3.4. Tensile tests
The dry tensile index values for the coated samples shown in
Fig. 5 suggestthatthere was about 15–20% decrease in tensile index
for PVA and modified DG gum coated samples from base paper at
14 g/m2. No significant difference was observed among the tensile
index values for DG gum at different coating weights. Although
high tensile index was not expected with this particular coating
formulation, the comparison allowed ascertaining that in terms of
tensile index DG gum based coatings and PVA based coatings were
comparable.
Wet strength of the samples is expressed in terms of wet/dry
strength ratio, which is the wet tensile strength expressed as a
percentage of the dry tensile strength, shown for the samples in
Fig. 6. Paper consists of a network of cellulosic fibers, which are
bonded together by interfiber hydrogen bonds which are broken
when subjected to high humidity or wet conditions. Wet strength
is imparted on to paper by introducing covalent bonds by the addition
of “wet-strength additives”. A high wet/dry ratio is preferred
for applications such as packaging, towels or bags that will be used
under wet conditions. Low wet strength with increasing coating
grammage for all samples suggests that hydrogen bonding is still
the predominant basis of the structure. DG gum based samples
showed consistently lower values than PVA based samples, however
modified DG gum based samples were comparable to PVA
samples.