Method for making extrudable polyvinyl alcohol compositions

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TECHNICAL FIELD

The present invention relates to melt extrudable polyvinyl alcohol compositions and a method for their preparation.

BACKGROUND OF THE INVENTION

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