence
of catalyst at 110C for 15 minutes to yield 58.2%
l,l-dichloro-l-fluoroethane: 41.7~ l-chloro-l,l-
difluoroethane; 0.10~ l,l,l-trichloroethane; and 0.03%
l,l,l-trifluoroethane. A much longer batch cycle time
of greater than six hours is required in order ~o
convert 99.9~ or more of the vinylidene chloride. A
high conversion is desirable because it is difficult to
separate the starting vinylidene chloride from the
product HCFC-141b. Increasing the temperature in an
attempt to increase HCFC-141b yield just increases the
formation of a side product, HCFC-142b and thus,
reduces the selectivity for HCFC-141b.
Because long batch cycle times as required in
the foregoing non-catalytic reaction in order to
increase the yield of HCFC-141b are commercially
impractical, attempts were made in the prior art to
catalyze the foregoing reaction.
U.S. Patent 4,258,225 teaches the reaction of
vinylidene chloride and anhy~rous hydrogen fluoride in
the presence of tantalum pentafluoride. The reaction
was conducted at 25C for three hours. The yield of
HCFC-141b was about 36% while the remaining product was
a dark oil. The low HCFC-141b yield makes this process
commercially impractical also.
~0~ 3:1.
A.E. Feiring, J _of F'luorine Chem. 1 7 (1~79)
reports on the addition of hydrogen fluoride to tetra-
and trichloroethene and related compounds in the
presence of tantalum pentafluoride, niobium penta-
fluoride, titanium tetrachloride, or molybdenumpentachloride. The article concluded that tantalum
penta~luoride appeared to be the most active of the
materials tested. The article reported that the
reaction of 0.4 mole vinylidene chloride and 0.5 mole
anhydrous hydrogen fluoride in the presence of 0.01
mole tantalum pentafluoride at 25C for three hours
yielded only 40~ HCFC-141b; the remaining product was
tar. This process is commercially unattractive because
the removal o~ tar trom the product is expensive and
again, a low HCFC-141b yield is undesirable.
U.S. Patent 2,005,708 teaches the preparation of
chlorofluoroethanes, but no~ l,l-dichloro-l-fluoro-
ethane, by the reaction of chlorinated ethanes or
ethenes with hydrogen f luoride in the presence of an
antimony halide at temperatures above 95C and under
pressure.
U.S. Patent 2,452,975 teaches the preparation of
chlorofluoroethanes, but not l.l-dichloro-l-fluoro-
ethane, by the reaction of chloroethanes with hydrogen
fluoride in the presence of stannic chloride.
U.S. Patent 4,147,733 teaches the preparation of
fluorocarbons, but not l,l-dichloro-l-fluoroethane, by
the reaction of chlorinated hydrocarbons with
hydrofluoric acid in the presence of an aluminum,
chromium or nickel fluoride catalyst at 275 to 425C.
4~3~1.
U.S. Patent 2.49s,407 teaches that ~ichloro-
ethylene and anhydrous hydrogen fluoride were reacted
in the presence of stannic chloride for 1.75 hours to
produce l,~-dichloro-l-fluoroethane with a 32.7~
conversion and a 43% yield based on dichloroethylene.
Again, a low HCFC-141b yield is commercially
unappealing.
U.S. Patent 4,766,258 teaches that the reaction
of vinylidene chloride and anhydrous hydrogen fluoride
in the presence of a stannic halide catalyst and water
for one hour produced HCFC-141b in a 29~ yield. The
reference also states that when a halide, other than a
stannic halide, such as titanium tetrachloride is used
to produce a hydrocarbon fluoride by reacting a
hydrogen-containing hydrocarbon halide and anhydrous
hydrogen fluoride, by-products such as higher boiling
substances, oligomers and black precipitates form in
even greater amounts than when the reaction is
performed with stannic halide as the catalyst. Again,
a low HCFC-141b yield is undesirable.
