ABSTRACT
In order to provide additional interfiber bonding, however, synthetic (polymer-based) binding materials, e.g., maleated polyolefins, thermosetting resins, and latex, are incorporated in the web [11-14]. In practice, the amount of synthetic binder used for dry-formed paper sheets can be of the order of 20%, and for other nonwovens it may even be higher (15, 16]. For economic reasons, it would be desirable to substantially decrease this level. For instance, Youngquist et al. [14] reported the bonding of air-formed wood-polypropylene fiber composite in conjunction with maleated polypropylene. The process for producing a flexible mat using thermoplastic fiber in combination with a thermosetting resin have been developed by two different companies [17, 18]. Generally, the mat is fed through an oven to melt and set the thermoplastic fiber without affecting the setting of the thermosetting resin component. Most binders used are latex polymers that depend on crosslinking during the curing stage to develop the required high-molecular weights. Although the curing operation is rather simple, in practice it is one of the most misunderstood and difficult steps of the entire process [II]. Moreover, the application of a synthetic binder gives rise to one of the more serious complications of the drying and curing operations. The cost of the binders represents one of the significant cost factors in the air-laid process. For this reason, great care is given to the adhesive application with the goal of minimizing the quantity to be applied for a maximum of web strength.
