PHOSPHORUS CONTAINING OLIGOMERS AND POLYMERS

Abstract

The present invention exploits reactive organophosphorus compounds containing unsaturated vinyl groups, which can be used in a flexible and highly controlled manner to prepare various macromolecular derivatives either via radical reactions or via Michael addition with suitable nucleophiles. Based on the fact that secondary amine groups on the one hand and vinyl groups on the other hand can work as mutual linking sites, an arsenal of novel and useful addition products can be built up. By selecting the number of secondary amine sites and vinyl sites of the participating reaction partners, very different addition products can be formed. In particular, one can form either linear chain type macromolecules (i.e. linear oligomers or polymers) or highly crosslinked network polymers.

Claims

1. A phosphorus containing linear oligomer or polymer comprising general formula (A) ##STR00038## wherein n is at least 2; R.sub.1 is selected from the group consisting of phenyl, substituted phenyl, benzyl, substituted benzyl, a linear or branched alkyl group with up to 5 carbon atoms, and a linear or branched alkoxy group with up to 5 carbon atoms; —X—R′—X— is selected from the group consisting of ##STR00039## wherein R″ is a linear or branched alkyl group with up to 5 carbon atoms in ortho, meta or para position, or —X—R′—X— is —NR.sub.2—R″—NR.sub.2— wherein R.sub.2 is a methyl or alkyl group and R′″ is selected from the group consisting of: a linear or branched alkyl group with up to 5 carbon atoms, ##STR00040##

2. The method of preparing a phosphorus containing oligomer or polymer (A) according to claim 1, the method comprising an addition reaction of a divinyl posphine oxide (B) ##STR00041## and an amine compound (C) containing two secondary amine groups according to
H—X—R′—X—H  (C).

3. A phosphorus containing cross-linked network polymer comprising: (i) general formula (D) ##STR00042## wherein R is selected from the group consisting of ##STR00043## or (ii) general formula (G) ##STR00044## wherein R is selected from the group consisting of ##STR00045## or (iii) general formula (J) ##STR00046## wherein R is selected from the group consisting of ##STR00047## and wherein R.sub.1 is selected from the group consisting of ##STR00048## a linear or branched alkyl group with up to 5 carbon atoms, and a linear or branched alkoxy group with up to 5 carbon atoms.

4. A method of preparing a cross-linked network polymer (D) according to claim 3, the method comprising an addition reaction of trivinyl posphine oxide (E) ##STR00049## and an amine compound (F) containing two secondary amine groups according to
H—N—R—N—H  (F) wherein —N—R—N— is selected from the group consisting of ##STR00050##

5. (canceled)

6. A method of preparing a cross-linked network polymer (G) according to claim 3, the method comprising an addition reaction of trivinyl posphine oxide (E) ##STR00051## and an amine compound (H) containing three secondary amine groups according to
—HN—R(NH—)—NH—  (H) wherein —N—R(N—)—N— is selected from the group consisting of ##STR00052##

7. (canceled)

8. A method of preparing a cross-linked network polymer (J) according to claim 3, the method comprising an addition reaction of a divinyl posphine oxide (K) ##STR00053## and an amine compound (H) containing three secondary amine groups according to
—HN—R(NH—)—NH—  (H) wherein —N—R(N—)—N— is selected from the group consisting of ##STR00054##

9. A phosphorus containing hydrogel or organogel, comprising a cross-linked network polymer according to claim 3 in water or in an organic solvent.

10. The method according to claim 2, wherein the method is carried out during thermal processing of a blend comprising a thermoplastic base polymer and an admixture of: the posphine oxide (B) containing at least two vinyl groups; and an amine compound (C) containing the at least two secondary amine groups.

11. The method according to claim 10, wherein the thermoplastic base polymer is a polyamide, polyolefin, polyester or polycarbonate.

