CA2007204A1

CA2007204A1 - Solid gel dispensers for achieving controlled release of volatile liquid materials and method for preparing same

Info

Publication number: CA2007204A1
Application number: CA002007204A
Authority: CA
Inventors: Daniel Graiver; Robert Edward Kalinowski
Original assignee: Dow Corning Corp
Current assignee: Dow Silicones Corp
Priority date: 1989-01-26
Filing date: 1990-01-05
Publication date: 1990-07-26
Also published as: EP0380103A2; DE69004978T2; JPH02237567A; EP0380103B1; AU4873890A; US4888364A; DE69004978D1; EP0380103A3; KR900011798A; AU625978B2

Abstract

SOLID GEL DISPENSERS FOR ACHIEVING CONTROLLED
RELEASE OF VOLATILE LIQUID MATERIALS AND
METHOD FOR PREPARING SAME

ABSTRACT

Aesthetically attractive, free standing dispensers for the controlled release of volatile liquid materials such as perfumes and fragrances into the atmosphere consist essentially of composite hydrogels comprising a continuous phase of a solubilized polyvinyl alcohol, a dispersed phase comprising a polyelectrolyte and an aqueous solution of the volatile liquid that is distributed between said continuous and dispersed phases. The exterior surfaces of the dispenser remain dry to the touch throughout its useful life.

Description

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SOLID GEL DISPENSE~S FOR ACHIEVING CONTROLLED
RELEASE OF VOIIATILE LIQUID MATERIALS AND
METHOD FOR PREPARING SAME

This invelltion relates to the controlled release of volatile materials. More particularly, this invention relates to improved dispensers for volatile liquid materials 3uch as an air freshener fragrance or a biologically active composition. The dispenser con~istsl essentially of a solid polyvinyl alcohol hydrogel containin~ a di~per~ed poly~
electrolyte and an aqueoll~ solution of the volatile material.
In accordance with one aspect of this invention aesthetically attractive, free standing dispensers :for the controlled relea~e of volatile liquid materials such as perfumes and fragrances into the atmosphere are prepared by 1) formin~ a first aqueous composition comprising a solubilized polyvinyl alcohol, water, a polyelectrolyte and an amount of a water-mi~cible liquid sufficient to maintain said polyelectrolyte as a di~persed phase while avoiding precipitation o the polyvinyl alcohol, said water-miscible liquid being ~e~ected from the group consisting of monohydric alcohols and dimethyl sulfoxide, 2) cooling the resultant aqueous composition to below room temperature to form a hydrogel in the shape of the final dispenser, 3) when dimethyl sulfoxide is an ingredient of said aqueous composition, immersing the hydrogel in methanol for a sufficient time to Pxtract said dimethyl sulfoxide, and then 4) immersing said hydrogel in a second aqueous composition comprising water~ said volatile liquid material and a monohydric alcohol containing from 1 to about 3 carbon atoms in an amount sufficient to solubilize the volatil~
ingredient.
The present dispensers contain up to 99 weight percent o~ a aqueous mixture containing the volatile liquid . .