Thus, a need exists in the art for a process for
the preparation of l,l-dichloro-l-fluoroethane wherein
the reaction time is shorter than that of known
processes and l,l-dichloro-l-fluoroethane is produced
in a higher yield than in known processes.
S~ARY OF THE I NVENTI ON
The present invention responds to the foregoing
need in the art by providing a process for the
preparation of l,l-dichloro-l-fluoroethane. The
process comprises the step of: reacting anhydrous
hydrogen fluoride and vinylidene chloride by using
excess anhydrous hydrogen fluoride relative to the
831.
vinylidene chloride in the presence of a catalytic
amount of a catalyst of the formula:
RmAclnFp
wherein A is titanium, zirconium, or hafnium: R is an
alkyl, alkenyl, or aryl group; m is 0 to 2: n is O to
4; and p is 0 to 4 with the proviso that at least one
of n an~ p is not zero and mln~p satisfies the valence
of A, for a time and at a temperature sufficient to
form l,l-dichloro-l-fluoroethane. The term "excess
anhydrous hydrogen fluoride~' as used herein means that
the molar ratio of anhydrous hydrogen fluoride to
vinylidene chloride is greater than 1:1. In other
words, a deficiency of vinylidene chloride is used.
Regarding the non-catalyzed reaction of
vinylidene chloride and anhydrous hydrogen fluoride as
discussed in the aforementioned Henne article and U.S.
Patent 3,833,676, the present process is more
advantageous because it reduces reaction time by more
than 75~ to obtain 99.9% conversion of vinylidene
chloride, Compared with the processes of the
aforemeneioned U.S. Patent 4,253,225 and Feiring
article which use tantalum pentafluoride to catalyze
the reaction of vinylidene chloride and anhydrous
hydrogen fluoride, the present process produces
l,l-dichloro-l-fluoroethane in a higher yield in a
shorter reaction time. Compared with the process of
the previously discussed U.S. Patent 2,495,407 which
uses stannic chloride to catalyze the reaction of
dichloroethylene and anhydrous hydrogen fluoride, the
present process produces l,l-dichloro-l-fluoroethane in
a higher yield in a shorter reaction time.
~0~831
Although the above-mentioned U.S. Patent
4,766,258 mentions the use o~ titanium tetrachloride in
the fluorination of hydrogen-containing hydrocarbon
halides, the reference does not teach the present
reaction. Further, because U.S. Patent 4,766,258
indicates yields of less than 2g% when titanium
tetrachloride is used to fluorinate hydrogen-containing
hydrocarbon halides, the reference would not lead a
person of ordinary skill in the art to the present
process.
Although the above-mentioned Feiring article
teaches the use of titanium tetrachloride in the
fluorination of tetrachloroethylene, the reference does
not teach the present reaction. The purpose of the
paper was to test the usefulness of catalysts including
tentalum pentafluoride, niobium pentafluoride, titanium
tetrachloride and molybdenum pentachloride in the
fluorination of tetra-, tri- and dichloroethene. The
paper concluded that tantalum pentafluoride appears to
be the most active catalyst and reported that the
reaction of 0.4 mole vinylidene chloride and 0.5 mole
anhydrous hydrogen fluoride in the presence of tantalum
pentafluoride at 25C for three hours yielded only 4Q%
HCFC-141b. Based on these teachings, the reference
would not lead a person of ordinary skill in the art to
the present process.
As such, the present invention provides a
process for the preparation of l,l-dichloro-l-fluoro-
ethane wherein the reaction time is shorter than that
of known processes and 1,1-dichloro-1-fluoroethane is
produced in a higher yield than in known processes.
Other advantages of the present invention will
become apparent from the following description and
appended claims.
DETAILED DESCRIPTI~N OF THE PREFERRE~ E 30DIMENTS
Commercially available anhydrouS hydrogen
f luoride and vinylidene chloride may be used in the
present invention. The addition of anhydrous hydrogen
fluoride to the carbon-carbon double bond of the
vinylidene chloride is a stoichiometric reaction in
which one mole of anhydrous hydrogen fluoride is
required for each mole of vinylidene chloride used.
