Abstract
Flexural properties of laminates prepared by hot pressing commercial papers and laboratory handsheets with films of polyolefins were measured following exposure to different levels of moisture. Prior to their incorporation in the laminates, the fibers, either individually or as preformed paper, were acetylated or crosslinked with formaldehyde to reduce their sensitivity to moisture. The results are compared with earlier measurements which showed that the properties of laminates based on paper precoated with polyethylene declined sharply at high relative humidities. For laminates comprising laboratory handsheets made from acetylated fibers, the decline in properties is less but the initial values under dry conditions are lower also. However, acetvlation or crosslinking of preformed paper improves up to threefold the retention of flexural modulus by the laminates under the wettest conditions with no concurrent loss in this property under dry conditions, although some embrittlement of the component paper sheets is observed. Enhanced fiber alignment or the addition of 5% glass fibers to the paper does not arrest the decline in flexural properties of the laminates with increasing relative humidity. This decline differs little when other polyolefins are used but is accentuated when ethylene–vinyl acetate copolymer is the binder.