12. A method of forming a phosphorous containing polymer, the method comprising: a) providing a mixture of a base polymer and a phosphine oxide (L) containing at least one vinyl group ##STR00055## wherein, R.sub.1 is selected from the group consisting of a linear or branched alkyl group with up to 5 carbon atoms, a phenyl group and a vinyl group, and R.sub.2 is independently selected from the group consisting of a linear or branched alkyl group with up to 5 carbon atoms, a phenyl group, a vinyl group, a linear or branched alkoxy group with up to 5 carbon atoms and a phenoxy group; said mixture optionally containing a free radical initiator; and b) subjecting said mixture to a radical initiation, thereby effecting an addition reaction wherein a phosphine oxide is grafted to the base polymer.

13. The method according to claim 12, wherein said base polymer is a polyamide or a polyester.

14. The method according to claim 12, wherein said radical initiation is effected by a) thermal activation, or b) ultraviolet or electron beam irradiation of a thin layer or fiber of said mixture.

15. A method for retarding flames comprising providing a composition comprising the phosphorus containing oligomer or polymer according to claim 1, applying the composition to a flame and retarding the flame.

16. A polymeric material with improved flame resistance, comprising the phosphorus containing oligomer or polymer according to claim 1 admixed in a melt processable polymer.

17. The method according to claim 4, wherein the method is carried out during thermal processing of a blend comprising a thermoplastic base polymer and an admixture of: the posphine oxide (E) containing at least two vinyl groups; and the amine compound (F) containing at least two secondary amine groups.

18. The method according to claim 6, wherein the method is carried out during thermal processing of a blend comprising a thermoplastic base polymer and an admixture of: the posphine oxide (E) containing at least two vinyl groups; and the amine compound (H) containing at least two secondary amine groups.

19. The method according to claim 8, wherein the method is carried out during thermal processing of a blend comprising a thermoplastic base polymer and an admixture of: the posphine oxide (K) containing at least two vinyl groups; and the amine compound (H) containing at least two secondary amine groups

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better understood by reference to the following description of various embodiments of this invention taken in conjunction with the accompanying drawings, wherein are shown:

[0070] FIG. 1 an overview of approaches used to make flame retardant polymers;

[0071] FIG. 2 the release behavior of Gel A for Acid Blue 80 at different pH-values, after 24 hours; and

[0072] FIG. 3 the release behavior of Gel A for methylene blue at different pH-values, after 4 hours.

DETAILED DESCRIPTION OF THE INVENTION

[0073] An overview of functional additives playing an important role in the present work is given in the following Table 1:

TABLE-US-00001 TABLE 1 Chemical structures of functional phosphorus compounds and piperazine derivatives used in this work. No. Chemical structure Chemical name DPVPO [00020] diphenyl(vinyl)phosphine oxide DVPPO [00021] divinylphenylphosphine oxide TVPO [00022] trivinylphosphine oxide Neucleo- philes [00023] Piperazine/Piperidine derivatives

Production of Flame-Retardant Polymer Systems

[0074] As shown schematically in FIG. 1, non-leaching flame retardant polymer systems can be obtained by permanently immobilizing phosphorus additives with flame retardant moieties via three different approaches: [0075] 1) The first approach consists of grafting the vinyl containing phosphorus additives (DPVPO, DVPPO and TVPO) to the backbone of a base polymer by radical reaction in an extruder or kneader (reactive extrusion). The radical reaction can be self-catalyzing or can be promoted by addition of a free radical initiator:

##STR00024## [0076] In this manner the flame-retardant phosphorus containing moiety symbolized as “P” is directly attached to the backbone of the base polymer. [0077] 2) The second approach consists of in situ polymerization (reactive extrusion) of DVPPO or TVPO with nucleophiles (Michael Addition) during the thermal processing of a base polymer.

##STR00025## [0078] This leads to formation of long chained phosphorus containing macromolecules entangled within a network of polymer chains forming the base polymer. Although the species with flame retardant effect is not attached to the backbone of the base polymer, it will be prevented from leaching out due to the entanglement. [0079] 3) The third approach consists of physical mixing of the vinyl containing phosphorus additives (DPVPO, DVPPO and TVPO) and a suitable base polymer in an extruder or kneader. The extruded polymer is then converted into thin walled materials like fibers and films and subsequently subjected to UV or e-beam irradiation leading to radical formation and thereby cross-link the additive to the backbone of the base polymer.

