Improvement of Corrugated Fiberboard Wet Properties Through Crosslinking in a Large-Scale Reactor

Improvement of corrugated fiberboard wet properties through crosslinking in a large-scale reactor

T. L. Young and D. F. Caulfield Chemical engineer and research chemist, respectively, USDA Forest Service, Forest Products Laboratory, Madison, Wis. 53705-2398

ABSTRACT The moisture sensitivity of fiberboard, which leads to a loss in KEYWORDS strength, stiffness, and dimensional stability is remedied by a chemical Stiffness crosslinking treatment developed at our laboratory that uses formaldehyde Wet strength and sulfur dioxide. The treatment was used to crosslink double-wall Crosslinking corrugated fiberboard in a large-scale, specifically designed reactor. We Corrugated board examined the effect of formaldehyde crosslinking on wet stiffness and wet Dimensional stability compressive strength of the linerboard component and on wet compressive Formaldehyde strength and dimensional stability of the combined board after 24-h water soaking. Fiberboard wet-to-dry property ratios increased proportionally with increasing bound formaldehyde content. Results encourage the development and use of crosslinked corrugated fiberboard in structural applications.

Fiberboard loses strength, stiffness, dioxide and dried at a high tempera- achieved at a low content of bound and dimensional stability when ex- ture. The crosslinking reaction occurs formaldehyde (about 1%) in a large- posed to moisture. Its sensitivity pre- in the dehydrating environment, scale reactor. vents its use in humid environments where the cellulose molecules are We examined corrugated fiber- as a structural material, as in disaster drawn together to allow the acetal board that was crosslinked in a spe- relief housing. If corrugated fiber- crosslinks to form. The use of wet- cially designed, large-scale reactor. board material of conventional wall- strengthening resins at additions of Linear regression analysis was used panel sizes can be made insensitive to 3-5% can increase effectively only the to evaluate the effect of formaldehyde moisture, it can be used in a wide tensile strength of wet paper. For wet crosslinking on (a) wet-to-dry stiffness range of engineered structures. tensile modulus, wet compressive and compressive strength ratios of the Research on this problem at our strength, or wet bending stiffness, linerboard component, (b) on wet-to- laboratory has focused on chemical marked improvement can only be ob- dry compressive strength ratio of com- modification of fiberboard to render tained by resins at high concentrations bined board, and (c) on dimensional it moisture insensitive. The treatment and high processing costs. Crosslink- stability of combined board. Because acts on the molecular structure of the ing is one way of obtaining striking previous work (4) showed that form- fiberboard’s cellulosic network to incor- changes in wet stiffness properties of aldehyde crosslinking improved wet porate strong, covalent acetal linkages paper with potentially low additions stiffness of the medium and linerboard (“crosslinks”) between cellulose mol- of chemical add-ens and low process- components of the combined board, ecules. These linkages physically block ing costs. we do not report the effects of cross- attachment between water and cel- The mechanism of crosslinking and linking on the medium component lulose molecules, preventing swelling the effects of process variables have here. and the loss of strength and stiffness been studied on a small scale (1-6). (Figs. 1 and 2). The chemical treat- These results indicate that wet stiff- Results and discussion ment, called the “SOFORM” process, ness and compressive strength can be is being developed for use on paper- substantially improved at a low con- Double-wall corrugated fiberboard sam- board products and utilizes formal- tent of bound formaldehyde. Our study ples were crosslinked in the large- dehyde and sulfur dioxide (as a cat- here on corrugated fiberboard was scale reactor under various process alyst). initiated to determine if wet stiffness conditions. The extent of crosslinking, Moisture-conditioned fiberboard is and wet strength properties greater measured as percent bound form- exposed to formaldehyde and sulfur than 30% of the dry properties can be aldehyde, and fiberboard properties

