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Tuesday, March 8, 2016

A Story from the Testing Lab: What Affects Opacity?

Several factors affect the opacity of paper, some have a larger affect than others:
  • Basis weight
  • Dyes
  • Beating and wet pressing
  • Fillers
  • Pulp fibers
Basis Weight
Increasing the sheet weight will increase the opacity due to the increase in light scattering by the sheet.

Even a small amount of dye tends to increase the opacity by absorbing part of the incident light. Since opacity is measured in the 'green' or middle of the visible spectrum (e.g. TAPPI T425 at 572 nm and ISO 2471 at 557 nm), nearly all dyes will have some affect on opacity with yellow dyes resulting in the smallest opacity increase.

Beating and Wet Pressing
The opacity of a sheet of paper made from cellulose fibers with no additives is due primarily to the scattering of the light as it passes from air to fiber and back to air. When pulp is beaten two things occur: total surface area of the fibers increases opacity and increased bonding between fibers (no scattering occurs) which reduces the opacity. Generally, beating will decrease opacity since the increase in bonding is more important than and increase in surface area.  Wet pressing will increase the bonding and, therefore, decrease opacity.

Fillers will increase the opacity since they increase the amount of scattering in the sheet. The higher the index of refraction, the higher in increase in opacity.

Pulp Fibers
The opacifying power of a pulp is a direct function of the fineness of the fibers or the fiber diameter. The length of the fiber has little or no effect because it does not affect the number of fiber-air interfaces.  Also, unbleached pulps produce papers of higher opacity than bleached pulps due to the lower reflectance of unbleached pulps.

Dyes, fillers and fibers in paper absorb and scatter light independent of each other. Their respective opacifying effects are additive.

Reference: Scott, W., Trosset, S., "Properties of Paper: An Introduction," p.84-85, (1989).

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