##STR00026## [0080] The third approach potentially leads to similar structures as the first approach, although via a different route.

Preparation of Gels

[0081] The gels can be prepared by reacting either divinyl or trivinyl phosphorus compounds with appropriate nucleophiles (see Scheme 1) via a Michael addition reaction in water suitable organic solvents.

##STR00027##

[0082] The general structures of gels made from trivinyl phosphorus derivatives (TVPO) are shown in Scheme 2:

##STR00028##

[0083] The general structure of gels made from divinyl phosphorus derivatives are shown in Scheme 3:

##STR00029##

Examples

1. Flame Retardant Polymers

Processing (Approach 1)

[0084] All phosphorus-based additives used in this work contain one or more vinyl groups that can react with the methylene groups (—CH2-) of PA6 and PET by radical reaction. Table 2 summarizes the list of processing trials performed on the reactive phosphorus additives. In order to promote the radical reaction, a radical initiator was added in some experiments.

[0085] The kneading trials were performed at 240° C. for PA-6 and 260° C. for PET and 30 rpm using a Brabender-mixer. The polymer was fed first and the additives were fed after two minutes, the total mixing time was 10 minutes.

TABLE-US-00002 TABLE 2 Details of formulations of kneading trials (Approach 1) Additive 1 Additive 2 Base content content Sample name polymer [wt %] [wt %] Reaction PA6 PA-6 — — PA6/DP PA-6 5.0% — Radical DPVPO PA6/DP/DCP PA-6 5.0% 0.1% Radical DPVPO DCP PA6/TVPO PA-6 5.0% — Radical TVPO PA6/TVPO/DCP PA-6 5.0% 0.1% Radical TVPO DCP PA6/DVPPO PA-6 5.0% — Radical DVPPO PA6/DVPPO/DCP PA-6 5.0% 0.1% Radical DVPPO DCP PET PET — — PET/DPVPO PA-6 5.0% — Radical DPVPO PET/DPVPO/DCP PA-6 5.0% 0.1% Radical DPVPO DCP PET/TVPO PA-6 5.0% — Radical TVPO PET/TVPO/DCP PA-6 5.0% 0.1% Radical TVPO DCP PET/DVPO PA-6 5.0% — Radical DVPPO PET/DVPPO/DCP PA-6 5.0% 0.1% Radical DVPPO DCP % wt = weight percent

Processing (Approach 2)

[0086] The vinyl groups of the phosphorus additives used in this work can react with the amino groups (—NH—) of PA-6 and/or Piperazine by Michael addition reaction. Table 3 explicates the trial performed on the reactive phosphorus additives. The trial was performed at 240° C. and 30 rpm using a Brabender-mixer.

TABLE-US-00003 TABLE 3 Details of formulation of extrusion trials (Approach 2) Additive 1 Additive 2 Base content content Sample name polymer [wt %] [wt %] Reaction PA6/DVPPO/PIP PA-6 5.0% 2.5% Michael DVPPO PIPERAZINE addition (1:1 mol) % wt = weight percent PIP = Piperazine

Processing (Approach 3)

[0087] The third approach consists of physical mixing of the flame-retardant additive and the polymer by kneading and subsequently e-beam treatment to crosslink the additive to the polymer backbone. This physical mixing is not considered reactive extrusion; however, we cannot avoid reaction of some vinyl groups of the phosphorus additives with the polymer, even in absence of any radical initiator. Some of the materials obtained via approach 1 and 2 were used in the post crosslinking experiments; the list is shown in Table 4.

[0088] Plates (150*50*0.5 mm) of the above materials have been made by compression molding at 260° C. for PA6 and 290° C. for PET. The plates were subsequently exposed to electron irradiation. The energy supplied through the electron beams allows the reaction between the vinyl groups present in the additives and the polymer chains.