December 1986 Tappi Journal 71 are given in Tables I and II. Regardless 7), may be used to indicate the degree equal to 0.82, indicating that 82% of of process conditions, wet stiffness, of linerboard stiffness. This stiffness, the variability in wet stiffness can be wet compressive strength, and dimen- represented by an apparent sonic mod- ascribed to the percentage of bound sional stability appeared strongly re- ulus, is closely related to Young’s formaldehyde. According to Eq. 1, a lated to the bound formaldehyde con- modulus (7). Wet stiffness can be ex- linerboard with a bound formaldehyde tent of the crosslinked samples. Bound pressed as the ratio of wet-to-dry ap- content of 1.3% is expected to possess a formaldehyde content ranged from parent sonic moduli. Results for the wet stiffness equal to 45% of its dry 0% (untreated) to 1.3%. linerboard component of the corru- stiffness. gated fiberboard indicate that this Stiffness can also be evaluated from Stiffness stiffness ratio, SR, is related to the linerboard’s response to uniaxial com- Viscoelastic characteristics of the cel- linerboard bound formaldehyde con- pressive stress, represented by a com- lulose network of fiberboard are in- tent, F b, in g formaldehyde × 100 ÷ g pressive modulus (8). The improve- fluenced by both the network’s fiber fiber, by ment in wet uniaxial compressive structure and moisture content. Form- properties of crosslinked linerboard SR = 0.0612 + 0.2995 Fb (1) aldehyde crosslinking of the cellulosic can be expressed as the ratio of wet-to- 2 network permanently alters the fiber- with r , the correlation coefficient, dry uniaxial compressive moduli board’s stiffness properties, whereas wax treatments, for example, act only as temporary moisture barriers. Crosslinking improves the stiffness of dry (50% RH, 22.8°C) fiberboard by approximately 10%. However, by limiting the swelling of fiberboard, crosslinking can improve its wet stiffness to several times that of wet fiberboard that has not been crosslinked. The elastic characteristics of a fiberboard, measured by the square of the velocity of sonic pulse propagation (3,

72December 1986 Tappi Journal (CSR). Results for the linerboard stability, DS. DS is found by sub- must change at higher content because component of the corrugated fiber- tracting the change in dimension in Eqs. 1-4 predict wet-to-dry property board indicate that CSR is related to crosslinked fiberboard from the ratios greater than unity at bound the bound formaldehyde content by change in uncrosslinked fiberboard, formaldehyde content of only 3%. Nev- dividing by the change in uncross- ertheless, regression Eqs. 1-4 indicate CSR = 0.1005 + 0.3586 Fb (2) linked fiberboard, and multiplying that, at a bound formaldehyde content