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~:007;Z04 material, which i9 dispensed at a uniform, controllable rate into the atmosphere ad~acent to the di~penser. The surface of the dispenser remains dry to the touch throughout the dispensing proces~.
This invention provi~es an improved di~penser for achieving controlled release over an extended period of time o~ a volatile liquid into the environment adjacent to the dispenser, the dispenser comprising a hydrogel fo~med from an aqueous solution of polyvinyl alcohol. The improvement compri~es (1) the presence in said hydrogel of from 1 to 20 percent, ba~ed on the total weight of said hydrogel, of a polyelectrolyte that is present as a finely divided dispersed phase within a continuous phase of said hydrogel and a monohydric alcohol containing rom one to fo~r carbon atoms in an amount sufficient to solubilize said volatile liquid and without precipitating the polyvinyl alcohol, (2) a method for preparin~ said dispenser whereby at least a portion of said volatile liquid is incorpora~ed into said dispenser by immer~ing it in an aqueous mixture comprising said volatile liquid and (3) a free standing dispenser possessing an exterior surface that is initially dry to the touch and remains so throughout the useful life of said dispenser.
The characterizing features of this invention include (1) a di~penser that is free standing, remains dry to the touch throughout its useful life and is fabricated from a hydrogel comprising a continuous phase containing from 5 to 25 percent, ba~ed on the weight of said dispenser, of a 301ubilized polyvinyl alcohol (PVA) and a finely divided dispersed phase containing from 1 to 20 percent, based on the weight of said dispenser, of a polyelectrolyte, and (2) up to 99 percent, based on the total weight of the hydrogel, of an aqueous composition compr.ising water, a volatile liquid to be released from said dispenser and at least one water-miscible ,'~' ' .

aliphatic monohydric alcohol containirlg from ]. to about 4 carbon atoms, the concentration of said alcohol being sufficient to solubilize said volatile liquid.
The polyvinyl alcohol that comprises the polymer portlon ~f the continuous pha~e in the present dispenser3 is typically prepared by hydroly~is or saponification of poly~inyl acetate. The degree of hydrolysis varies depending upon the intended end use of the polymer. The vinyl alcohol polymers of this in~ention are preferably fully hydrolyzed and are linear or contain at mo~t a minimal degree of branching. The reason for this preference is to achieve the maxim~lm degree of hydrogen bonding. The formatlon of hydrogen bonding between hydroxyl groups on adjacent polymer molecules and crystallization of the polymer are considered responsible for the development of the excellent physical properties associated with this type of polymer.
The molecular weight of the PVA used to prepare the gel and, ultimately, the dispenser is determined by the desired physical and aesthetic properties of the dispenser.
The molecular weight of the PVA should not be less than 44,000, preferably not le~s than 75,000. Commercially available polymers with molecular weights of from 75~000 to 440,000 are preferred for preparing the present dispensers, particularly those polymers containing relatively large concentrations of syndiotactic or isotactic ~egments within the polymer molecules.
To maximize the concentration of active ingredient, i.e. the fragrance or other volatile liquid, in the dispenser the concentration of PVA in the .initial solution should be as low as will allow formation of a hydrogel that retains its integrity and a dry surace in the pre~ence of the solubilized active ingredient. The operable and preferred `
, , 2 ~0 concentration ranges for the PVA will be at lea~t i.n part dependent upon the molecular weight of this polymer.
Typically the propertie~ of PVA gels, particularly tensile strength and elongation at break, increase with increasing concentration and/or molecular weighk of the polymer. PVA concentrations of below about 10 weight percent are preferred, although higher concemtrations of polymer can be used if it is desired to improve physical properties at the cost of reducing the concentration of liquid ingredients in the final dispenser.
The ability o the present dispensers to absorb an amount of water equal to up to 8000 times the weight o the polymeric ingredients present in the hydrogel is due to the presence of a polyelectrolyte that is present a~ a Pinely divided dispersed phase within a matrix of solubilized polyvinyl alcohol.
Suitable polyelectrolytes are capable of being ionized and dissociated on contact with water, but are insoluble in the liquid PVA compositions used to prepare the present hydrogels and dispensers. Examples of suitable polyelectrolytes include but are not limited to polyacrylic acid, polymethacrylic acid, sodium, potassium and other metal salts-of polyacrylic acid and polymethacrylic acid, polyethyleneimine, polymethacrylamide, partially hydrolyzed polyacrylamide, partially hydrolyzed polyalkylacrylamide, polyphosphoric acid, polyethylenesulfonic acid, polystyrene-sulfonic acid, polyvinylamine, polyvinylsulfonic acid, polyvinylpyridines, poly-2-acrylamido-2-methylpropanesulfonic acid, copolymers of monomers havang an ionizable atomic group such as copolymers of vinyl alcohol and acrylic acid, water-soluble derivatives of natural substance~ and polymers thereof. These polyelectrolytes can be used singly or in the ~orm of mixtures o:E two or more of them. Alkali metal and alkaline earth metal salts, especially sodium salts, of polymerized ethylenically unsaturated acids such as acrylic acid are preferred.
The polyelectrolyte portion of the present hydrogel~ e~ists as a dispersed particulate phase within a matrix of PVA. The particle size range of the poly-electrolyte is typically from about 10 to about 50 microns.
The presence of the polyelectrolyte as a discrete phase can be recognized when the water is removed form the hydrogel.
The polyelectrolyte is preferably crosslinked to the extent that it will expand but not dissolve when immersed in water. The reason for this preference is to avoid migration of the polyelectrolyte together with the liquid material that diffuses to the surace during release of the volatile liquid from the dispenser. Under these condi.tions the quantity of linear polyelectrolyte reaching the surface of the dispenser may be sufficient to impart a stic~ or tacky touch to the surface.
A second route by which composition of the hydrogel can be altered is by leaching out of water soluble linear polyelectrolytes when the dispenser is regenerated by standing in excess water. For certain end-use applications it may be desirable to refill the present gel dispensers when the initial supply of liquid materials is depleted. This is conveniently accomplished by immersing the depleted hydrogel in an aqueous solution of the volatile liquid and allowing the gel to expand as the solution i9 absorbed. Thi~
immersion process can be repeated when the initial supply of volatile liquid is depleted by eyaporation.
Hydrogelq con~aining the preferred cross linked polyelectrolytes can be sub~ected to more of these expanding and contracting cycles without significant changes in composition re~ulting from solubilization of the ,.