Generally, the anhydrous hydrogen fluoride and
vinylidene chloride are reacted with an excess of
anhydrous hydrogen fluoride relative to the vinylidene
chloride. It has been found that because the
undesirable tars or high boilers are mainly formed by
coupling or polymerization reactions among vinylidene
chloride molecules, the amount of tars or high boilers
formed during the reaction can be minimized by
minimizing the amount of starting ~inylidene chloride
used. As such, the use of excess vinylidene chloride
is disadvantageous. Preferably, the anhydrous hydrogen
fluoride and vinylidene chloride are reacted in a molar
ratio of about 1.5:1 to about 5:1. The use of
anhydrous hydrogen fluoride in an amount greater than
the molar ratio of 5:l causes other fluorinated alkanes
to form. More preferably, the anhydrous hydrogen
fluoride and vinylidene chloride are reacted in a molar
ratio of about 1.5:1 to about 4:1 and most preferably
abou~ 1.5:1 to about 3:1.
The catalyst used is of the formula:
RmAclnFp
whersin A is titanium, zirconium, or hafnium: R is an
alkyl, alkenyl, or aryl group; m is O to 2; n is O to
4: and p is O to 4 with the proviso that at least ons
~.0~
of n and p is not zero and m+n+p satisfies the valence
of A. For R, examples of useful alkyl groups include
methyl, ethyl, isopropyl, butyl, pentyl, hexyl, heptyl
and octyl. For R, examples of useful aryl groups
include phenyl and naphthyl. The aryl group may also
be substituted so as to include groups such as benzyl,
tolyl and halogenated phenyls. For R, examples of
useful alkenyl groups include cyclopentadienyl and
pentamethylcyclopentadienyl.
When A is zirconium, preferred catalysts include
zirconium tetrachloride, zirconium tetrafluoride,
zirconium tetrabromide and zirconocene dichloride.
When A is hafnium, preferred catalysts include hafnium
tetrachloride, hafnium tetrafluoride and hafnocene
dichloride. When A is titanium, preferred catalysts
include titanium trichloride, titanium tetrachloride,
titanium trifluoride, titanium tetrafluoride, titanium
tetrabromide and titanocene dichloride. The most
preferred catalyst is titanium tetrachloride.
Commercially available zirconium tetrachloride.
zirconocene dichloride, hafnium tetrachloride,
hafnocene dichloride, titanium tetrachloride, titanium
tetrabromide and titanocene dichloride may be used in
practicing the present invention.
A catalytic amount of catalyst is used.
Generally, the amount of catalyst used is about 0.01 to
about 50 percent mole of catalyst per mole of
vinylidene chloride used. Preferably, the amount of
catalyst used is about 0.1 to about 10 percent, more
preferably about 0.2 to about S percent and most
preferably about 0.3 to about 2 percent.
31.
The reaction is preferably conducted at a
temperature of about 30 to about 100C. At reaction
temperatures below this limit, the reaction becomes too
slow to be useful. At reaction temperatures above this
limit, the product yield is reduced by the formation of
by-products. More preferably, the reaction is
conducted at a temperature of about 50 to about 65C:
in this range, the highest selectivity for
l,l-dichloro-l-fluoroethane exists.
Operation o~ the reaction at atmospheric
pressure is convenient. Operation at a pressure of
about 10 to about 130 psig (69 to 897 KPa) is
preferred. Means may be provided ~or venting of the
excess pressure caused by hydrogen chloride formed in
the reaction and offers an advantage in minimizing
formation of side products.
A useful reaction vessel is constructed from
materials which are resistant to the action of
anhydrous hydrogen fluoride. Examples include metallic
materials and polymeric materials. For reactions at
temperatures above the boiling point of vinylidene
chloride which is abou~ 30C, a closed reaction vessel
is used to minimize the loss of vinylidene chloride and
anhydrous hydrogen fluoride.
