POLYVINYL ALCOHOL COATINGS WITH ENHANCED PROPERTIES

Abstract: The present invention is a method of producing low oxygen-transmissive films having PVOH coatings through the utilization of a PVOH solution which includes at least two PVOH resins having differing degrees of hydrolysis. The present invention is also a low oxygen-transmissive film manufactured by this method.

Description: "POLYVINYL ALCOHOL COATINGS WITH ENHANCED PROPERTIES' The present invention relates to films having poly(vinyl alcohol) coatings, and, more particularly, to enhancing the oxygen barrier properties and processing characteristics of cross-linked poly(vinyl alcohol) coatings. Poly(vinyl alcohol) coatings have been applied to various substrates in the past, and are known to provide a barrier to the transmission of oxygen. However, poly(vinyl alcohol) is soluble in water and as a result susceptible to attack by moisture. Various attempts have been made in the prior art to decrease the sensitivity of poly(vinyl alcohol) to moisture. As known to those skilled in the art, poly(vinyl alcohol) (PVOH) is typically produced by hydrolyzing poly(vinyl acetate) . Specifically, the acetate groups of poly(vinyl acetate) are replaced with alcohol groups through a hydrolysis reaction. The more acetate groups that are replaced, the greater the hydrolysis of the PVOH resin. For example, in a 95% hydrolyzed PVOH resin 5% of the acetate groups remain unchanged. Similarly, in a 99% hydrolyzed PVOH resin, 1% of the acetate groups remain unchanged. One known method of decreasing the sensitivity of a PVOH coating to moisture is to cross-link the PVOH. For example, a cross-linking agent and catalyst may be applied along with the PVOH such that the agent interconnects and thereby cross-links the PVOH molecules as the coating dries. The catalyst is present to aid in the cross-linking process which otherwise may not occur without it. Another method of decreasing the sensitivity and thereby fortifying the oxygen barrier characteristics of a PVOH coating is by utilizing a highly hydrolyzed PVOH resin. It is believed that the presence of more alcohol groups, i.e., greater hydrolysis, directly corresponds to better barrier properties and better moisture resistance. As a result, many of the PVOH coatings found in the prior art utilize PVOH resins having a degree of hydrolysis 98% or greater. However, highly hydrolyzed PVOH resins have been found to create difficulties during application as a coating. Highly hydrolyzed PVOH resins exhibit a greater tendency to foam when pumped. Consequently, microscopic bubbles are created in the coating. This results in imperfections in the PVOH coating which may reduce the coating's moisture resistance and impermeability to oxygen. Further, highly hydrolyzed PVOH resins exhibit a poorer "wet out" characteristic on a substrate as compared to the "wet out" exhibited using a less hydrolyzed PVOH resin. "Wet out" means the tendency of a coating to spread over a substrate. Highly hydrolyzed PVOH is less efficient during spreading than is less hydrolyzed PVOH. Therefore, there is a need in the art for PVOH coatings which exhibit a high degree of oxygen impermeability and moisture resistance, while at the same time are easier to apply. Specifically, there is a need for PVOH coatings having the moisture resistance and high impermeability associated with highly hydrolyzed PVOH resins while, at the same time, possessing the lower foaming tendency and the better wet out associated with less hydrolyzed PVOH resins. The present invention provides PVOH coatings which exhibit oxygen impermeability and moisture resistance associated with highly hydrolyzed PVOH resins, while at the same time, exhibiting lower foaming tendency and the better wet out associated with less hydrolyzed PVOH resins. The present invention also provides a low oxygen transmissive film utilizing this coating. The present invention comprises a method of coating at least one side of a substrate with a PVOH solution which includes a blend of at least two PVOH components. The first PVOH component is a PVOH resin having a high degree of hydrolysis, preferably of at least 98%. The second PVOH component is a PVOH resin having a low degree of hydrolysis, preferably from 80% to 90%. The blend utilized in the present invention, preferably, has a ratio of the first component to the second component ranging from 1:2 to 20:1. A cross-linking agent and an optional catalyst may also be utilized with the present invention. The present invention also includes a low oxygen- transmissive film manufactured by the method of the present invention. As a result of the present invention, a new method of making a low oxygen-transmissive film, and a new low oxygen-transmissive film, are provided. The method and article of manufacture of the present