Patent Application
20080044635
This invention relates to an improved gas impermeable barrier membrane having a configuration that is flexible while retaining its integrity when applied to a substrate that stretches, flexes and/or compresses. The inventive membrane is useful for significantly reducing air permeability in inflated articles that are flexible and require a significant amount of stretch to function. The invention is also directed to the composition, construction and manufacturing process of such a membrane.
To maintain good gas impermeability, barrier membrane manufacturers apply barrier materials such as polyvinylidene chloride (PVDC), Polyvinyl alcohol (PVOH), Ethylene vinyl alcohol (EVOH), nylon, or nano-clays to tough plastic or similar non stretchable substrates. The reason for using these materials (i.e. the hard crystalline structure in the case of plastics like PVDC or the mineral in the case of clay) on tough plastic or otherwise low elastic substrates is that they tend to crack, break and/or flake off if the substrate is stretched.
PVOH is a highly crystalline polymer that forms a film by drying the polymer molecules from a water solution at a temperature higher than the minimum film forming point so that while drying into a film, the molecular motion moves the long chain molecules together into close proximity (1 or 2 Angstroms distance apart) and then aligns them into linear arrays that are held together by hydrogen bonding or van der Waals forces.
Films formed by this process maintain very good barrier properties but are very crystalline and of little practical use on flexible substrates because they are brittle and can easily be damaged through stress cracking or due to poor adhesion to any supporting material. Delamination of the barrier coating is a problem that is induced by the stress created by the difference in the stress/strain relationship between the barrier and substrate materials. Manufacturers use various plasticizers in the barrier coatings in an attempt to overcome these issues and do so quite successfully in the cases where the production processes are mild enough to prevent plasticizer migration and loss or where the application is less demanding in terms of mechanical stress or barrier requirements. However, when these commercially plasticized films are use in production processes that require laminates with cure temperatures above 80° C. the plasticizer tends to migrate out of the film or into localized regions of higher concentration in a way that creates highly crystalline regions where there is little plasticization. The PVOH coating then cracks and peels from its substrate. The problem is further exacerbated by the loss of residual water from the film that has a significant effect on polymer crystallinity and brittleness. One example of this is the use of a commercial PVOH film in a play ball construction that is plasticized with 20% weight glycerine and has approximately 8% by weight water. When processed at temperatures higher than 80 deg C. the water is driven off and the resulting build up of crystallinity creates channels for glycerol (plasticizer) migration. The loss of plasticizer creates regions of very highly crystalline hard segments that have improved barrier properties but are too brittle to be of any practical use. When a ball of such a construction is bounced, the PVOH film is brittle and inelastic, creating an undesirable rebound. Further, upon bouncing of such a ball, the film itself breaks with the subsequent loss of the barrier properties.
In general when a standard barrier coating such as a nano-clay or PVDC is applied to a stretchable or flexible substrate, the integrity of the barrier coating becomes compromised each time the substrate is stretched. Just like in the case of the plasticized and heat treated PVOH coating, the crystalline structure is brittle and when stretched, the forces holding the molecules together easily break, thereby destroying the barrier's integrity. Under such stretch forces the crystals form shear planes that crack into the platelet-like structures and the molecules separate, creating large gaps between the crystalline regions or break and even flake off the substrate that is used to support the barrier.
There is thus a need for a gas impermeable barrier film that can retain its integrity while undergoing standard product manufacturing processes for an inflatable articles, and continue to maintain its integrity through the rigors of flexion and compression that inflatable articles undergo.
The invention is directed to a barrier membrane for flexible articles comprising a) a first stretchable film layer, wherein said first stretchable film layer is substantially water impervious; b) a barrier film layer, wherein said barrier film layer is substantially gas impermeable; and c) a second stretchable film layer, wherein said second stretchable film layer is substantially water impervious; wherein said barrier film layer comprises a residual moisture level of between 1% and 20% by weight.