Gene~ally, vinylidene chloride, anhydrous
hydrogen fluoride and catalyst are introduced in any
order into the reaction vessel as long as an excess of
anhydrous hydrogen fluoride relative to the vinylidene
chloride is used. The contents of the vessel are
raised to the appropriate reaction temperature and are
agitated by sha~ing or stirring for a time sufficient
for the anhydrous hydrogen fluoride and vinylidene
chloride to react. The reaction progress may be
~`0~
-- 10 --
monitored by withdrawing samples periodically.
Preferably, the reaction time is less than about 2
hours. More preferably, the reaction time is about 0.1
to about 1.5 hours.
Preferably, a catalytic amount of the catalyst
is dissolved in the anhydrous hydrogen fluoride and
this mixture is heated to about 50 to about 65C before
the addition of vinylidene chloride thereto. Anhydrous
hydrogen fluoride has a vapor pressure of about ~0 psig
at 60C and a boiling point of 19.5C; at atmospheric
pressure, titanium tetrachloride has a boiling point of
136.4C. When a catalytic amount of titanium
tetrachloride was mixed with anhydrous hydrogen
fluoride at 60C. the mixture had a vapor pressure of
around 65-70 psig. This pressure increase suggested
that anhydrous hydrogen fluoride reacted with titanium
tetrachloride to produce a low boiling compound such as
HCl. It is believed that several titanium species also
formed. The following equation summarizes the probable
reaction.
4TiC14 + lOHF --~ lOHCl + TiC13F + TiC12F2 + TiC1~3 + TiF4
As such, it is believed that titanium tetrachloride can be
completely fluorinated to titanium tetrafluoride or
partially fluorinated to compounds such as TiC13F,
TiC12F2 and TiClF3. The mixture of titanium
tetrachloride and anhydrous hydrogen fluoride could
contain any combination of these four fluorinated titanium
species. It is believed that the reaction between
vinylidene chloride and anhydrous hydrogen fluoride can be
catalyzed by any of the five titanium species or any
combination thereof. As such, the catalyst of the formula
RmAclnFp
83~L
11 --
wherein A, R, m, n and p are as pre~iously defined covers
TiC13F, TiC12F2 and TiClF3.
The l,1-dichloro-1-fluoroethane product may be
isolated by any of a variety of known techniques. The
contents of the reaction vessel may be discharged onto ice
and the organic layer collected, washed with water and
dried with a drying agent. The product may be analyzed by
standard techniques including gas-liquid chromatography,
NMR spectroscopy and mass spectrometry.
The present process may be conducted in batch or
continuous fashion.
The 1,1-dichloro-1-fluoroethane produced by the
present process is particularly useful as a blowing agent
for the production of rigid urethane thermoinsulation
foam. See for example U.S. Patents 4,652,589; 4,686,240;
4,699,932: 4,701,474: 4,717,518: and 4,727.094.
The present invention is more fully illustrated by
the following non-limiting Examples.
For each Example and Comparative, the molar ratio
of anhydrous hydrogen fluoride to vinylidene chloride was
about 1.9:1. Each run was agitated at 2400 RPM.
In Table 1 below, "Cat." stands for catalyst. "VC"
stands for vinylidene chloride. 142b is known in the art
and is l-chloro-l,l-difluoroethane. 140a i8 also known in
the art and is l,l,l-trichloroethane. "HB's" stands for
High Boilers. In the column for "Total HB's", the number
shown in parenthesis indicates the number of high boilers
observed.
- 12 -
Co~parative_l
200 grams anhydrous hydrogen fluoride were charged
to a l liter autoclave at room temperature. The autoclave
5 was heated to the reaction temperature, 60OC, with
agitation. Five hundreds grams of vinylidene chloride
were then pumped into the autoclave at a constant rate.
The progress of the reaction was monitored by withdrawing
the organic layer periodically by using a dip leg
installed in the reactor. The sample was quenched with an
ice/water slush immediately. The organic layer was then
isolated, dried and analyzed on a gas chromatograph. The
reaction was considered to be completed when 99.9%
vinylidene chloride was converted.