invention provide a low oxygen-transmissive film which exhibits the impermeability and moisture resistance typically associated with highly hydrolyzed PVOH resins, and exhibits the improved processability typically associated with less hydrolyzed PVOH resins. It has now been discovered that a PVOH solution, which includes a blend of at least two PVOH resins having different degrees of hydrolysis, exhibits improved oxygen impermeability, moisture resistance and processability when applied as a coating in comparison to coatings utilizing either of the PVOH components by themselves. Specifically, the PVOH coatings utilized in the present invention exhibit an oxygen impermeability comparable to, if not better than, the PVOH coatings utilizing the highly hydrolyzed PVOH component alone. While at the same time, the PVOH solution of the present invention exhibits easier processability in the coating process through reduced foaming and a better wet out. The two processing characteristics are typically found in PVOH resins which have a low hydrolysis level normally precluding such resins from use as a barrier coating. As previously mentioned, the first component of the PVOH blend utilized in the present invention is a PVOH resin having a high degree of hydrolysis. Preferably, the PVOH resins which are be utilized as the first component in the present invention have a hydrolysis level of at least 98%. In other words, at least 98% of the acetate groups of the poly(vinyl acetate) have been replaced with alcohol (OH) groups. In the most preferred embodiment of the present invention, a PVOH resin having at least 99% hydrolysis is utilized. The second component of the PVOH blend utilized in the present invention is a PVOH resin having a low degree of hydrolysis. This hydrolysis level ordinarily would normally preclude the use of these resins in a barrier coating. Preferably, a PVOH resin having a hydrolysis level of from 80% to 90% is utilized. In a preferred embodiment of the invention, a PVOH resin of from 85% to 86% hydrolysis is used. It has been found that a blend of the two components in a ratio of from 1:2 to 20:1, preferably from 2:1 to 5:1, and most preferably from 2.5:1 to 3.5:1, exhibits an increased oxygen impermeability in the applied coating while at the same time exhibiting better processability. In the most preferred embodiment of the present invention, the first component and the second component are in a ratio of 2.5:1 to 3.5:1. In other words, the PVOH resin blend is 71% to 78% of the highly hydrolyzed PVOH resin and 22% to 29% of the lower hydrolyzed PVOH resin. Further, in another most preferred embodiment of the present invention, the two components of the blend are 71% to 78% of a 99.3% hydrolyzed PVOH resin and 22% to 29% of an 86% hydrolyzed PVOH resin. The PVOH resins utilized as first and second component can range from low molecular weight resins to high molecular weight resins depending on the viscosity of the applied coating desired. Specifically, there is a direct relationship between the molecular weight of the PVOH polymer and the viscosity of the PVOH/water solution. PVOH resins having molecules of a high molecular weight also exhibit a high viscosity, and PVOH resins having low molecular weight molecules exhibit a lower viscosity. Preferably, the PVOH resins utilized in the present invention are those with a low molecular weight. Also, as known in the art, a cross-linking agent and an optional catalyst can be utilized with the PVOH solution to facilitate cross-linking. It is contemplated that any of the cross-linking agent and catalysts known in the art can be utilized with the present invention. For example, the cross-linking agent can be a melamine-formaldehyde or a urea-formaldehyde resin. Commercially available cross- linkers include PAREZ 613, a methylated melamine formaldehyde; CYMEL 373, a methylated melamine formaldehyde; CYMEL 401, a trimethelol melamine urea formaldehyde; glyoxal; borax; and others. The catalyst, which is usually an acid catalyst, can be ammonium sulfate, ammonium chloride, ammonium nitrate, sulfuric acid or nitric acid. In a preferred embodiment of the present invention, the cross-linking agent is glyoxal which does not require the use of a catalyst. It has been found that by utilizing glyoxal as the cross-linking agent, the pot life of the PVOH solution is also increased. The PVOH solution for use in the present invention, which includes a blend of at least two PVOH resins, is preferably an aqueous solution. The solution is prepared by adding the PVOH resins in the appropriate ratio to water at a sufficient temperature to dissolve the PVOH resins. The water and the dissolved PVOH are then cooled. The cross-linking agent and the optional catalyst are then added to the cooled solution. The resulting solution