[0089] The plates were treated on both sides (the penetration of the electron beam is 200-250 μm) in N2-atmosphere at 200 kV with a speed of 6 m/min. In order to evaluate the amount of reacted additive as a function of the energy supplied doses of 50, 100 and 200 kGy were used.

TABLE-US-00004 TABLE 4 Details of formulation of kneading trials (Approach 3) Additive 1 Additive 2 Base content content Sample name polymer [wt %] [wt %] PA6/DP PA-6 5.0% — DPVPO PA6/DV PA-6 5.0% — DVPPO PA6/DV/PIP PA-6 5.0% 2.5% DVPPO PIPERAZINE (1:1 mol) PET/DP PET 5.0% — DPVPO

Thermal Data

[0090] Table 5 summarizes the thermal data of all polymer formulations obtained by Approach 1 and Approach 2. From the thermal gravimetric analysis (TGA) performed, it is clear that additives lower the decomposition temperature of the PA in air and nitrogen and this effect is more evident in the materials processed with DCP. No significant differences in melting and crystallization temperature were detected from differential scanning calorimetry analysis (DSC).

TABLE-US-00005 TABLE 5 Thermal data of all formulations TD- TD- Crystal- TD5%/ main/ TD5%/ main/ Melting lization Air Air N.sub.2 N.sub.2 Point Point Sample name [° C.] [° C.] [° C.] [° C.] [° C.] [° C.] PA6 354 428 388 453 222 195 PA6/DPVPO 325 417 365 426 223 189 PA6/DPVPO/DCP 361 409 370 405 224 188 PA6/TVPO 325 406 359 413 220 184 PA6/TVPO/DCP 337 409 358 417 221 191 PA6/DVPO 343 409 352 420 220 189 PA6/DVPO/DCP 362 422 381 423 217 191 *PA6/DVPO/PIP 326 424 347 426 220 180 PET 386 395 400 430 250 190 PET/DPVPO 374 390 379 399 254 205 PET/DPVPO/DCP 368 384 378 397 253 201 PET/TVPO 375 391 381 392 245 195 PET/TVPO/DCP 369 388 378 389 244 194 PET/DVPO 376 392 381 391 250 198 PET/DVPO/DCP 373 387 379 385 248 196 *Material obtained via approach 2

Evaluation of Phosphorus Content and Retention of Various Polymer Formulations (Approach 1 and 2)

[0091] The extruded polymers were then grinded and extracted with chloroform. The P-analysis using ICP instrument was done for each sample before and after extraction to calculate the flame retardant retained. The various formulations were evaluated for % P content using ICP-OES method. To estimate the % P retention the polymer formulations were extracted with chloroform at 100° C. for 1 hour and then estimated for % P retention. Table 6 presents the phosphorus content and its retention after solvent extraction.

TABLE-US-00006 TABLE 6 Phosphorus content and retention of all formulations Phosphorus Phosphorus Phosphorus content content content After Phosphorus Theoretical Actual Extraction** Retention Sample name [wt %] [wt %] [wt %] [%] PA6 — — — — PA6/DPVPO 0.68 0.51 0.13 25.5 PA6/DPVPO/ 0.68 0.62 0.48 77.4 DCP PA6/TVPO 1.20 1.14 0.90 78.9 PA6/TVPO/ 1.20 0.87 0.55 57.5 DCP PA6/DVPPO 0.87 0.77 0.33 42.9 PA6/DVPPO/ 0.87 0.71 0.51 71.8 DCP *PA6/DVPPO/ 0.87 0.74 0.43 58.1 PIP PET PET/DPVPO 0.68 0.60 0.01 1.3 PET/DPVPO/ 0.68 0.61 0.09 15.3 DCP PET/TVPO 1.20 0.86 0.43 49.5 PET/TVPO/ 1.20 0.86 0.59 68.6 DCP PET/DVPPO 0.87 0.76 0.34 44.7 PET/DVPO/ 0.87 0.74 0.30 40.5 DCP *Material obtained via approach 2 **The extraction was performed in Chloroform (1 ml solvent /100 mg material) at 100° C., 30 bars for 1 hour. The solution was stirred. % wt = weight percent