After the reaction was completed, the reactor was
cooled quickly to the ambient temperature and the organic
layer was recovered and washed with ice/water slush. The
washed crude product was then isolated, dried and
analyzed. The results are listed under "Cl" in Table l
below.
TABLE 1
VC % HOL
25 FEED FRAC.
TEHP. RATE (Cat./ PaES. TIHE PRODUCT ANALYSIS(%)
EX CAT. (C) (~/min? VC) (PSi~ (hr.? 142b 141b VC 140a TOTAL HB's
Cl NONE 60 2.16 - 80-120 6.1 1.3 93 0.10 4.2 1.2 (3)
C2 BF3 60 8.64 0.78 100-120 1.5 0.9 91 0.03 2.9 4.8 (5)
C3 BF3 49 8.08 1.23 75-80 2.0 0.3 87 0.15 2.7 10.2 (5)
30 C4 TaF5 60 8.64 0.76 100-120 1.25 0.9 94 0.00 2.3 2.2 (4)
C5 TaF5 60 9.09 0.38 100-120 1.5 0.6 94 0.03 2.4 3.2 (4)
C6 SnC1460 8.96 0.81 110-150 1.5 4.4 84 1.60 9.9 0.4 (1)
1 TiC14 60 10.20 0.83 120-130 1.25 0.4 96 0.10 1.6 1.6 (2)
2 TiC14 60 9.43 0.90 124-126 1.25 0.4 96 0.09 1.9 0.8 (3)
35 3 TiC14 50 8.77 0.97 87-95 1.50 0.4 97 0.20 1.4 1.1 (3)
4 TiC14 6010.870.92 102-113 1.25 0.7 97 0.10 1.0 1.3 (3)
33~
The mecric uni~s for the pressure are in Table 2
below.
TABLE 2
PRES.
F (KPa)
Cl 552-828
C2 690-828
10 C3 518-552
C4 690-828
C5 690-828
C6 759-1035
1 828-897
15 2 856-869
3 60~-656
4 704-780
Major by-products of the reaction include
20 HCFC-142b, HCC-140a, HCl and hiqh boilers. The High
boilers consisted of fluorinated 4-carbon and 6-carbon
compounds, ~ome polymeric materials and the stabilizer
used in the 6tarting vinylidene chloride. The crude
product compositions were determined by analyzing the
recovered liquid phase product. HCFC-142b has a
boiling point much lower than room temperature and
thus, the actual selectivity for HCFC-142b is slightly
higher than that indicated in Table 1. HCC-140a wa
generated from the reaction between vinylidene chloride
and hydrogen chloride. Hydrogen chloride and HCFC-142b
were formed from the reaction between HCFC-141b and
HF. High boilers were mainly produced from coupling
reactions and polymerization among vinylidene chloride
molecules.
3~.
-- lg --
The results indicate that ~he long reaction time
of 6.1 hours has to be used ~or a non-catalyzed
reaction.
S ComDaratives 2 and 3
In the same autoclave as in Comparative 1. two
separate runs were conducted under similar conditions.
Boron trifluoride was charged to the reactor after 200
grams of anhydrous hydrogen fluoride were charged. The
mixture was then heated to 60 and 49C respectively
prior to the vinylidene chloride feed (500 g). The
reactions were monitored in the same way as in
Comparative 1. The crude product was also recovered
and analyzed the same way. The results are listed
under "C2" and "C3" in ~able 1 above.
Boron trifluoride seemed to promote coupli~g
reactions and produced more high boilers (or tars). As
a result. it produced a lower selectivity for HCFC-141b
than the non-catalytic reaction of Comparative 1.
ComParatives 4 and S
Using the same apparatus as in Comparative 1.
the same reaction conditions as in Comparative 2 were
used for these two runs, except tantalum pentafluoride
was used as catalyst. The results are listed under
"C4" and "C5" in Table 1 above.