is then coated onto a substrate, which previously may have been treated with a primer to enhance adhesion of the PVOH coating thereto. In a preferred embodiment the aqueous PVOH solution includes from 2% to 20% by weight of solid and preferably, from 4% to 8% by weight of solid. This solid content is made up of from 20% to 98% by weight of the PVOH resins in their various ratios, from 2% to 20% of a cross-linking agent and an optional catalyst. The improved PVOH coatings of the present invention may be adhered to any number of substrates, including polymeric films, box boards, metallic films and paper. Preferably, the coating is adhered to a polymeric film such as a polyolefin. One particular preferred polyolefin is polypropylene. To ensure that the PVOH coatings of the present invention properly adhere to a substrate, the substrate preferably includes at least one side that is adapted for receipt of the coating. Particularly, the side of the substrate to be coated should have surface properties which facilitate the securing of a PVOH layer thereto. For example, the side to be coated may be treated with a primer such as poly(ethyleneimine) . However, other suitable primers may also be utilized. The side to be coated may also be adapted for subsequent receipt of a poly(vinyl alcohol) layer during formation of the substrate itself. For example, a polymeric substrate, e.g., polypropylene, may include a material such as maleic anhydride which improves the ability of poly(vinyl alcohol) to bond thereto. Finally, the substrate may be highly treated. Once the coating is applied to the substrate, the coated substrate is passed through a drying oven. A typical drying oven heats the film from 100"C to 130*C. As the film passes through the oven, the water in the applied coating is driven off which in turn increases the concentration of the solid content. At some point during the drying process, the cross-linking of the PVOH molecules is initiated. Depending on the cross-linking agent and/or catalyst utilized, the amount of time for the coating to fully cross-link will vary. Once the coating has fully cross-linked, the coated substrate (the resulting film) exhibits improved oxygen impermeability and moisture resistance. The following examples illustrate the improved processability and the enhanced barrier characteristics of films produced by the present invention. EXAMPLE I In Example I, a control sample 1 was prepared using a PVOH solution containing 100% of a 99.3% hydrolyzed PVOH resin. Specifically, the PVOH resin utilized was Elvanol 71-30 which is commercially available from DuPont Chemical Co. 1220 grams of PVOH resin was dissolved in 11,540 grams of water (H 2 0) . 485 grams of glyoxal-40 was added to the solution. The solution was then applied to an orientated polypropylene (OPP) substrate using a reverse direct gravure coater. The resulting film was dried using a dry- air oven at 110°C running at 38.1 m/min (125 ft/min) . The film was tested for oxygen transmission (T0 2 ) , which is measured in cc/100 cm 2 /24 hr (cc/100 in 2 /24 hr) , on a Mocon Oxtran apparatus. Oxygen transmission rates at both 0% relative humidity (RH) and 75% RH were taken. A substantial amount of foaming was observed during the applicating process. The large amount of air entrained in the solution made pumping the solution extremely difficult. Once the solution was pumped to the coater, the amount of foam was sufficient to overflow the coating pan and even come in contact with the substrate. The wet out of the applied coating onto the substrate was also not complete. The PVOH solution further exhibited a pot life of one hour. EXAMPLE II In Example II, control sample 2 was prepared using a PVOH solution containing 100% of a 86% hydrolyzed PVOH resin. Specifically, the PVOH resin utilized was Elvanol 52-22 manufactured by DuPont Chemical Co. 1040 grams of this PVOH resin was dissolved in 11,560 grams of H 2 0. 485 grams of glyoxal-40 was added to the solution. The solution was then applied to an OPP substrate using a reverse direct gravure coater. The resulting film was dried using a dry-air oven at llO'C running at 38.1 m/min (125 ft/min) . The film was tested for oxygen transmission at both 0% RH and 75% RH utilizing the apparatus described in Example I. The PVOH solution exhibited a low amount of foaming as evidenced by the low amount of foam present in the coating pan. Further, the wet out of the applied coating onto the OPP substrate surface was substantially complete. Thus, the level of foam was very low and did not interfere with the coating process. Example III In Example III, test sample 3 was prepared in accordance with the present invention using a PVOH solution containing a blend of 50% of the 86% hydrolyzed PVOH resin and 50% of the 99.3% hydrolyzed PVOH resin. The blend was prepared by dissolving 490 grams of 86% PVOH resin and 490 grams of 99.3% PVOH resin in 11,540 grams of H 2 0. 