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polyelectrolyte in the water that gradually diffuses out of the hydrogel as it contracts.
Another method for reducirlg loss of polyelectrolyte from the dispenser i~ to employ as the polyelectrolyte block copolymer~ containing a PVA block and a polyelectrolyte block. The~e block copolymers have a reduced tendency to diffuse out of the hydrogel composite during repeated expanding and contracting cycles because of the compatibility of the PVA block with the PVA present in the matrix portion of the present dispensers. It is believed that the PVA block forms hydrogen bonds with the PVA matrix in which the polyelectrolyte is dispersed. These hydrogen bonds act as anchors to hold the polyelectrolyte within the hydroge:l matrix. Block copolymers of vinyl alcohol and acrylic acid and block copolymers of vinyl alcohol and methacrylic acid are preferred.
The total solids concentration and the weight ratio of polyelectrolyte to PVA will affect a number of properties o~ the present composite hydrogels, particularly the maximum amount of water the g~l will absorb. To fully utilize the advantages of composite hydrogels for the controlled release of volatile liquids the hydrogel should contain the maximum amount of polyelectrolyte that will yield a self-supporting article. With the exception of the aforementioned block copolymers, a significant portion of non-crosslinked polyelectrolytes are leached out when the dispenser is placed in an aqueous solution of the volatile liquid. It is therefore possible to form self-supporting dispenser from compositions containing up to about 80 weight percent of these polyslectrolytes, based on the combined weight of polyelectrolyte and PVA. On the other hand, polyelectrolytes that are either crosslinked or block copolymers containing PVA segment~ are not readily leached out. The concentration ~)07~