These results are surprising in view of the
results reported by the aforementioned Feiring
reference. The article reported that the reaction of
0.4 mole vinylidene chloride and 0.5 mole anhydrous
hydrogen fluoride in the presence of tantalum
pentafluoride at 25C for three hours yielded only 40%
~00~31
HCFC-141b. It is believed that the better yield
results ~rom dissolving tantalum pentafluoride in the
anhydrous hydrogen fluoride and heating this mixture to
the reaction temperature before adding a deficiency of
vinylidene chloride thereto.
Tantalum pentafluoride improved the selectivity
and reduced reaction time. but still produced slightly
more tars.
ComDarative 6
Tin tetrachloride was studied in this run. The
number of moles of catalyst used was about the same as
lS in Comparative 2. The other reaction conditions were
also quite similar. The crude product was also
recovered in the same way. A third phase heavier than
the organic phase was also observed ~hich suggested
that tin tetrachloride was not very soluble in either
the anhydrous hydrogen fluoride or the organic phase.
The results of crude organic product analysis are
listed under "C6" in Table 1 above.
Tin tetrachloride reduced the amounts or high
boilers. bu~ did not catalyze the reaction as well as
the other catalysts. Tin tetrachloride promoted the
formation of HCFC-142b and HCC-140a. resulting in lower
selectivity for HCFC-141b.
EXAMPLES 1-3
The catalytic activity of titanium tetrachloride
was also studied in the same 1 liter autoclave. The
reaction conditions were similar to those used for
Co~paratives 4 and 5, except titanium tetrachloride was
used as catalyst instead of tantalum pentafluoride.
3 ~831.
- 16 -
The desired amounts of fresh titanium tetrachloride
were used ~or these three runs re.spectively. For
Examples 1 and 2, 60C was employed and 50C was used
for Example 3. The results of these three experiments
are listed under 1, 2 and 3 in Table 1 above.
When tantalum pentafluoride was used, the yield
of 141b was 94%. In the present process, the yield of
141b was 96-97%. As such, titanium tetrachloride is
more selective and produces less ta~s. This yield
increase represents a major economic advantage to a
commercial user of the present process who will be
producing substantial quantities of 141b. Savings of
over one million dollars per year are expected.
Titanium tetrachloride is also commercially available
and less expensive than tantalum pentafluoride.
EXAMPLE 4
The reaction conditions for this run were almost
identical to those of Example 3, except that titanium
tetrachloride and about 90 geams anhydrous hydrogen
fluoride were recycled from Example 3. 105 grams of
fresh anhydrous hydrogen fluoride were also added for
the reaction by assuming that 95% catalyst was retained
in the anhydrous hydrogen fluoride phase. The results
of this run are listed under 4 in Table 1 above. The
recycled titanium tetrachloride showed the same
catalytic activity as the fresh catalyst used in
Examples 1 and 2.
As illustrated by Examples 1 through 4, titanium
tetrachloride gave the highest selectivity for
l,l-dichloro-l-fluoroethane and did not produce more
high boilers than the non-catalytic reaction.
- 17 -
Exam~les 5-15
Using the same apparatus and reaction procedures
as in Comparative 2, the catalysts o~ Table 3 below are
run using the following conditions: temperature =
60OC: vinylidene chloride feed rate = ~0 g/m; catalyst/
vinylidene chloride ~ mole reaction = 0.8-1: pressure =
80-120 psig (552-828 KPa): and time - 1.5 hours.
TABLE 3
EX. CAT.
S ZrCl4
6 ZrF4
15 7 Zrcl2(c5H5)2
8 HfC14
~ HfF4
HfC12(C5H5)2
11 TiC13
2012 TiC14
13 TiF3
14 TiF4
lS TiC12(C5H5)
Having described the invention in detail and by
reference to preferred embodiments thereof, it will be
apparent that modifications and variations are possible
without departing from the scope of the invention
defined in the appended claims.
