GB2248627A - Process for purifying a vinylaromatic monomer - Google Patents
Process for purifying a vinylaromatic monomer Download PDFInfo
- Publication number
- GB2248627A GB2248627A GB9119728A GB9119728A GB2248627A GB 2248627 A GB2248627 A GB 2248627A GB 9119728 A GB9119728 A GB 9119728A GB 9119728 A GB9119728 A GB 9119728A GB 2248627 A GB2248627 A GB 2248627A
- Authority
- GB
- United Kingdom
- Prior art keywords
- monomer
- alumina
- purifying
- inhibitor
- ppm
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 239000000178 monomer Substances 0.000 title claims abstract description 35
- 238000000034 method Methods 0.000 title claims abstract description 21
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims abstract description 40
- 239000003112 inhibitor Substances 0.000 claims abstract description 15
- 239000002245 particle Substances 0.000 claims abstract description 10
- ISWSIDIOOBJBQZ-UHFFFAOYSA-N phenol group Chemical group C1(=CC=CC=C1)O ISWSIDIOOBJBQZ-UHFFFAOYSA-N 0.000 claims abstract description 5
- 238000006116 polymerization reaction Methods 0.000 claims abstract description 4
- PPBRXRYQALVLMV-UHFFFAOYSA-N Styrene Natural products C=CC1=CC=CC=C1 PPBRXRYQALVLMV-UHFFFAOYSA-N 0.000 claims description 35
- 229920002554 vinyl polymer Polymers 0.000 claims description 3
- 125000004432 carbon atom Chemical group C* 0.000 claims description 2
- 125000003011 styrenyl group Chemical group [H]\C(*)=C(/[H])C1=C([H])C([H])=C([H])C([H])=C1[H] 0.000 claims 1
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 6
- 230000000052 comparative effect Effects 0.000 description 6
- 238000000746 purification Methods 0.000 description 5
- 239000000463 material Substances 0.000 description 4
- HIGRAKVNKLCVCA-UHFFFAOYSA-N alumine Chemical compound C1=CC=[Al]C=C1 HIGRAKVNKLCVCA-UHFFFAOYSA-N 0.000 description 2
- 239000008187 granular material Substances 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 229920000642 polymer Polymers 0.000 description 2
- 238000005406 washing Methods 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- MYRTYDVEIRVNKP-UHFFFAOYSA-N 1,2-Divinylbenzene Chemical class C=CC1=CC=CC=C1C=C MYRTYDVEIRVNKP-UHFFFAOYSA-N 0.000 description 1
- CYLVUSZHVURAOY-UHFFFAOYSA-N 2,2-dibromoethenylbenzene Chemical class BrC(Br)=CC1=CC=CC=C1 CYLVUSZHVURAOY-UHFFFAOYSA-N 0.000 description 1
- MHKBMNACOMRIAW-UHFFFAOYSA-N 2,3-dinitrophenol Chemical class OC1=CC=CC([N+]([O-])=O)=C1[N+]([O-])=O MHKBMNACOMRIAW-UHFFFAOYSA-N 0.000 description 1
- JLBJTVDPSNHSKJ-UHFFFAOYSA-N 4-Methylstyrene Chemical compound CC1=CC=C(C=C)C=C1 JLBJTVDPSNHSKJ-UHFFFAOYSA-N 0.000 description 1
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical group [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 1
- 238000005299 abrasion Methods 0.000 description 1
- 230000002745 absorbent Effects 0.000 description 1
- 239000002250 absorbent Substances 0.000 description 1
- 239000011358 absorbing material Substances 0.000 description 1
- XYLMUPLGERFSHI-UHFFFAOYSA-N alpha-Methylstyrene Chemical compound CC(=C)C1=CC=CC=C1 XYLMUPLGERFSHI-UHFFFAOYSA-N 0.000 description 1
- 239000003957 anion exchange resin Substances 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 229910052593 corundum Inorganic materials 0.000 description 1
- 238000004821 distillation Methods 0.000 description 1
- 238000005265 energy consumption Methods 0.000 description 1
- 230000001788 irregular Effects 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- NWVVVBRKAWDGAB-UHFFFAOYSA-N p-methoxyphenol Chemical compound COC1=CC=C(O)C=C1 NWVVVBRKAWDGAB-UHFFFAOYSA-N 0.000 description 1
- OXNIZHLAWKMVMX-UHFFFAOYSA-N picric acid Chemical compound OC1=C([N+]([O-])=O)C=C([N+]([O-])=O)C=C1[N+]([O-])=O OXNIZHLAWKMVMX-UHFFFAOYSA-N 0.000 description 1
- 239000011347 resin Substances 0.000 description 1
- 229920005989 resin Polymers 0.000 description 1
- 239000011780 sodium chloride Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000011282 treatment Methods 0.000 description 1
- 229910001845 yogo sapphire Inorganic materials 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C7/00—Purification; Separation; Use of additives
- C07C7/12—Purification; Separation; Use of additives by adsorption, i.e. purification or separation of hydrocarbons with the aid of solids, e.g. with ion-exchangers
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C7/00—Purification; Separation; Use of additives
- C07C7/20—Use of additives, e.g. for stabilisation
Landscapes
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Analytical Chemistry (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Water Supply & Treatment (AREA)
- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
- Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)
- Solid-Sorbent Or Filter-Aiding Compositions (AREA)
- Polymerisation Methods In General (AREA)
- Low-Molecular Organic Synthesis Reactions Using Catalysts (AREA)
Abstract
A process for purifying a vinylaromatic monomer, which contains a phenolic polymerization inhibitor, by passing it over a fixed bed of alumina particles, characterized in that the particle size ranges from 0.5 to 3 mm, the monomer space velocity ranges from 0.5 to 7.5 h<-1>, and the monomer linear velocity is in the range from 0.125 to 45 m/h.
