The end use areas of polyvinyl alcohol (PVOH) have been limited despite its excellent strength, adhesive and barrier properties. This limitation is partly due to the fact that vinyl alcohol polymers in the unplasticized state have a high degree of crystallinity and show little or no thermoplasticity before the occurrence of decomposition which starts at about 170.degree. C. and becomes pronounced at 200.degree. C.
The crystallinity of PVOH with a degree of hydrolysis in the 98-99 +mole % range is 30-40% depending on the manufacturing conditions. The higher the heat history experienced by the PVOH resin, the higher the crystallinity as described by K. Toyoshima (Polyvinyl Alcohol Properties and Applications edited by C. A. Finch, John Wiley & sons Ltd. London 1973). The crystallinity of 87-89 mole % hydrolyzed PVOH is in the 12-18% range and is fairly independent of the manufacturing conditions used.
The melting of PVOH occurs by first melting the small and less than perfect crystals which melt at a temperature approximately 100.degree.-120.degree. C. lower than that of the perfect crystal. Thus a melt is generated by consecutive melting of crystals having a higher and higher melting point until only the perfect and highest melting crystals remain. These perfect crystals are extremely difficult to melt in an extruder as "particle flow" of these crystals is believed to occur. Particle flow is a phenomena which is widely known in the compounding of polyvinyl chloride and was first reported and described by A. R. Berens and V. L. Folt (Trans. Soc. Rheology 11, 95 (1967)). The theory developed by A. R. Berens et al. and adapted by the present inventors to the case of PVOH would predict that the perfect crystals would flow in the PVOH melt created by the melting of the less than perfect crystals and the amorphous material and remain virtually intact at the outlet of the extruder. The gels observed in the extruded PVOH products have been found to consist of unmelted crystalline areas. The crystals making up these areas are believed to be the perfect crystals and thus those with the highest melting point. Increasing the heat during the extrusion process to melt these perfect crystals would result in the formation of degraded and crosslinked material yielding gels having a structure similar to the crystalline-origin gels, the only difference being the presence of a chemical link instead of a physical one. Gels lead to imperfections in the final product which contribute to increased leakage of gas or liquid through the formed product and significantly reduces the mechanical and physical properties.
Resolution of this extrudability problem has been sought through the use of external plasticizers, such as ethylene glycol, trimethylene glycol, propylene glycol and 2,2,4-trimethyl-1,3-pentanediol (U.S. Pat. No. 3,148,202). However, the use of external plasticizer cannot provide sufficient lowering of the melting point to fully overcome the problem of thermal decomposition without leading to significant loss in physical properties such as tensile strength and elongation. Also, the tackiness of any article produced with high plasticizer levels leads to articles having little or no commercial value. Moreover, the addition of plasticizer contributes little, if anything, to resolving the problem of gels in the final product.
Water based extrusion or molding processes have been suggested to help lower the melting point and disperse the crystalline areas thus rendering a uniform and gel-free melt at a low temperature. This technique in essence forms a high solids solution of PVOH which is then cast into a film from which the moisture is removed through evaporation to form a solid, water-free sheet. This process can also be extended to tubular film blowing to produce a biaxially oriented film. However, the film thickness is limited by the rate by which the water can be removed from the film without the creation of air voids. In addition, the necessarily slow drying step results in the reappearance of crystallinity which may be advantageous depending on the particular application. The films produced in this manner are expensive because of the energy cost required to remove the water from the film and, most importantly, the films are prevented from being utilized in the area of coextruded structures. Further, the technique does not allow for injection molding or blow molding of articles as water removal under these circumstances is extremely difficult if not impossible. Thus, the number of end products which can be produced using the water based technique are extremely limited.
The prior art has addressed the problems associated with the melt processability (extrusion) of PVOH. The majority of the art deals with the use of external plasticizer to reduce the melting point of the polymer for the subsequent forming of a PVOH film. However, the art does not address the problems associated with the time/temperature history of the PVOH. Much like polyvinyl chloride, polyvinyl alcohol can withstand a finite heat history before degradation commences. The degradation is evidenced by a yellowing of the polymer and an increase in gels due to heat induced crystallization and crosslinking. Most attempts at thermal processing have involved the direct extrusion of PVOH into films or other articles in order to avoid prolonging the time at increased temperature. However, operating conditions for most forming operations are generally insufficient to remove gels and produce a uniform melt of the polymer. The result is either poor quality films or limited production runs due to material degradation with time in stagnant zones of the extruder.