[0092] The trial PA6/DV/PIP has showed promising processability characteristics as well as higher phosphorus retention, which led to the production of compound (PA6/DV/PIP-Comp) in kilogram quantity and subsequent fibers (PA6/DV/PIP-FB) with the same concentration of additives. The virgin PA-6 was previously dried in a vacuum oven at 100° C. for 12 hrs and then physically premixed with the additives for 30 mins. This compound was obtained using a corotating twin screws (16 mm) compounder; the processing temperature and the temperature of the die were respectively 265° C. and 251° C. at 110 rpm. The output rate of the compounder was 500 g/hr. The same compound was used to produce fibers; the processing temperature and the temperature of the spin pack were 275° C. and 245° C. respectively and the output rate was 360 cm.sup.3/hr. The resulting filament was drawn up to a draw ratio of 4 which led to final fiber diameter of 70 μm.

TABLE-US-00007 TABLE 7 Phosphorus content and retention of compound and fibers Phosphorus Phosphorus Phosphorus content content content After Phosphorus Theoretical Actual Extraction Retention Sample name [wt %] [wt %] [wt %] [%] PA6-Virg — — — — PA6/DV/PIP 0.87 0.74 0.43 58.1 PA6/DVPPO/ 0.87 0.73 0.43 58.9 PIP-Comp PA6/DVPPO/ 0.87 0.73 0.54 74.0 PIP-FB % wt = weight percent Virg = virgin, Comp = Compound, FB = Fiber

[0093] As shown in Table 7, compounding and kneading (PA6/DV/PIP-Comp and PA6/DV/PIP) gave similar results in terms of phosphorus retention. Instead, the % P retention significantly increased for the fibers, it is probably due to the longer processing of the material, which leads to a higher reaction yields (Michael Addition).

Mechanical Property of Fibers

[0094] The mechanical properties of the fibers were investigated; they are summarized in Table 8 and data presented are the average over 20 measurements.

TABLE-US-00008 TABLE 8 Mechanical properties of the obtained fibers Force at Elongation Young's break at break Modulus Sample name [cN/dtex] [%] [cN/dtex] PA6-FB 6.0 52.3 23.0 (Stdev %) (2.4) (5.5) (3.8) PA6/DVPPO/ 4.1 40.1 34.2 PIP-FB (4.8) (11.7) (6.4) (Stdev %)

Evaluation of Phosphorus Content and Retention of Various Polymer Formulations (Approach 3)

[0095] The e-beam treated plates were then grinded and extracted with chloroform. The P-analysis using ICP instrument was done for each sample before and after extraction to calculate the flame retardant retained. The various formulations were evaluated for % P content using ICP-OES method. To estimate the % P retention the polymer formulations were extracted with chloroform at 100° C. for 1 hour and then estimated for % P retention. Table 9 summarizes the various polymer formulations, e-beam treatments intensity and their % P content and retention.

TABLE-US-00009 TABLE 9 Phosphorus content and retention of the ebeam-treated materials (Approach 3) Phosphorus Radiation Phosphorus content energy content After Phosphorus absorbed Actual Extraction Retention Sample name [kGy.sup.#] [wt %] [wt %] [%] PA6/DPVPO — 0.51 0.13 25.5  50 0.51 0.46 90.2 100 0.48 0.46 95.8 200 0.51 0.47 92.2 PA6/DVPPO — 0.77 0.33 42.9  50 0.76 0.70 92.1 100 0.77 0.70 90.9 200 0.78 0.72 92.3 *PA6/DVPPO/ — 0.74 0.43 58.1 PIP  50 0.69 0.70 100 100 0.70 0.69 98.6 200 0.67 0.68 100 PET/DPVPO — 0.60 0.01 1.3  50 0.61 0.07 12.2 100 0.59 0.07 11.7 200 0.60 0.09 15.7 .sup.#The “gray” (Gy) is defined as the absorption of one joule of radiation energy per kilogram of matter.