The invention is further directed to a barrier membrane for flexible articles comprising a stretchable film layer, wherein said stretchable film layer is substantially water impervious, and a barrier film layer, wherein said barrier film layer is substantially gas impermeable, and wherein said barrier film layer is encapsulated by said stretchable film layer in the configuration of a closed, gas-filled chamber.
The invention is also directed to processes for preparing the barrier membrane of the invention, such as a process comprising the steps of a) providing a first, water impervious stretchable film layer; b) applying a gas impermeable barrier film layer onto said first stretchable film layer; c) applying a second, water impervious stretchable film layer onto said barrier film layer; and (d) adhering the layers together using high pressure and high temperature, whereby said barrier film layer comprises a residual moisture of 1% to 20% by weight.
The invention is directed to a flexible barrier membrane for use on flexible articles comprising a barrier film layer sandwiched between a first stretchable film layer and a second stretchable film layer, or otherwise encapsulated by a stretchable film layer.
The invention is also directed to a process for preparing the flexible barrier membrane of the invention comprising the steps of a) providing a first stretchable film layer b) applying a barrier film layer onto said first stretchable film layer; c) adhering said layers to form a multi-film membrane; and d) applying a second stretchable film layer onto the barrier film layer of the multi-film membrane whereby said barrier film layer is sandwiched between said first stretchable film layer and said second stretchable film layer.
Alternatively, the invention can be prepared such that the barrier film is applied onto an elastomeric (substrate) film that forms the inside of a chamber or cavity of an inflatable article, and, when sealed, the barrier film is totally encapsulated within the chamber or cavity, whereby virtually no residual moisture is lost from the chamber.
Our invention addresses the deficiencies of existing barrier films or membranes that lose their integrity when exposed to significant stretch or fatigue resulting from dynamic flexing or direct stretch and compression cycles like those to be experienced in an inflatable article such as a ball, tire, air mattress, float or the like. In particular, the barrier membrane of the invention is capable of retaining its integrity to a much greater extent than barrier films of the prior art, and, as a result, significantly improves gas retention within flexible inflatable articles.
The barrier film layer of the barrier membrane of the invention preferably comprises a gas impermeable material, such as a material made from highly crystalline polymer materials including, but not limited to Poly Vinyl Alcohol (PVOH), Ethylene-Vinyl Alcohol (EVOH), and polyvinylidene chloride (PVDC). In a preferred embodiment, the barrier membrane of the invention comprises barrier layer which is either sandwiched between two stretchable films or adhered to a single stretchable film and encapsulated in a chamber. The barrier film is adhered to the stretchable film(s) by drying or by otherwise laminating or adhering said layers to form a laminated or multi-film membrane The stretchable film(s) can accommodate the stretch requirements of the inflatable article without losing the performance integrity of the stretchable film(s). The stretchable film(s) are preferably moisture impermeable and made from a flexible material such as, for example, an elastomeric film such as natural rubber, synthetic rubber or other elastomeric or plasticized plastic material such as PVC or the like. The stretchable film(s) of the invention often serve as a substrate onto which the barrier film is adhered.
When the term “PVOH” is used in this document, it is intended to refer to any water soluble polyhydroxy compound with repeating units of CH2.CHOH in the polymer chain. The weight average molecular weight of the PVOH can range from about 10,000 to about 200,000. In a preferred embodiment, PVOH has a weight average molecular weight greater than 80,000 and is used at a concentration in water of about 5 to about 14 percent by weight. The invention encompasses derivatives of water-soluble polyhydroxy compounds, particularly those which are partially hydrolyzed in which some of the hydroxyl groups are acetylated. The percent of acetate groups replaced by hydroxyl groups is the degree of hydrolysis. The invention further encompasses PVOHs with a degree of hydrolysis greater than 87 percent. To enhance elasticity in the barrier film, the preferred embodiment employs a PVOH which is about 87 to about 89 percent hydrolyzed. Other polyvinyl alcohols having a degree of hydrolysis up to over 99 percent can also be used, such as, for example, Elvanol 70-31 sold by Du Pont de Nemours Inc. Another preferred material is Elvanol 51 (87% hydrolyzed) sold by Du Pont de Nemours Inc. Other types of PVOHS that are chemically modified or otherwise altered to provide the benefits of the invention are also included. For example, water resistance of a dried PVOH film can be enhanced by cross linking the backbone polymer of the PVOH using aldehydes or aldehyde donors in the formulation. Other modifications of the PVOH backbone include reaction of the hydroxyl groups to form ethers, esters, acetals and ketals.