of these polyelectrolytes is therefore limited to a maximum of about 70 percent, based on the combined weight of polyelectrolyte and PVA if the final dispen~er is to be self-supporting.
Hydrogels of the present type can be prepared by uniformly dispersing the polyelectrolyte throuehout a solution of PVA. The PVA is dissolved in a mixture of water and a water miscible liquid that can be dimethyl sulfoxide or a monohydric alcohol containing from 1 to 4 carbon atoms.
The concentration of dimethyl sulfoxide or alcohol i~
sufficient to maintain the polyelectrolyte as a dispersed phase within the ~olubilized PVA without causing precipitation of the PVA. For dimethyl sulfoxide this concentration range i9 typically from 50 to about 85 weight percent, based on the combined weight of this solvent and water, while for alcohol this range is typically from 10 to about 40 percent by weight, preferably from 20 to 35 percent by weight. Alcohol concentration ~reater than about 40 weight percent often result in precipitation of the PVA.
The solution of PVA and dispersed polyelectrolyte in one of the a~orementioned water/alcohol or water/DMS0 mixtures is cooled to a temperature lower than 25C., preferably below 0C., to form a hydrogel. This low-temperature treatment promotes gelation and yields a solid gel wherein the polyelectrolyte is dispersed in as finely divided solid particles within a PVA matri~. The container in which the ~olution is cooled preferably conforms to the shape of the final dispenser.
The cooling period required to form a hydrogel from the PVA solution is cooled depends upon a number of variable3, includi~g the concentration and molecular weight oi` the polymer, the rate at which the solution is cooled and the rate at which the resultant gel is returned to ambient 2~07~

temperature. Taking all of these variable~ into account, this time period is generally from 2 to 16 hours.
The present inve~tors found that for the types and concentrations of polymers and the range of processing condition~ they evaluated no significant additional improvement in properties of the final dispenser was apparent after 16 hours of cooling.
When the water miscible l:iquid present in the initial PVA solution is dimethyl su.lfoxide (DMS0), the next step in preparing the present dispenser is replacement o the DMS0 with a volatile alcohol such as methanol or ethanol by placing the hydrogel in contact with an excess of this alcohol. Typically the hydrogel is place in a container filled with the alcohol and allowed to remain there for a period o.~ time suficient to extract the DMS0. This time period is usually from 24 to 48 hours.
The final step in th~ method for preparing the present dispensers is to place the hydrogel, which is preferably in the form of the final dispenser, in an aqueous 801ution of the volatile liquid to be released during operation of the dispenser. If the volatile liquid is not soluble in water, the solution also contains an amount of a monohydric alcohol sufficient to solubili~e the volati.le liquid. Because the alcohol is not a swellin~ agent for the polyelectrolyte and may thereby reduce the weight of liquid capable of being absorbed by the dispenser, tha amount of alcohol should be limited to the minimum required to solubilize the volatile ingredient. Becau3e many volatile ingredients, particularly fragrances and biologically active compounds, are not readily soluble in a water-alcohol mixture, one or more water-miscible organic compounds such as ketones or amides such as N,N-dimethylformamide and N,N-dimethylacetamide may also be required.

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~9-During this step of the proce59 the di~penser may swell to 2 or more times its original volume.
As disclosed in preceding sections of this specification, the liquid ingredient:s constitute up to about 98 weight percent of the final hydrogel and the dispensers fabricated from these hydrogels. The maximum amount of liquid material will depend at least: in part upon the relative concentrations oP polyelect:rolyte and water itl the hydrogel.
The fragrance or other volatile active ingredient to be released rom the present dispen~er~ typically constitutes from about 5 to about 25 weight percent of the liquid, i.e. non-polymer, ingredients present in the dispensers of this invention. When the volatile material is a fragrance, a variety of both nat-1ral and synthetic Ploral and herbal ~ragrances are commercially available and can be used for this purpose.
An alternative method for preparing the final dispenser is cooling the initial PVA/polyelectrolyte mixture to form a rod or similarly ~haped elongate article that is subsequently placed in the solution of volatile ingredient and allowed to qwell, after which it is cut into pellets.
Because the gel behaves a~ a thermoplastic material, the pellets can be melted and fabricated into the desired shape.
The pellets themselves can be placed in a suitable decorative container having at least one aperture through which the volatile liquid contained within the hydrogel is released into the atmo~phere.
The advantage of the present hydrogels containing up to 95 percent by weight or more of a solubilized volatile ingredient as ~he liquid phase is their ability to be used directly as a fre~ standing dispenser for an air freshener fragrance or other volatile liquid. The gel does not have to , 2~07~