Description
PROCESS FOR PURIFYING A VINYLAROMATIC MONOMER
This invention relates to a process for purifying a vinylaromatic monomer, and more particularly for removing phenolic inhibitors from a vinylaromatic monomer.
As is known, in order to be able to store and transport vinyl aromatic monomers, for example styrene, methyls tyrene, alpha-methylstyrene, butylstyr enes, divinyl benzenes, and mono- and dibromo-styrenes, as well as mixtures thereof, it is necessary to add polymerization inhibitors at concentrations from 5 to 50 ppm. The most commonly used inhibitor is para-tert.butyl catechol (TBC), but other inhibitors can be used, such as hydroquinone monomethylether, dinitrophenols and picric acid. However, such inhibitors are not always consistent with the quality of the polymeric compounds to be produced, and therefore it is sometimes necessary to remove them in advance from the monomer.Several processes suited to this purpose have been already described, for example: i) Distillation of the monomer, in order to remove (at the end) the phenolic inhibitor contained herein; this process is very expensive as it involves a high energy consumption and the use of complex apparatus.
ii) Washing with a water solution of sodium hydroxide; this process requires a subsequent intense washing with water in order to remove the sodium hydroxide still contained in the monomer, and usually requires a final dryirg to eliminate the residual moisture.
iii) Passing over a bed of anion exchange resins, having tertiary and/or quaternary terminations, both in the free form and in the salified form; the drawbacks which are very frequent - of such process are a very short life of the material, if used in the saline form, and the disagreeable smell of the treated monomer, due to the presence of traces of aminic compounds, if the resin is utilized in the free form.
There are also known purification processes, which comprise causing the vinyl aromatic monomer to pass on fixed beds of alumina. These treatments generally involve high operating costs, which are not always compatible with the process economy owing to the short life of the absorbing material. Therefore it is necessary to often replace the bed, which requires complex operations such as the removal of the monomer absorbed by the material, discharge and disposal of the exhausted material and charging fresh material.
In order to prolong the life of the alumina, and so to reduce the frequency of the operations for replacing it, a recent document (Japan patent publication 57538/1988) suggests to alternate normal space velocities with very low space velocities (of the monomer), stating for example that the alternation of a period at a space velocity equal to 0. 8 h with a period at a space velocity equal to or lower than 0. 01 h 1 results in a longer life of the alumina, as a result of which the obtainable amounts of purified monomer per weight unit (or volume unit) of alumina, before alumina becomes de-activated, are much higher. For "normal" space velocities, values from 0. 1 to 10 h 1 are indicated.
However, this method compels the operator to follow complicated principles for operating the purification column; furthermore, the total amounts of purified monomer obtainable up to the alumina de-activation threshold are not very high, and the periods at "low" space velocity (below 0. 1 h l) involve undesirable production losses.
We have now ascertained that an accurate selection of some operative conditions permits avoidance of the alternation of the space velocities, eliminating the periods at low space velocity, and a surprisingly high increase in the total amounts of purified monomer to be obtained.