It is preferred that the barrier film of the invention preferably contains between about 1% and 20%, and more preferably, about 2% to about 14%, weight residual water upon formation of the barrier membrane of the invention. It is also preferred that the barrier film contains between about 5% and about 50% weight of a plasticizer that is solid at room temperature and does not migrate during barrier membrane formation.
Preferred plasticizers (also referred to as “solids” in this document) for this invention include sugar alcohols such as dextrose, maltitol, erythritol and sorbitol. These compounds include mono- and di-saccharides including glucose, dextrose including the monohydrate, sucrose, arabinose, lactose, mannose, maltose, fructose, galactose, amylose, allose, altrose, talose, gulose, idose, erythrose, threose, lyxose, xylose, arabinose, rhamnose, and cellobiose. Low molecular weight water soluble polysaccharides such as starches are also included within the scope of this invention. Other plasticizers that can be included in this invention's formulation are glycerol, various phthalates, 2,3 butandiol, and polyethylene glycols.
The barrier film of the invention is initially formed as a solution, and can then either be directly applied as a solution to the stretchable film substrate as a coating and dried thereon, or pre-dried into a film (i.e. the barrier film) and then formed into appropriate shape for adhesion to the stretchable film substrate.
It may be appropriate, and often is preferred, to incorporate additional ingredients to the barrier film solution that would add one or more beneficial characteristics to the barrier solution, and ultimately the barrier membrane of the invention. Some examples of beneficial ingredients would include water-shedding compounds that give up water at a very slow rate, for example, taking longer than 20 minutes while subject to temperatures greater than 100° C. as would be experienced in rubber curing or plastic laminating processes (See Example 5). In such a case, the water that evaporates from the barrier film or coating during the curing or laminating process is replenished by that which is given up by the water shedding compound which prevents the barrier film from being over-dried to potentially a brittle state. Examples of water-shedding compounds include silica gel, aluminum trihydrate, magnesium hydroxide, activated carbon and various other highly hydrated organic and inorganic compounds.
Other beneficial ingredients that can be added to the barrier film solution in small amounts without substantially altering the function of the barrier membrane performance include antioxidants, biocide, anti-mold materials, coloring fillers and viscosity thickeners. For example, up to 0.5% by weight of anti-mold ingredients like sodium diacetate, calcium proprionate and potassium sorbate can be added to the solution to prevent spores and mold growth on the barrier film.
To prevent gelation of the barrier solution before use, non-ionic surfactants and in particular those containing alkyl ethoxylated formulations such as Triton-x® (Rohm & Haas) surfactants can be added up to about 0.3 or 0.4% by weight. Viscosity stability of the barrier coating solution can be increased by adding small amounts of viscosity stabilizers such as thiocyanate, phenol, butyl alcohol or isopropyl alcohol using standard methods known in the art. For improved processing, the viscosity of the barrier solution can be increased or thickened and extended using thickeners or extenders such as starch, methyl cellulose, polyacryl amide, alginic acid soda, clays, calcium carbonate, dextrin or casein.
Small amounts of nano-clays of the montmorillonite type as sold under the trade name Cloisite® (Southern Clay Products, Inc.) can also be added to enhance solution stability, extend viscosity and to affect drying rates and improve barrier performance. (see Example 6).
If desirable, up to about 10% of aqueous polyurethane or other elastic/elastomers can be added to the barrier solution as a latex to lower overall Tg (glass transition temperature) and improve the elastic behavior of the barrier film.