be confined in a container to avoid leakage of liquid material and/or requiring the ultimate user of the dispenser to handle an exposed wet surface or view an aesthetically unattractive article at any time durin~ the useful life of the dispenser. As disclosed in the preceding specification, the exterior surface of the initial dispenser i9 dry to the touch and remains dry until the all o the air freshener fragrance or other active ingredient: has been released.
It should be apparent that the external dimensions of articles formed from the present air freshener compositions will decrease sub3tantially as the mixture of active ingredient and volatile solvents evaporates into the atmosphere. Under given conditions of temperature and humidity the rate at which the active ingredient is released into the atrnosphere will be directly proportional to the external dimensions of the dispenser. The reduction in external dimensions will cease when the supply of active ingredien~ is exhausted, thereby providing an obvious signal to the ~ser that the useful life of the dispenser is complete. A second me'thod for determining the end of the useful life is to include in the composition used to prepare the dispenser a small amount of a basic or acidic material and a pH indicator such as thymol blue that will unde'rgo a color change within the pH range o from 5 to about 8. The present inventor~ have found that the indicator will undergo a color change as the concentration o~ active ingredient approaches zero.
Volatile liquid materials other than pPrfumes and fraKrances that can be release~ ,at a controlled rate using the PYA dispensers o~ this invention include but are not limited to deodorizers and physiologically or biologically active materials such as insect attractants or repellents, 2~0~ 4 pheromones, disinfectants, pesticides, pres~rvatives and vaporized medications.
The following examples are intended to describe preferred embodiments of the present invention and should not be interpreted as limiting the scope of the invention as defined in the a~companying claims. Unless otherwise specified all parts and percentages ~pecified in the examples are by weight and viscosities were measured at 25C.
Examp1e This example de~cribes the preparation of a preferred type of air freshener dispenser.
A dispenser of this in~ention was prepared by blending the following ingredient~ to homogeneity in a glass reactor equipped with a mechanically operated stirrer, water cooled condenser and ther~ometer:
15 parts of a polyvinyl alcohol with a degree of hydrolysis of at least 99.8 mole % and exhibiting a number average molecular weight of 86,000;
30 parts of a copolymer of polyvinyl alcohol and polyacrylic acid, available as SP-S10 from Sumitomo Chemical as the polyelectrolyte;
228 parts dimethyl su~foxide; and ~ 57 parts water.
The resultant slurry was heated or 30 minutes at 100C. to dissolve the polyvinyl alcohol. The poly-electrolyte remained dispersed in the solution. The composition wa~ allowed to cool to 60C., at which time it was pourad into a mold fabricated from silicone rubb~r. The filled mold was then ~tored for about 16 hours in the freezer compartment of a residential type refrigerator-freezer that was maintained at a temperature of -20C. The resultant molded dispenser of this invention, which was in the shape of a fro~ (Sample 1), was removed from the mold and placed into .