[0096] As shown in Table 9, even with low e-beam irradiation, phosphorus retention higher than 90% was achieved for all the PA6-based materials. Thus, this procedures offer a novel way of permanently immobilizing FR additives in the polymer. For PET-based material, after the e-beam irradiation, the phosphorus retention increased significantly but the values are still low.

Fire Tests Materials Obtained Via Approach 2

[0097] Small-scale fire tests were performed on various formulations to evaluate their fire behavior. Limiting oxygen index (LOI) test and vertical burning test (BKZ— Swiss standard) were performed on plates of PA6-MB and PA6/DV/PIP-Comp. For both tests, plates (150*50*0.5 mm) of the above materials have been made by compression molding at 260° C.

[0098] The LOI is the minimum concentration of oxygen, expressed as a percentage, which will support combustion of a polymer; it is measured by flowing a mixture of oxygen and nitrogen over a burning specimen, the test is repeated reducing the oxygen concentration until the flame does not propagate.

[0099] The test consists in putting in contact the lower edge of the samples with a propane gas flame (40±2 mm in length) for 15 s. The burner is inclined by 45° relative to the vertical line. The damaged length and the afterglow time are measured.

TABLE-US-00010 TABLE 10 Fire test results Limiting Oxygen Damaged Afterglow Index .sup.□ length .sup.□ time .sup.□ Sample name [%] [cm] [s] PA6-MB 25.6 4.80 18 PA6/DVPPO/ 30.2 2.27 1 PIP-MB .sup.□ The values are the average of 3 tests.

[0100] Thus, it is clear such flame-retardant modifications improve the fire protections of polyamide 6 remarkably.

2. Gels

Synthesis of Gel-A

[0101] TVPO (64.02 mg, 0.50 mmol) and piperazine (64.60 mg, 0.75 mmol) were added to water (2.5 ml). The resulting mixture was stirred at 90° C. for 0.5 h and a colorless transparent gel was obtained. The solvents were evaporated by freeze-drying.

##STR00030##

Synthesis of Gel-A1

[0102] TVPO (64.02 mg, 0.50 mmol), piperazine (64.60 mg, 0.75 mmol) Polyethylene glycol (20K) (12.80 mg, 10%) were added to water (2.5 ml). The resulting mixture was stirred at 90° C. for 0.5 h. The solvents were evaporated by freeze-drying.

##STR00031##

Synthesis of Gel-A2

[0103] TVPO (64.02 mg, 0.50 mmol), piperazine (64.60 mg, 0.75 mmol) Polyethylene glycol (200K) (6.40 mg, 5%) were added to water (2.5 ml). The resulting mixture was stirred at 90° C. for 0.5 h. The solvents were evaporated by freeze-drying.

##STR00032##

Synthesis of Gel-A3

[0104] TVPO (64.02 mg, 0.50 mmol), piperazine (64.60 mg, 0.75 mmol) Polyethylene glycol (300 K) (6.90 mg, 5%) were added to water (2.5 ml). The resulting mixture was stirred at 90° C. for 0.5 h. The solvents were evaporated by freeze-drying.

##STR00033##

Synthesis of Gel-B

[0105] TVPO (128 mg, 1 mmol) and DPP (157.7 mg, 0.75 mmol) were added to Ethanol (2.5 ml). The resulting mixture was stirred at 80° C. for 1.5 h. The solvents were evaporated by freeze-drying.

##STR00034##

Synthesis of Gel-C

[0106] TVPO (64.02 mg, 0.50 mmol) and 1,10-di(piperazin-1-yl)decane (232.89 mg, 0.75 mmol) were added to ethanol (5 ml). The resulting mixture was stirred at 85° C. for 8 h and a colorless transparent gel was obtained. The solvents were evaporated by freeze-drying.