As preliminarily described above, the barrier film or coating of the invention is preferably sandwiched between elastomeric film(s) or encapsulated within an inflatable chamber. It is preferred that the barrier film is contained within a controlled environment where relatively no significant moisture either enters or leaves the film. Consequently, after curing/drying the film remains in a continuously plasticized state due to both the residual water content and the plasticizer content, and as a result, the film does not exhibit embrittlement that typically occurs upon loss of the film's residual water content and the subsequent creation of too much crystallinity in the less plasticized and drier regions. It is therefore preferred that the flexible film(s) of the invention sandwich or encapsulate the barrier film in such a way that, when the barrier membrane of the invention is in use, the barrier film retains a residual water content of about 1% to 20% weight range, and more preferably, about 2% to 14% weight range so that no embrittlement occurs. In addition, it is preferred that the plasticizer(s) also remains within the range of about 5% to about 50% as the plasticizer attracts water and helps maintain polymer flexibility, as well as enhance barrier properties due to the plasticizer's crystallinity.
A resulting advantage of the invention's barrier film properties is that they remain the same or better because the barrier film remains primarily intact while under stress and because the plasticizers enhance barrier properties. Otherwise stated, a unique advantage of this encapsulated barrier membrane composition and configuration is that the barrier material does not breakdown, degrade or yellow with a subsequent loss in performance (See Example 6).
Inflatable products that incorporate the barrier membrane of the invention can be subjected to many cycles of flexing and stretching without becoming embrittled and without cracking or significant loss of the membranes barrier properties. The membrane of the invention can be elongated to 350% and can undergo more than 12,000 cycles of a one inch deflexion without cracking or breaking (see Example 4). Thus an inflatable article like a ball, tire or air bed can undergo many cycles of use and maintain their air retention for up to one hundred times or more their normal inflation period.
An inflated article, such as a sports ball or the like, incorporating the flexible barrier membrane of the invention will ultimately remain inflated much longer than inflated products that incorporate conventional flexible barriers. In an example of the application of this technology to inflatable balls, our tests indicate an improvement over conventional inflatable balls of at least 20 to 100 times better inflation retention. Similar tests using standard 9.7 K80 air bed PVC showed improvements in air retention of more than 30 times that or PVC without the coating (See Examples 7 and 8). Such products should retain the functional air pressure for more than a year (See Example 9).
In addition to increased air pressure retention, the flexible barrier membrane of the invention has the added advantage that it uses less material than is typically used in barrier membranes of the past. To address stretchable products, the incumbent or past applications of barrier technology used, for example, nano-clay platelet structures. The nano-clays were placed in aqueous emulsions or dispersions of rubbers, or into solvated rubber compounds as suspended clay platelets. These solutions require high clay loading, from 5% to more than 25%, to achieve good barrier properties. The resulting structures are rigid and non uniform brittle films that break and fatigue poorly. When damaged in a flexible or stretch application, they are permanently damaged and incapable of air retention thereafter. The end result is the formation of very non uniform films full of crests and troughs and pin hole type defects when deployed onto a product. The flexible barrier film of the invention does not suffer from these deficiencies as described earlier.
Another distinct advantage of the invention, particularly when the barrier film is applied to the substrate as a solution to form a coating, is that the polymer will plug any pinholes in the substrate and prevent gas leakage. This is particularly important to manufacturers who experience defective articles because of imperfections in the material stock being used to make the article. The same effect holds true in alternative embodiments of the invention such as there where the barrier film is pre-dried and then laminated onto the substrate (e.g. the inside of ball bladders that are cured under pressure), or in the case of dip coated bladders such as may be used in a soccer ball's natural rubber latex bladder (discussed below). Production defects derived from pin hole leaks in all inflatable articles would be eliminated by any of the embodiments of the invention.