The present invention provides a process for purifying a vinylaromatic monomer having from 8 12 carbon atoms, and containing a phenolic polymerization inhibitor, by causing it to pass over a fixed bed of alumina particles, characterised in that the average particle size ranges from 0. 5 to 3 mm, -the monomer space velocity ranges from 0. 5 to 7. 5 h 1, and the monomer linear velocity ranges from 0. 125 to 45 and preferably from 1. 2 to 45 m/h.
Space velocity means the ratio: 3
m@/h of monomer W =
m or apparent volume occupied by alumina where the "apparent" volume is the volume of the (empty) purification chamber (before the filling with alumina), which, of course, is different from the proper volume of the A1203 particles. Linear velocity means the ratio between volumetric flow of the monomer and normal section of the purification chamber. "Monomer" means both the monomer alone, for example styrene, and mixtures thereof with other vinylaromatic monomers.
The preferred inhibitor for the process of the present invention is para-tert.butyl-catechol, and the inhibitor amount in the monomer to be purified is preferably from 5 to 50 ppm, more preferably from 5 to 15 ppm.
The alumina surface area is preferably from 150 to 270 2 m /g.
The apparatus for the purification of the monomer according to the invention can be of various type; excellent results have been obtained by feeding the monomer to be purified to a vertical cylindrical column, filled with a fixed bed of alumina and having the inlet preferably on the bottom. The alumina can be spherical (with very little losses due to abrasion) or granular (granules with irregular edges). The alumina life is the longer, the smaller is its size, this result being independent of the shape of the utilized alumina (balls, small cylinders, granules, etc. ); this fact was never observed previously.
The invention will be further described with reference to the following illustrative Examples. In the tests, the break-through threshold (end of the absorbent life or "de-activation threshold") is conventionally made to coincide with the point at which the para-tert.butyl catechol in the treated monomer is equal to 2 ppm, this limit being much more precautionary (with regard to the requirements of many polymeric compounds) than the limit indicated in the cited Japan patent publication 57538/1988, the threshold whereof being equal to 8 ppm.
Example 1 (comparative test)
A granular alumina, commercially known as "COMPALOX"
AN/V - 850", produced by the company Martinswerk GmbH,
Bergheim (Germany), having an average size from 5 to 10 mm, was charged into a vertical column having a diameter of 2. 5 cm and a height of 25 cm; then, at room temperature (20 C), styrene monomer containing 12 ppm of para-tert.butyl catechol was fed at a space velocity of 6hl (linear velocity = 1. 5 m/h), and it was possible to purify only 150 volumes of styrene per volume of alumina in a time of 1.04 days, i.e. in the time preceding the de-activation threshold. The obtained data and results are given in Table 1 below.
Example 2
Under the same conditions described in Example 1, but using an alumina having sizes ranging from 1 to 3 mm, commercially known as "COMPALOX AN/V-813", having a surface area of about 170 m2/g, it was possible to purify a much higher styrene amount (1800 volumes of styrene per volume of alumina) in a time of 12. 50 days.
Example 3
Under the same conditions described in Example 2, but operating at a space velocity of 0. 5 h 1 (linear velocity = 0. 125 m/h), it was possible to purify 4, 500 volumes of styrene per volume of alumina in a time of 375 days.
Example 4
Under the same conditions described in Example 2, but using an alumina having an average size from 0. 5 to 2 mm, commercially known as "COMPALOX AN/V-802", more than 2, 500 volumes of styrene per volume of alumina were purified in a time of more than 17. 36 days.
Example 5 (comparative test)
Under the same conditions described in Example 2, but at a much higher space velocity (14 h 1) (linear velocity = 3.5 m/h), 1,550 volumes of styrene per volume of alumina were purified in a time of 4. 61 days.
Example 6
The granular alumina of Example 2 was charged into a column having a diameter of 4. 5 cm and a height of 6 m.
Styrene monomer containing 10 ppm of para-tert.butyl catechol was fed at a space velocity of 7. 1 h (linear velocity .= 42. 6 m/h), and it was possible to purify 2,200 volumes of styrene per volume of alumina in a time of 12. 91 days.
Example 7 (comparative test)
The granular alumina of Example 2, having sizes from 1 to :; mm, was charged into a column having a diameter of 4. 5 cm and a height of 3 m. Styrene monomer containing 10 ppm of para-tert.butyl catechol was fed at a space velocity of 14 h 1 (linear velocity = 42.0 m/h), and 1, 250 volumes of styrene per volume of alumina were purified in a time of 3.72 days.
Example 8 (comparative test)
A spherical alumina, commercially known as "ALUMINE
ACTIVEE A 2-5", manufactured by Rhone Poulenc (France), having particle sizes from 2 to 5 mm, was charged into a column having a diameter of 4. 5 cm and a height of 6 m.