485 grams of glyoxal-40 was added to the solution. The solution was then applied to an OPP substrate and dried utilizing the same procedure as described in Example I. Oxygen transmission rates were also measured utilizing the apparatus described in Example I. The amount of foaming observed was substantially less than the amount of foaming observed during the application of the solution in Example I. The level of foam was very low and did not interfere with the coating process. Similarly, the wet out exhibited by this PVOH solution was superior to the wet out observed in Example I. Example IV In Example IV, test sample 4 was prepared in accordance with the present invention using a PVOH solution containing a blend of 25% of the 86% hydrolyzed PVOH resin and 75% of the 99.3% hydrolyzed PVOH resin. The blend was accomplished by dissolving 244 grams of 86% PVOH resin and 732 grams of 99.3% PVOH resin in 11,540 grams of H 2 0. 487 grams of glyoxal-40 was then added to the solution. The coating was applied to an OPP substrate, dried and oxygen transmission measurements taken utilizing the procedure described in Example I. The amount of foaming observed was also substantially less than the amount of foaming observed in Example I. The level of foam was very low and did not interfere with the coating process. The wet out on the OPP substrate was also superior to the wet out observed in Example I. Lastly, the PVOH solution exhibited a pot life of 72 hours. COMPARATIVE TESTS RESULTS The results from Examples I to IV are shown in Table I. Table I Sample Hydrolysis Level Oxygen Transmission 99 . 3% 86% 0% RH 75% RH cc x 10 _3 /100 cm 2 /24 hr (cc/100 in 2 /24 hr) 1 100 0 4 .96 (0. 032) 251. 1 ( 1. 62 ) 2 0 100 66. 65 (0. 430) 465. 0 (3 . 00) 3 50 50 8. 37 (0. 054 ) 308.45 (1. 99) 4 75 25 2.79 (0. 018) 201. 5 ( 1. 30) As can be seen from Table I, surprisingly Test Sample 4, which utilized a PVOH solution containing a blend of the 99.3% and the 86% hydrolyzed PVOH resins in a ratio of 75% to 25% (3:1) exhibited the lowest rate of oxygen transmission, 201.5 x 10 ~3 cc/100 cm/24 hr (1.30 cc/100 in/24 hr) at 75% RH. This was quite unexpected because knowledge and experience in the art would have dictated that Control Sample 1, which utilized a PVOH solution containing 100% of the 99.3% hydrolyzed PVOH resin, should exhibit the lowest oxygen transmission rate. However, Control Sample 1 exhibited an oxygen transmission rate of 251.1 x 10 -3 cc/100 cm 2 /24 hr (1.62 cc/100 in/24 hr) at 75% RH. Furthermore, experience in the art would predict a theoretical average oxygen transmission rate for the blend of Test Sample 4 to be 305.35 x 10 -3 cc/100 cm 2 /24 hr (1.97 cc/100 in 2 /24 hr) , ((.75) (T0 2 Control 1) + (.25)(T0 2 Control 2)). Quite unexpectedly, however, the oxygen transmission rate was a low 201.5 x 10" 3 cc/100 cm 2 /24 hr (1.30 cc/100 in 2 /24 hr) . The blend of Test Sample 4 also exhibited better processability in terms of reduced foaming and a better wet out onto the substrate, in comparison to the solution of Control Sample 1. Also surprisingly Test Sample 3, which utilized a PVOH solution containing a blend of the 99.3% and the 86% hydrolyzed resins in a 50% to 50% (1:1) ratio, exhibited an oxygen transmission rate of 308.45 x 10" 3 cc/100 cm 2 /24 hr (1.99 cc/100 in/24 hr) at 75% RH. This rate is improved over the oxygen transmission rate of Control Sample 1 at 251.1 x 10 -3 cc/100 cm 2 /24 hr (1.62 cc/100 in 2 /24 hr) at 75% RH. In fact, knowledge in the art would have predicted Test Sample 3 to exhibit an oxygen transmission rate of 358.05 x 10 *3 cc/100 cm 2 /24 hr (2.31 cc/100 in 2 /24 hr) at 75% RH. This rate would be the theoretical average oxygen transmission rate expected utilizing a solution containing 1:1 PVOH blend of the 99.3% and 86% hydrolyzed resins, ((.5)(T0 2 Control 1) + (.5)(T0 2 Control 2)). The blend of Test Sample 3 also exhibited improved processability in terms of reduced foaming and a better wet out on the substrate, in comparison to the solution of Control Sample 1. Thus, as demonstrated, the PVOH Solutions utilized for Test Samples 3 and 4 offer improvements in oxygen transmission, in addition to improvements in processability, over the PVOH solutions currently used in the art. The PVOH solution for Test Sample 4 also exhibited a dramatic improvement in the pot life of the solution. The pot life of the solution for Control Sample 1 was relatively short, only one hour. The pot life of the solution for Test Sample 4 was 72 hours. Thus, the use of the PVOH blend increased the life of the prepared batch and will thereby reduce waste in the production of PVOH coated films due to the limited pot life of the PVOH solution.