Manufacture of Flexible Barrier Membrane
As best depicted in
Another preferred process for manufacturing the barrier membrane (10) of the invention employs a first stretchable film layer or substrate (12) of a natural rubber, synthetic rubber or other elastomeric or any plasticized plastic material such as PVC or Nylon. Once the first stretchable film layer (12) is in position, a coating of barrier material (14), such as one containing PVOH and, if desired, one or more of the ingredients of this invention, is applied in the form of a solution that is formulated with adequate solids concentration of between 5 and 50% and viscosity between 50 and 10,000 cp to wet and easily adhere to the substrate (12) when sprayed, brushed, poured or otherwise applied to the surface to form the barrier layer (14). The concentration and viscosity of the barrier solution can be readily altered to create different thicknesses of the final dried coating by altering the solids content of the solution. For example a 45% solids and 9000 cp viscosity solution will result in a minimum 1.5 mil thick coating after drying if poured over the surface of a butyl rubber substrate at an angle of 45 degree to horizontal plane. This subcomponent consisting of the elastomeric substrate (12) and the barrier coating (14) is then placed in a drying oven and dried at temperatures between 60 and 200 deg C. at various pressures for period ranging between 2 and 30 minutes. The dried barrier coating (14) should preferably contain coating containing between about 1 and 20% residual water content is then covered by another layer of elastomeric material (16) that seals the barrier coating (14) between an inner (12) and outer elastomeric layer (16). An adhering formulation may be used at this point to place the second elastomeric film (16) on to the barrier coating (14). The whole elastomeric sphere component consisting of a elastomeric inner sphere (12), an elastic barrier layer (14) and an outer elastomeric sphere (16) is then cured or laminated in a mold at temperatures between 120 & 200 deg C. while applying air pressure between 1 kg/cm2 and 14 kg/cm2 to the internal surface of the inner elastomeric sphere. The shape of the membrane can be flat, or any shape or cut length. This example uses a sphere to demonstrate the art as applied to ball manufacture.
Another preferred process for manufacturing the flexible barrier membrane of the invention (10) is depicted in
Depicted in
In an alternative application of this method, a fully formed component of an inflatable product, in any shape (e.g. a ball bladder that is spherical), is covered with strips of barrier film (14) that are applied with polymer cement to the substrate (12). The strips are placed north to south pole on the sphere and are overlapped and cemented at the edges as they are located onto the spherical elastomeric substrate. Eventually, it is preferred that they cover the full surface of the component like petals of a flower cover a bud. This configuration consisting of the rubber substrate (12) and overlaid barrier films (14) is then covered by a second rubber film (16) that is laid onto the barrier film (14) in the same way that the barrier film (14) was laid onto the initial elastomeric substrate (12). An adhering formulation may be used at this point to adhere the second elastomeric film (16) on to the barrier film (14). Alternatively a malleable and curable elastomeric formulation can be used as in the case of natural or synthetic rubber. The whole elastomeric sphere component consisting of an elastomeric inner sphere (12), an elastic barrier layer (14) and an outer elastomeric sphere (16) is then cured or laminated in a mold at temperatures between 120° and 200° C. while applying air pressure between 1 kg/cm2 and 14 kg/cm2 to the internal surface of the inner elastomeric sphere. The shape of the membrane can be flat, or any shape or cut length. This example uses a sphere to demonstrate the art as applied to ball manufacture.
In particular 9.7 mm thick soft PVC sheet covered with a 1.5 mil barrier coating of this invention and tested on an air seepage tester that measures the volumetric air diffusion rate through a fixed area of film. The results are in cc/hr/m2. See chart 1. The barrier coating reduced air seepage through the PVC film by approximately 30 fold. An standard consumer PVC air mattress using this same barrier membrane configuration would stay inflated for more than a year.
The 9.7 mm thick soft K80 PVC sheet used in example 1 was cut into 3 parts. One was covered with a 1.5 mil barrier coating of this invention using a fully hydrolyzed PVOH, another with a partially hydrolysed film and the third was used as a control. The membranes were each tested for the air diffusion tester. The results showed that the fully hydrolyzed membrane was 30 times better than the control and almost 3 times better than the partially hydrolyzed PVOH formulation. See chart 2.