Styrene monomer, containing on average 9 ppm of para-tert.butyl catechol, was fed at a space velocity of 6.4 h (linear velocity = 38.4 m/h), and 1,200 volumes of styrene per volume of alumina were fed in a time of 7. 81 days.
Example 9
A spherical alumina having particle sizes from 1. 5 to 2 2. 5 mm and a surface area of about 270 m /g, commercially known as "ALUMINE ACTIVEE A 1. 5-2. 5", was charged into a column having a diameter of 4. 5 cm and a height of 6 m. Styrene monomer containing on average 9 ppm of para-tert.butyl catechol was fed at a space velocity of 6.8 h-1 (linear velocity = 40.8 m/h); it was possible to purify 2,200 volumes of styrene per volume of alumina in a time of 13.4 days.
Example 10 (comparative test)
A spherical alumina, having a size of 1/4 inch (6. 4 mm), commercially known as Alcoa H151 and produced by Alcoa
Chemicals Division (USA), was charged into a column having a diameter of 28 mm and a height of 25 cm.
Styrene monomer, containing on average 12 ppm of para-tert.butyl catechol, was fed at a space velocity of 4 h 1 (linear velocity = 1.0 m/h); only 150 volumes of styrene per volume of alumina were purified in a time of 1. 56 days.
The data and results of all the examples are given in
Table 1 below.
T A B L E 1
EXAMPLE 1(*) 2 3 4 5(*) 6 7(*) 8(*) 9 10(*) Form of Al2O3 G G G G G G G S S S #AL2O3 (mm) 5-10 1-3 1-3 0,5-2 1-3 1-3 1-3 2-5 1,5-2,5 6,4 #column (cm) 2,3 2,3 2,3 2,3 2,3 4,5 4,5 4,5 4,5 2,8 H column (m) 0,25 0,25 0,25 0,25 0,25 6 3 6 6 0,25 Inhibitor(ppm) 12 12 12 12 12 10 10 9 9 12 Space velocity (h-1) 6 6 0,5 6 14 7,1 14 6,4 6,8 4,0 Linear velocity (m/h) 1,5 1,5 0,125 1,5 3,5 42,6 42,0 38,4 40,8 1,0 Purified styrene more than (vol/vol) 150 1800 4500 2500 1550 2200 1250 1200 2200 150 Desactivation 1,04 12,50 375 more than 4,61 12,91 3,72 7,81 13,48 1,56 threshold (days) 17,36 G = Granular
S = Spherical (*) comparative
Claims (9)
- Claims 1. A process for purifying a vinyl aromatic monomer having 8 to 12 carbon atoms, containing a phenolic polymerization inhibitor, by passing over a fixed bed of alumina particles, wherein the monomer space velocity is from 0. 5 to 7. 5 h 1, the average size of the particles is from 0. 5 to 3. 0 mm, and the linear velocity of the monomer is from 0. 125 to 45 m/h.
- 2. A process as claimed in claim 1, wherein said linear velocity is from 1. 2 to 45 m/h.
- 3. A process as claimed in claim 1 or 2, wherein said vinylaromatic monomer is styrene.
- 4. A process as claimed in any of claims 1 to 3, wherein said inhibitor is para-tert.butyl catechol.
- 5. A process as claimed in any of claims 1 to 4, wherein the amount of inhibitor is from 5 to 50 ppm.
- 6. A process as claimed in claim 5, wherein the amount of inhibitor is from 5 to 15 ppm.
- 7. A process as claimed in any of claims 1 to 6, wherein the alumina surface area is from 150 to 270 m/h.
- 8. A process as claimed in any of claims 1 to 7, wherein the alumina particles have a spherical shape.