##STR00035##

Synthesis of Gel-D

[0107] TVPO (64.02 mg, 0.50 mmol) and 2,4,6-tri(piperazin-1-yl)-1,3,5-triazine (166.72 mg, 0.50 mmol) were added to ethanol (5 ml). The resulting mixture was stirred at 85° C. for 1 h and a colorless transparent gel was obtained. The solvents were evaporated by freeze-drying.

##STR00036##

Synthesis of Gel-E

[0108] DVPO (133.54 mg, 0.75 mmol) and 2,4,6-tri(piperazin-1-yl)-1,3,5-triazine (166.72 mg, 0.50 mmol) were added to ethanol (5 ml). The resulting mixture was stirred at 85° C. for 8 h and a colorless transparent gel was obtained. The solvents were evaporated by freeze-drying.

##STR00037##

Properties of Gels

1. Swelling Behavior

[0109] Swelling ratio of cross-linked gels was measured by soaking the gel in a particular solvent till to reach equilibrium swelling. Then, the swelled gel was taken out by spatula on a butter paper carefully, blotted quickly with a moist tissue paper (in respective solvent) and weighed. The solvent uptake ratio (swelling ratio, SR) of swelled gel was determined following the formula:

[00001]$\mathrm{Swelling}\mathrm{Ratio}\left(\mathrm{SR}\right)=\frac{\mathrm{Ws}-\mathrm{Wd}}{\mathrm{Wd}}$

[0110] Where, Ws and Wd represent the weight of swelled and dry crosslinked gels respectively. The swelling behavior of synthesized gels have been investigated in solvents having different polarities and results are summarized in Table 11.

TABLE-US-00011 TABLE 11 Swelling ratios of gels in different solvents Gels Toluene DCM EtOH Water Gel-A  1.91 16.56 11.2 20.91 Gel-A1 — — — 9.7 Gel-A2 — — — 10.19 Gel-A3 — — — 9.2 Gel-B 1.9 16.5  11.6 0.5
2. pH-Responsiveness of Hydrogels

[0111] pH responsive properties of hydrogels were performed at room temperature in the range of pH 2 to 7.4. pH of the aqueous media was adjusted by 0.1 N NaOH or 0.1 N HCl solution. A measured amount of gel was soaked at a particular pH till equilibrium swelling was achieved, then, taken out, blotted quickly with a moist tissue paper and weighted. Swelling ratio was calculated as mentioned earlier.

TABLE-US-00012 TABLE 12 Effect of pH on swelling ratio of Gel A pH 2 pH 3 pH 4 pH 5 pH 6 pH 7 pH 7.4 23.65 13.85 12.42 7.35 5.78 20.91 4.93

3. Drug Release Property of Gels

[0112] The release behavior of a gel describes the release of certain substances trapped in the gel matrix. This can be influenced by different factors such as pH, temperature, ionic strength, electric field or specific analyte concentration gradients. Depending on the factor, the gels are suitable for different application areas. Healthy human skin is slightly acidic due to secretion of lactic acid and sebum and has a pH of about 5. In certain injuries, the pH changes to neutral or basic 10 (blood pH=˜7.4). Such stimuli can be utilized to trigger the release of active ingredients. Once the skin has regenerated, the pH drops and the release is inhibited or stopped altogether. Gels that are pH-sensitive can be applied to wounds.

[0113] To study the drug release behavior, Methylene blue and Acid Blue 80 were chosen as the model molecules. The model molecule was initially dissolved in water (1 mg/1 ml) at room temperature and then gels were soaked in it. After 4 h the gels were removed from the solution and washed with water repeatedly till colorless water was obtained. As qualitative estimation for gel A as an example, the release of Acid Blue 80 was estimated visually. The figure (FIG. 2) clearly indicates release of Acid Blue 80 preferentially at pH higher than 7.

[0114] Similarly, the qualitative estimation of release behavior of gel A containing methylene blue (model basic drug) was also recorded. The figure (FIG. 3) clearly indicates higher release of Methylene blue preferentially at lower pH.