U.S. Pat. No. 3,365,413 discloses a process for the blow extrusion of clear water dispersible PVOH tubing. A critical plasticized polyvinyl alcohol composition having a residual acetate content of from 25 to 40 wt % is melted at temperatures in the range of 375.degree. to 425.degree. F. to form a plastic mass having a viscosity within the range of 100 to 20,000 poises. This material is extruded through a ring die and blown while in the plastic state. Upon cooling, a thin wall, continuous, non-tacky film is obtained.
U.S. Pat. No. 3,607,812 discloses a method of manufacturing a PVOH film insoluble in water at a temperature below 40.degree. C. by adding 13 to 5 parts by weight of a polyhydric alcohol plasticizer to 87 to 95 parts by weight of PVOH resin which has a polymerization degree of from 700 to 1500 and a hydrolysis degree of at least 97 mole % and contains less than 0.5 % by weight of sodium acetate, drying the mixture to reduce the moisture content to less than 2 wt % and finally melt extruding the mass into a film with a die heated to temperatures of from 190.degree. to 250.degree. C.
U.S. Pat. No. 3,997,489 discloses PVOH compositions of improved melt flow characteristics obtained by the use of extrusion aids comprising a combination of a low molecular weight hydrocarbon oil or wax and a higher molecular weight ethylene homo or copolymer. The improvement is even more pronounced in the presence of a plasticizer.
U.S. Pat. No. 4,119,604 discloses films prepared by melt extrusion or aqueous casting from compositions consisting essentially of a (i) resin mixture containing a low molecular weight PVOH and a medium molecular weight PVOH, and optionally, a copolymer of vinyl alcohol and an ethylenically unsaturated ester and (ii) as a plasticizer a polyethylene glycol.
U.S. Pat. No. 4,206,101 discloses films which are rapidly and completely soluble in cold water, and which are suitable for use as packaging film in automatic packaging equipment prepared by conventional melt extrusion processes from a composition consisting essentially of 5-20 parts by weight of a polyethylene glycol (having an average molecular weight in the range between 325 and 550) in 100 parts by weight of a partially hydrolyzed low molecular weight PVOH.
U.S. Pat. No. 4,244,914 discloses a process for preparing coupled and coextruded multilayer articles made of thermoplastic materials. Part of the process comprises the steps of (a) feeding, to an extruder connected with a coextrusion head, a PVOH having a high degree of hydrolysis mixed with an aqueous mixture of plasticizing compounds and heated under pressure to bring it to the plastisol state, and (b) subjecting the plastisol PVOH to rapid decompression while simultaneously venting the evolved vapors, before it enters the coextrusion head and while it is at a temperature at least equal to, and preferably higher than, the temperature of the coextrusion head.
U.S. Pat. No. 4,469,837 discloses a thermoplastic composition based on PVOH adapted for thermoplastic molding and extrusion comprising a substantially dry mixture of PVOH with at least one or more solid polyhydroxylated monomeric alcohols.
U.S. Pat. No. 4,529,666 discloses plasticized PVOH containing one or more 1,4-monoanhydrohexitols and/or one or more 1,4-3,6-dianhydrohexitols and its use for the production of composite films by coextrusion, coating, doubling and lamination.
U.S. Pat. No. 4,611,019 discloses the addition of small amounts of selected polyamides or polyesters to plasticized, melt extrudable PVOH homopolymer.
U.S. Pat. No. 4,672,087 discloses formed articles made from PVOH by forming PVOH containing a non-hydroxyl plasticizer in a substantially water-free condition and cooling at a rate sufficiently slow to provide enhanced impermeability.
JP86/095,053 discloses a method for producing PVOH-type polymers with excellent thermostability by incorporating acetic acid, sodium acetate, and a partial alkali metal salt of an organic polybasic acid with a limited pk.sub.a value in a PVOH-type polymer.
JP86/095,054 discloses a method for producing PVOH-type polymers with excellent thermostability by incorporating acetic acid, sodium acetate, and a partial alkali metal salt of an inorganic polybasic acid with a limited pk.sub.a value in a PVOH-type polymer.