2 by 100. (The dimensional change is of 1.3%, stiffness ratios of 0.45 (SR) with r equal to 0.89. the wet dimension minus the dry and 0.57 (CSR) and compressive The high correlation between stiff- dimension.) strength ratios of 0.73( UCR) and 0.45 ness ratios SR and CSR and bound For the combined board, the im- (CR) can be achieved. formaldehyde content suggests that provement in dimensional stability the two ratios are also highly corre- with crosslinking is given by lated to one another. Data shown in Conclusions Fig. 3 indicate that SR is directly DS= 15.13 + 58.40 Fb (5a) Double-wall corrugated fiberboard proportional to CSR. 2 can be successfully crosslinked with with r equal to 0.77, for the thickness Compressive strength low percentages of bound formalde- direction, and by hyde in a large-scale reactor (Figs. 5 Compression loading to failure (com- DS = 29.88 + 46.32 Fb (5b) and 6) to improve wet stiffness, wet pressive strength) for both the wet compressive strength, and dimension- linerboard component and the wet with r2 equal to 0.64, for the flute al stability. These fiberboard proper- combined corrugated fiberboard also direction. ties increase proportionally with in- appear directly related to the degree There is less correlation between creasing bound formaldehyde content of crosslinking. For crosslinked liner- dimensional stability and bound form- over the range studied. Dimensional board, the ratio of wet uniaxial com- aldehyde content than there is between swelling was reduced by 80% with pressive strength to dry uniaxial strength and stiffness properties and approximately 1% bound formalde- compressive strength, UCR, measured bound formaldehyde content. Over hyde, and wet stiffness and wet com- by the vacuum restraint method (8), is the range of bound formaldehyde per- pressive strength (of both the com- related to bound formaldehyde content centages studied, only 70% of the vari- bined board and linerboard compo- by: ability in dimensional stability can be nent) were greater than 30% of the dry UCR = 0.1427 + 0.4487 Fb (3) ascribed to bound formaldehyde con- stiffness and dry compressive strength tent. Nevertheless, dimensional sta- values. The development and use of with r2 equal to 0.95. bility of approximately 80% was crosslinked corrugated fiberboard in The wet edgewise compressive achieved with about 1% bound form- structural applications is encouraged strength of crosslinked combined aldehyde. by these results. board (TAPPI T-811 os-79), expressed The higher correlation coefficients as the ratio of wet compressive between both strength and stiffness Experimental procedures strength to dry compressive strength ratios and bound formaldehyde con- Corrugated fiberboard (CR), is related to bound formalde- tent indicate that over 80% of the hyde content by: improvements in these properties can The double-wall corrugated fiber- be ascribed to crosslinking. The ap- board panels, measuring 0.61 m × 1.2 CR= 0.1226 + 0.2553 Fb (4) parent linear dependence of property m, consisted of commercial 1028 -g/m2 with r2 equal to 0.91. ratio on bound formaldehyde content fiberboard, with the linerboard made Data shown in Fig. 4 indicate that is admittedly limited to the narrow from kraft Pulp and the corrugating linerboard and combined board com- and low range of bound formaldehyde medium made from neutral sulfite pressive strength, measured by UCR percentages studied. This dependence semichemical pulp. The B-C-fluted and CR, respectively, are linearly related. The high correlation coefficient indicates that over 90% of the improvement in wet compressive strength in linerboard is reflected in a similar improvement in combined board. The direct relationship between the compressive strengths of the combined board and the component linerboard is expected for a uniformly treated corrugated fiberboard, based on the principle of load sharing between linerboard and corrugating medium (10). Dimensional stability Swelling restraints can be measured in terms of linear dimensional change. The percent reduction in swell ingrela- tive to an uncrosslinked fiberboard can be expressed as its dimensional December 1986 Tappi Journal 73 corrugating medium was glued to the ranging from 61% to 100% (30°C); in a 4% solution of sodium hydroxide linerboards with a commercial, un- exposure to sulfur dioxide gas result- to remove unbound formaldehyde. modified starch. ing in a molar ratio of sulfur dioxide to formaldehyde of 1:2; exposure to Literature cited Crosslinking reactor reactant for 2-6 h; and dehydration at The large-scale reactor used to cross- 120° (±5°C) for 15 min. Specific pro- link corrugated fiberboard consists of cess variable values and the effects of a 24-m3(840-ft 3) chamber, gas recircu- these variables on the extent of cross- lating ducts, air intake/gas exhaust linking achieved—applicable to large- ducts, heating units, a scrubber, and scale treatment—are discussed for instrumentation (Figs. 5 and 6). Re- small-scale treatment elsewhere (1-6). actants-formaldehyde in the form of formalin and sulfur dioxid—are in- Property measurements jected upstream of the chamber into Linerboard stiffness ratios, SR (mea- the common duct and are exhausted sured sonically) and CSR (measured downstream of the chamber through in compression), were determined the scrubber tower. from measurements of sonic wave The reactor is equipped with 2 velocities through linerboard samples electric resistance (cal-rod-type) by methods previously described (3, heating units for gas and air heating 7) and from measurements of uniaxial, to 150°C, located upstream of the or flute direction, compressive modu- chamber. Both ducts have fans for lus using the vacuum restraint method moving gases and air and for dis- (8). Linerboard compressive strength persing reactants. A steam injection was determined using the UCR ratio nozzle is also located upstream of the from the Gunderson apparatus (8). chamber for humidification anti for Wet specimens were prevented from stripping out unreacted chemicals. drying during measurement by cover- The following process variables are ing the area of the linerboards between controlled and monitored with the aid the tester jaws with wet blotters. The authors wish to thank Peter Abits and of a microcomputer relative hu- Combined-board edgewise compres- Marguerite Sykes, physical science technicians midity, formaldehyde, and sulfur sive strength was measured according at the Forest Products Laboratory, for technical dioxide charges and concentrations; to TAPPI T-811 os-79 (9). Wet speci- assistance. corrugated fiberboard reactant pick- mens were measured after a 24-h Received for review Nov. 20, 1985. up reaction temperature; injection water soak. Accepted Feb. 10, 1986. and reaction times; airstream velocity Dimensions were measured in the and steam injection and stripping thickness and flute directions of both times. dry (50% RH, 22.8°C) and wet (24-h soak) combined board. Process conditions Each load of 12 corrugated fiberboard Bound formaldehyde content panels, uniformly spaced in the reactor Bound formaldehyde content was an- chamber, was reached under various alytically determined using a chro- process conditions that included con- motropic acid method (11). Samples ditioning with formalin (37% formal- were prepared for determining bound dehyde in water) at a relative humidity formaldehyde content by prewashing

74 December 1986 Tappi Journal