Surface modification of polyolefins for hydrophilicity and bondability: Ozonization and grafting hydrophilic monomers on ozonized polyolefins
Authors: Dasgupta Sunil 
Published: April 2003

Ozonization is a very efficient, economical, and potentially practical method to modify polypropylene (PP) as well as polyethylene (PE) surfaces. Carbonyl, carboxyl, and hydroperoxide groups can be very conveniently generated by reacting these polymers with ozone. In the case of fibers and powders, surface treatment with ozone in an aqueous medium offers advantages: water can be used as a carrier in commercial processes, and since water enhances generation of hydroperoxide it may be used for potential reaction purposes. The extent of reaction of polyolefins with ozone depends on the extent of agitation of the slurry, the slurry temperature, solid consistency, and ozone concentration in the gas. Within the limits of the present work, the ozone uptake of fibers increases linearly with time. The carbonyl (%) and carboxyl (meq/g) contents are approximately 0.08 and 0.04 times the ozone uptake (%) of the PP fibers. Hydroperoxide generation is found optimum at a slurry temperature of 60–80°C, and the amount of active oxygen (%) at 60°C is about 0.1 times the ozone uptake (%) of the PP pulp. Spurted polypropylene fibers show two different kinds of crystallinity in X‐ray diffraction analysis, namely monoclinic and smectic. The type of fibers produced depends on the spurting conditions. The monoclinic fibers have a mostly rodlike structure while the smectic fibers have a ribbonlike appearance. The fiber types give different sheet properties in blends with cellulose, and show different grafting rate behavior. The smectic fibers provide relatively stronger sheets, and graft acrylamide monomers at a higher rate. Ozonization degrades the chain length of polyolefins as shown by intrinsic viscosity of treated material. It is possible to have an additive in the polymer to reduce degradation rate of the chain length. Further work in this area is needed, however, for better understanding of the reaction, and to achieve complete control over the degradation process to make the product commercially viable. Effects of some additives on ozone reaction to enhance the generation of functional groups also opens an area for further study. Handsheets were prepared with blends of 30% ozonized polypropylene pulp and 70% wood pulp. About 90% of the strength of a 100% wood pulp sheet can be achieved by either grafting acrylamide (25–30%) on the polypropylene pulp, or by disk refining with 5% of a bonding resin. Polypropylene pulp significantly raises the bulk, opacity, and brightness of the sheets. High levels of acrylamide grafted to pulp reduce opacity and caliper of sheets. Grafting with a redox system of hydrophilic monomers, such as acrylamide, to ozonized pulps containing a reasonable amount of hydroperoxide (0.1–0.5% active oxygen) also provides an effective method for surface modification. The surface of both ozonized and grafted pulps becomes highly hydrophilic. Polyethylene, in the limited results obtained in this work, behaves very similarly to polypropylene. The activation energy of ozone reaction with both polypropylene and polyethylene is approximately 7 kcal/mol.