To maintain the barrier membranes flexibility it is important to provide a formulation of barrier material that can retain its moisture content between 2 and 14% by weight after going through a curing process. Sugar alcohols, Dextrose monohydrate and cloisite clay are advantageous in this respect. Barrier membranes comprising 80% Elvanol 70-31(fully hydrolyzed) and 20% dextrose or sorbitol and up to 5% weight nano-clay(Cloisite-Na+) were compared to a partially hydrolyzed PVOH and a commercially available fully hydrolyzed PVOH, by Kuraray Inc.
The test measured the percent of initial moisture content remaining in the membrane per minute at 150 deg C.—a typical curing temperature employed by rubber manufacturers. The results showed that the sugar alcohol or saccharide plasticized PVOH retained more than 40% of their original moisture for up to 20 minutes while the other gave up 100% of their water in less than 10 minutes. In a typical curing process lasting up to 12 minutes the barrier membranes of this invention retain moisture, remain stretchable and maintain their gas impermeability. The other films become brittle and crack while under many cycle of flexing. (See
A film comprising 80% filly hydrolysed PVOH, 20% glycerol plasticizer was incorporated into a ball and cured at 150° C. for 10 min. No sugar alcohol or saccharide plasticizer of this invention was included. The bass was put through a standard ball manufacturers 12,000 cycle durability test and inspected.
This test checked the rate of water generation in a 5 g sample of aluminum trihydrate. The analysis shows that A12(OH)3(ATH) can be used to generate water in films to keep them plasticized and flexible so that they do not crack when stretched. In this test moisture was given off for up to 30 minutes at 200° C. A typical processing cure cycle only lasts 10 minutes at such temperatures. For very high temperature processes addition of a hydrating agent like ATH is a good method for maintaining film moisture. ATH provides up to 34% by gross weight as water of hydration (See
Comparison of the rate as a percentage total moisture loss through various membrane barrier films. The samples were subjected to 150° C. for up to 30 minutes at atmospheric pressure. The sugar alcohol formulations comprising 20% by weight Dextrose Monohydrate and those comprising 20% Sorbitol and 5% weight Cloisite (montmorillinate clay) performed the best with slowest rate of moisture loss. At the end of a 10 minute cure cycle the membranes still contained close to 40% of their initial moisture content and remained subtle and flexible. These samples comprised 80% weight Elvanol 7130. Partially hydrolyzed PVOH film and POVAL 1.5 mil thick film lost 100% of their initial water content after being heated to 150° C. for extended period of time as would be experienced in an elastomer curing cycle. The films became embrittled and were easily damaged and cracked. See
Comparison of membrane seepage performance between a PVC sheet used in air mattresses and PVC sheets treated with different formulations of the barrier films of this invention. 80% PVOH, 20% Sugar alcohols barrier films. (See
Comparison of the rate of air diffusion through various membrane materials treated and not treated with the barrier material of this invention. For a Polyurethane thermoplastic (TPU) cut from a 15.5 mils thick film that was coated with a 1.5 mil film of this invention the improvement in air retention was 7 fold. For a PVC sheet cut from a 9.7 mils thick film that was coated with a 1.5 mil barrier film of this invention the improvement in air retention was 30 fold. (See
Comparison of air retention performance of inflatable articles made from PVC, PVC treated with a highly hydrolyzed PVOH and a partially hydrolyzed PVOH barrier material of this invention. 80% PVOH, 20% glycerol plasticizer barrier films. (See
Comparison of the rate of barrier film moisture loss for various PVOH films of this invention. PVOH p-H1 and PH-2 are partially hydrolyzed films, PVOH FH is a fully hydrolyzed film. All the films were subjected to a simulated curing operation at 150° C. for up to 40 minutes. Without the sugar alcohols/saccharide plasticizers of this invention the barrier membranes were 100% dry within 10 minutes. They became brittle, easily cracked and loss their air barrier properties. (See
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
The present application claims the benefit of U.S. Provisional Application Ser. No. 60/811,924 filed Jun. 8, 2006, the disclosure of which is expressly incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60811924 | Jun 2006 | US |