2~ 7~

a methanol bath to extract the dimethyl sul:Eoxide. The methanol was replaced twice during a two-day period, following which the di~pen~er wa~ removed from the bath and allowed to dry.
The di~penser was then placed in a solution of a floral type fragrance dissolved in a 65/35 weight ratio mixture of waterlethanol. The di~pen~er was removed Erom the fragrance solution after 24 hours. The dispen~er weighed 1$8 grams.
To determine the rate at which the ragrance and other volatile liquids were released, the dispen er was allowed to remain under ambient conditions while the weight of the dispenser wa9 measured periodically using an analytical balance. The rate of weight 1089 between each of the weight measurements was calculated and appears in Table 1. The dispenser remained dry to the touch throughout the entire testing period.
For purposes of comparison, the rate of weight loss exhibited by a di~penser prepared without a polyelectrolyte was determined. The dispen3er wa~ prepared by blending to homogeneity 36 parts each of water and a finely divided polyvinyl alcohol ~PVA) exhibiting a mol~cular weight of 86,0~0. The mixture was heated with stirring under ambient pres3ure. When the temperature reached 90C., 60 parts of a solution of a floral type fragrance in a 65/35 weight ratio water/ethanol mi~ture wa~ added to the reactor. The re~ultant mi~ture was heated at a temperature of 87C. for about ten minutes until the polymer had completely dis~olved, at which time 26~ parts of the fragrance 301ution were added and heating wa~ continued for an additional 20 minute~. An additional 19.38 parts o-f PVA and 19.4 part~ o~ ethanol were then added to achieve a PVA concentration of 9.070 and a 65/35 wei~ht ratio of water/ethanol in the final solution.

2 [)~7204 The PVA solution was allowed to cool to 60C., at which time it was poured into the same mold u~ed to prepare sample 1. The filled mold was then stored Eor about 64 hours in the freezer compartment- of a residential type refrigerator-freezer that was maintained at a temperat~re of -20C. The resultant dispen~er was removed and tested for release rate. These data appear in Table 1 under the heading "Comparative Sample".

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, 2~)0~20 These data demonstrate that the initial relea~e rate for the present dispenser wa9 larger than the rate for the comparative sample by a factor of almost 5 and remained greater throughout the test period.
Example 2 _ This e~ample demon~trates the direct relationship between ~he ratio of polyvinyl alcohol to polyele~troly~e and the rate at which a volatile liquid composition is released from the present dispenser.
Three films were prepared by blending to homogeneity polyvinyl alcohol (PVA), the polyelectrolyte describecl in the yreced.ing E~ample 1 (SP-S10), dimethyl sulfoxide (DMSO) and water to form a slurry. The amounts of these ingredients used to prepare the three compositions are listed in Table 2. The slurry was heated for half an hour at a temperature of 100C. then poured into a petri dish to form a 4.8 mm-thick layer. The compositions were gelled by placing the dishes in the freezer compartment of a residential refrigerator-freezer maintained at a temperature of -20C. for about 16 hours. The resul~ant film was then placed in a container of methanol to extract the dimethyl sulfoxide. The methanol was replaced twice over a period of 48 ho~rs. The film~ were then dried to remove the methanol and divided into samples weighlng ~rom 0.1 to 0.4 grams each.
The weight of each sample was recorded prior to placing it into a solution of a floral type fragrance in a 65/35 weight ratio water/ethanol mixture and remained in this solution under ambient conditions until no additional increase in weight was observed, which required 7 days. The amount of fragrance solution absorbed during this period, expressed as a percentage of the weight of the film prior to being placed in the fragrance solution, is recorded in Table 2.

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The samples were then allowed to remain under ambient conditions and weighed periodically to determined the amount of material that had been released. The percentage of the fragrance mixture remaining in each sample at each of these weighings was calculated. The calculated data are recorded in Table 3.

T_le 2 Sample 2 3 4 PVA (parts) 10 10 5 SP-510 ~parts) 2.5 5 10 DMS0 (parts) 90 108 108 Water (parts) 22.527 27 Percent Weight Increase During Liquid Absorption 5731046 3445 26~(~7204 Table 3 Elapsed Time Weight Percent of Liquid Remaining (Hour~) Z 3 4 O 100 100 lO~
6.82 27.Z7 56.~6 70.46 22.60 2.27 17.21 4g.63 31.28 2.27 6.56 28.48 46.47 0 1.23 12.22 54.47 0.82 6.82 118.47 - O 0.71 126.19 - - O