- 9. A process according to claim 1 for purifying a vinylaromatic monomer, substantially as herein described in any of the foregoing Examples.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IT02149590A IT1245845B (en) | 1990-09-17 | 1990-09-17 | PROCESS FOR THE PURIFICATION OF A VINYLAROMATIC MONOMER. |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB9119728D0 GB9119728D0 (en) | 1991-10-30 |
| GB2248627A true GB2248627A (en) | 1992-04-15 |
| GB2248627B GB2248627B (en) | 1994-05-18 |
Family
ID=11182672
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9119728A Expired - Fee Related GB2248627B (en) | 1990-09-17 | 1991-09-16 | Process for purifying a vinylaromatic monomer |
Country Status (5)
| Country | Link |
|---|---|
| CN (1) | CN1061020A (en) |
| GB (1) | GB2248627B (en) |
| HU (1) | HU209668B (en) |
| IT (1) | IT1245845B (en) |
| TN (1) | TNSN91079A1 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2758553A1 (en) * | 1997-01-22 | 1998-07-24 | Rhodia Chimie Sa | PROCESS FOR REMOVING POLYMERIZATION INHIBITORS FROM MONOMER MIXTURES USING DOPED ALUMINA |
| FR2758554A1 (en) * | 1997-01-22 | 1998-07-24 | Rhodia Chimie Sa | PROCESS FOR REMOVING POLYMERIZATION INHIBITORS FROM MONOMER MIXTURES USING OPTIMIZED FORM ALUMINA |
| FR2758552A1 (en) * | 1997-01-22 | 1998-07-24 | Rhodia Chimie Sa | PROCESS FOR REMOVING POLYMERIZATION INHIBITORS FROM MONOMER MIXTURES USING OPTIMIZED ALUMINA |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1073977C (en) * | 1998-11-18 | 2001-10-31 | 中国石油化工集团公司 | Purification method of phenylethylene monomer |
| US7651635B1 (en) * | 2009-02-05 | 2010-01-26 | Nalco Company | Polymer inhibition of vinyl aromatic monomers using a quinone methide/alkyl hydroxylamine combination |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6357538A (en) * | 1986-08-28 | 1988-03-12 | Mitsui Toatsu Chem Inc | Purification of aromatic vinyl monomer |
-
1990
- 1990-09-17 IT IT02149590A patent/IT1245845B/en active IP Right Grant
-
1991
- 1991-09-16 HU HU912978A patent/HU209668B/en not_active IP Right Cessation
- 1991-09-16 GB GB9119728A patent/GB2248627B/en not_active Expired - Fee Related
- 1991-09-17 CN CN91109195.5A patent/CN1061020A/en active Pending
- 1991-09-17 TN TNTNSN91079A patent/TNSN91079A1/en unknown
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6357538A (en) * | 1986-08-28 | 1988-03-12 | Mitsui Toatsu Chem Inc | Purification of aromatic vinyl monomer |
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2758553A1 (en) * | 1997-01-22 | 1998-07-24 | Rhodia Chimie Sa | PROCESS FOR REMOVING POLYMERIZATION INHIBITORS FROM MONOMER MIXTURES USING DOPED ALUMINA |
| FR2758554A1 (en) * | 1997-01-22 | 1998-07-24 | Rhodia Chimie Sa | PROCESS FOR REMOVING POLYMERIZATION INHIBITORS FROM MONOMER MIXTURES USING OPTIMIZED FORM ALUMINA |
| FR2758552A1 (en) * | 1997-01-22 | 1998-07-24 | Rhodia Chimie Sa | PROCESS FOR REMOVING POLYMERIZATION INHIBITORS FROM MONOMER MIXTURES USING OPTIMIZED ALUMINA |
| WO1998032717A1 (en) * | 1997-01-22 | 1998-07-30 | Rhodia Chimie | Method for eliminating inhibitors of polymerisation of monomer mixtures using an alumina with optimised form |
| WO1998032715A1 (en) * | 1997-01-22 | 1998-07-30 | Rhodia Chimie | Method for eliminating inhibitors of polymerisation of monomer mixtures using an optimised alumina |
| WO1998032716A1 (en) * | 1997-01-22 | 1998-07-30 | Rhodia Chimie | Method for eliminating inhibitors or polymerisation of monomer mixtures using a doped alumina |
| US6174480B1 (en) | 1997-01-22 | 2001-01-16 | Rhodia Chimie | Method for eliminating inhibitors of polymerization of monomer mixtures using an alumina with optimized form |
| US6288299B1 (en) | 1997-01-22 | 2001-09-11 | Rhodia Chimie | Method for eliminating inhibitors of polymerization of monomer mixtures using an optimized alumina |
Also Published As
| Publication number | Publication date |
|---|---|
| TNSN91079A1 (en) | 1992-10-25 |
| IT1245845B (en) | 1994-10-25 |
| HU912978D0 (en) | 1992-01-28 |
| HU209668B (en) | 1994-10-28 |
| GB2248627B (en) | 1994-05-18 |
| IT9021495A0 (en) | 1990-09-17 |
| IT9021495A1 (en) | 1992-03-17 |
| HUT59897A (en) | 1992-07-28 |
| CN1061020A (en) | 1992-05-13 |
| GB9119728D0 (en) | 1991-10-30 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19950916 |