Pulps artificially aged

Materials. A number of papermaking pulp samples, including bleached and unbleached, sulfite and sulfate, hardwood and softwood, and semichemical varieties, were analyzed thermogravimetrically. Some of these pulps, artificially aged at elevated temperatures and humidity, were also included. A list of the pulps and the codes assigned to them for easier reference are given in Table I. 

Natural or artificially accelerated aging of papermaking pulps is characterized by two important reactions, scission of the polymeric cellulose chains and some cross-linking reaction (7,8,9), the exact nature of which remains unknown. Since the mechanical properties of aged paper are modified by these two simultaneous reactions, it was of interest to determine whether these chemical effects influence the thermograms of artificially aged papers. 

In this paper, the thermogram profiles of a number of papermaking pulps are examined. Different methods of presenting such curves are discussed, and the effects of changes in the rate of heating on the curves are examined. Where possible, the effects of such aspects as wood species, fiber preparation, crystallinity, or artificial aging on the thermograms are also discussed. 

Figure 5A. Differential thermograms of three artificially aged pulps. Rate of...

Figure SB. Differential thermograms of two artificially aged sulfite semichemical pulps. Rate of heating 40°C/min.

Differential rate of weight loss vs. weight fraction plots indicate reaction orders between 0 and 1. Only minor differences between some artificially aged and control pulps were noted, whereas other pulp types showed an increased rate of weight loss with a slight displacement of the thermograms toward lower temperatures. No clear-cut interpretation of these differences is yet possible. 

Figure 2. Comparative degradation of two unaged and two artificially aged papermaking pulps. Rate of heating averaged over all heating rates.

Figure 2 shows that over most of the conversion range, the two artificially aged pulps, BBC aged and BPC aged, are more resistant to further thermal degradation than their unaged counterparts. 

Table VII shows that the activation energies of artificially aged pulps BBC and BPC are consistently lower than for the control samples. The lower values indicate that the rate-determining reaction can proceed more readily after artificially aging. This may be a result of the introduction of carbonyl groups or of the introduction of chain ends as a result of hydrolysis of the hemicellulose and cellulose. Such modifications of the original pulp may act as weak links or sites at which further degradation is facilitated.

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