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Claims

1. In a dispenser for achieving controlled release over an extended period of time of a volatile liquid into the environment adjacent to the dispenser, the dispenser comprising a hydrogel formed from an aqueous solution of polyvinyl alcohol, the improvement comprising (1) the presence in said hydrogel of from 1 to 20 percent, based on the total weight of said hydrogel, of a polyelectrolyte as a finely divided dispersed phase within a continuous phase of said hydrogel and a monohydric alcohol containing from one to four carbon atoms in an amount sufficient to solubilize said volatile liquid and without precipitating the polyvinyl alcohol, (2) a method for preparing said dispenser whereby at least a portion of said volatile liquid is incorporated into said dispenser by immersing it in an aqueous solution of said volatile liquid and (3) a free standing dispenser possessing an exterior surface that is initially dry to the touch and remains so throughout the useful life of said dispenser.

2. A dispenser according to claim 1 where said volatile liquid is a fragrance, disinfectant or a biologically active material, the solvent for said polyvinyl alcohol is a mixture comprising water and an alcohol, where said alcohol constitutes less than 40 weight percent of said solvent, the molecular weight of the polyvinyl alcohol is from 75,000 to 440,000, the polyvinyl alcohol constitutes less than 10 percent by weight of said hydrogel, said polyelectrolyte is selected from the group consisting of alkali metal salts of ethylenically unsaturated acids and block copolymers of said salts and polyvinyl alcohol, the particle size of said polyelectrolyte is from 10 to 50 microns, the polyelectrolyte when uncrosslinked and substantially free of polyvinyl alcohol constitutes no more than about 80 percent of the combined weight of polyvinyl alcohol or no more than about 70 percent of said combined weight when crosslinked or in the form of a block copolymer containing polyvinyl alcohol segments and said dispenser contains an acidic or basic material and a pH indicator that undergoes a color change within the pH range of from 5 to about 8 as a means for signaling the end of the useful life of said dispenser.

3. A method for preparing a free standing dispenser for the controlled release of a volatile liquid material, said method comprising the following steps:

1) heating under atmospheric pressure a mixture comprising from 5 to 25 weight percent of polyvinyl alcohol, a polyelectrolyte, water and at least one water-miscible liquid selected from the group consisting of monohydric alcohols and dimethyl sulfoxide, the concentration of water-miscible liquid being sufficient to maintain said polyelectrolyte as a dispersed phase in a solution of said polyvinyl alcohol, 2) placing said dispersion in a container corresponding in contour to the shape of the final dispenser and cooling the resultant dispersion to below room temperature to form a hydrogel in the shape of the final dispenser, 3) when said water-miscible liquid is dimethyl sulfoxide, placing said hydrogel in contact with methanol or ethanol for a period of time sufficient to replace substantially all of the dimethyl sulfoxide with said methanol or ethanol,

4) placing said dispenser in an aqueous solution of said volatile liquid for a period of time sufficient to absorb an amount of said solution equivalent to at least 95 weight percent of the final dispenser, replace at least a portion of the water present in said dispenser with said solution, and

5) removing the final dispenser from said aqueous solution.

4. A method according to claim 3 where said volatile liquid is a fragrance, disinfectant or a biologically active material, the water miscible liquid is dimethyl sulfoxide and constitutes from 50 to 85 weight percent of the combined weight of water and said water-miscible liquid, the molecular weight of the polyvinyl alcohol is from 75,000 to 440,000, the polyvinyl alcohol constitutes less than 10 percent by weight of said hydrogel, said polyelectrolyte is selected from the group consisting of alkali metal salts of ethylenically unsaturated acids, the particle size of said polyelectrolyte is from 10 to 50 microns, the polyelectrolyte when uncrosslinked and substantially free of polyvinyl alcohol constitutes no more than about 80 percent of the combined weight of polyvinyl alcohol or no more than about 70 percent of said combined weight when crosslinked or in the form of a block copolymer containing polyvinyl alcohol segments and said dispenser contains an acidic or basic material and a pH indicator that undergoes a color change within the pH range of from 5 to about 8 as a means for signaling the end of the useful life of said dispenser.