Claims: HAT IB CIAIMg 19 1. A method for producing poly(vinyl alcohol) coated films having enhanced properties, comprising: coating a substrate with a poly(vinyl alcohol) solution which includes a blend of at least two poly(vinyl alcohol) components, a first component being a poly(vinyl alcohol) resin having a hydrolysis level sufficient to provide said coated film with oxygen barrier properties and a second component being a poly(vinyl alcohol) resin having a hydrolysis level less than said first component and which facilitates the processability of said poly(vinyl alcohol) solution. 2. The method of claim 1, wherein said first component and said second component are in the ratio of from 1:2 to 20:1. 3. The method of claim 2, wherein said first component and said second component are in a ratio of from 2:1 to 5:1. 4. The method of claim 3, where said first component and said second component are in a ratio of from 2.5:1 to 3.5:1. 5. The method of claim 1, 2, 3 or 4, wherein said first component is a poly(vinyl alcohol) resin having at least 98% hydrolysis, and said second component is a poly(vinyl alcohol) resin having from 80% to 90% hydrolysis. 6. The method of claim 5, wherein said first component is a poly(vinyl alcohol) resin having at least 99% hydrolysis, and said second component is a poly(vinyl alcohol) resin having from 85% to 86% hydrolysis. 7. The method of claim 1, 2, 3, 4, 5 or 6 wherein said poly(vinyl alcohol) solution further comprises a cross-linking agent. 8. The method of claim 7, wherein said cross-linking agent is glyoxal. 9. The method of claim 7, wherein said cross-linking agent is melamine-formaldehyde, urea-formaldehyde, borax or derivatives thereof. 10. The method of claim 7, wherein said poly(vinyl alcohol) solution further comprises a catalyst to facilitate cross-linking. 11. The method of claim 10, wherein said catalyst is ammonium sulfate, ammonium chloride, ammonium nitrate, sulfuric acid, nitric acid or mixtures thereof. 12. The method of claim 1, 2, 3, 4, 5, 6 or 7, wherein said substrate is polymeric film, box board, metallic film or paper. 13. The method of claim 12, wherein said polymeric film is an olefin. 14. The method of claim 13, wherein said olefin is polypropylene. 15. A film comprising: a substrate; and a coating applied as a blend of at least two poly(vinyl alcohol) components, a first component being a poly(vinyl alcohol) resin having a hydrolysis level sufficient to provide said coated film with oxygen barrier properties and a second component being a poly(vinyl alcohol) resin having a hydrolysis level less than said first component and which facilitates the processability of said poly(vinyl alcohol) solution. 16. The film of claim 15, wherein said first component and said second component are in a ratio of 1:2 to 20:1. 17. The film of claim 16, wherein said first component and said second component are in a ratio of from 2:1 to 5:1. 18. The film of claim 17, wherein said first component and said second component are in a ratio of from 2.5:1 to 3.5:1. 19. The film of claim 15, 16, 17 or 18, wherein said first component is a poly(vinyl alcohol) resin having at least 98% hydrolysis, and said second component is a poly(vinyl alcohol) resin having from 80% to 90% hydrolysis. 20. The film of claim 19, wherein said first component is a poly(vinyl alcohol) resin having at least 99% hydrolysis, and said second component is a poly(vinyl alcohol) resin having from 85% to 86% hydrolysis. 21. The film of claim 15, 16, 17, 18, 19 or 20, wherein said coating further comprises a cross-linking agent. 22. The film of claim 21, wherein said cross-linking agent is glyoxal. 23. The film of claim 21, wherein said cross-linking agent is melamine-formaldehyde, urea-formaldehyde, borax or derivatives thereof. 24. The film of claim 21, wherein said coating further comprises a catalyst to facilitate cross-linking. 25. The film of claim 24, wherein said catalyst is ammonium sulfate, ammonium chloride, ammonium nitrate, sulfuric acid, nitric acid or mixtures thereof. 26. The film of claim 15, 16, 17, 18, 19, 20 or 21, wherein said substrate is polymeric film, box board, metallic film or paper. 27. The film of claim 26, where said polymeric film is an olefin. 28. The film of claim 27, wherein said olefin is polypropylene.

Inventors: BLAIS MARCIA, KNOERZER ANTHONY ROBERT, BLAIS, MARCIA, KNOERZER, ANTHONY, ROBERT
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1995
Main classification: B32B23
IPC codes: B05D7, B32B27, C08J7, C08L29, C09D129