THE DEVELOPMENT OF A RESEARCH APPROACH TO THE SCIENTIFIC STUDY OF CELLULOSIC AND LIGNEOUS MATERIALS
HELEN D. BURGESS, & NANCY E. BINNIE
4 SELECTION OF ANALYTICAL TECHNIQUES
ALL OF the analyses discussed in this section are being carried out on the 25 different fiber samples (tables 3 and 4). Fumigated samples are tested alongside analogous unfumigated (control) material. Suitable quantities of all the fumigated and unfumigated samples are subjected to accelerated thermal aging. Aging is allowed to proceed at 70°C and 50% RH for 8–12 weeks, the length determined individually for each fiber. The factors used in these decisions include:
- age of sample
- fiber content, especially the quantity of lignin
- degree of degradation, as determined by average DP¯ and visual inspection for color change.
The main goal in the accelerated aging is to stress the samples enough so that significant chemical and/or physical changes result but not to the point at which the rate of degradation has almost ceased. This is a particular problem for many physical testing methods based upon the determination of the strength of fibers. In extreme cases involving naturally aged ligneous material, statistically significant physical strength measurements may be difficult or even impossible to obtain. As fibers reach the micro-crystalline level, the rate of change in molecular and chemical properties, such as DP¯ or carbonyl level, slows down considerably.
The chemistry of the fumigant and the two substrates, cellulose and lignin, indicates that the analytical procedures should be able to monitor changes in acid content, general deterioration, and fumigant residues. Indicators of degradation of fibrous polymeric materials include polymer length, degree of oxidation, color change, and physical strength. The techniques chosen to investigate all of these parameters are outlined below along with comments regarding anticipated benefits or disadvantages. Information is also given concerning the accelerated aging procedures used in this project.
4.1 DETERMINATION OF ACIDITY AND ALKALINITY
- Surface pH(TAPPI 1982)A value is obtained for the surface pH of a substrate by putting a drop of water on the sample placing a flat-headed electrode on the wetted area and taking the reading from a pH meter.Benefits:
for some samples, changes in surface pH (due to surface adsorption of acidic chemicals) may be differentiated from changes in pH that reflect absorption of acid into the substrate;procedure is relatively quick and easy to do;analysis gives good, relative, semiquantitative values;procedure has been used frequently in scientific projects in the conservation field.Disadvantages:
many substrates are too thin or absorbent to give data that can be related to surface effects (especially problematic for textiles);many samples (especially ligneous or degraded papers) have low initial pH, and since pH is a log scale, significant changes may not be observed (similar problems may exist with highly buffered materials);many replicates are necessary to obtain statistically significant results.
- Cold Extracted pH(TAPPI 1977b)Samples are cut into small pieces; suspended in pure water at room temperature, and incubated for a predetermined length of time. The pH is determined by combination electrode.Benefits:
relatively large quantities of the sample are used, which tends to help “even out” inhomogeneities within the sample material;procedure is relatively quick and easy to do;analysis gives good, relative, semiquantitative values;procedure has been used frequently in scientific projects in the conservation field.Disadvantage:
many samples (especially ligneous or degraded papers) have low initial pH, and since pH is a log scale, significant changes may not be observed (similar problems may exist with highly buffered materials).
- Iodometric Total Acid(Nabar and Padmanabhan 1950; Slavik et al. 1967; Achwal and Murali 1985)Samples are pretreated with acid to ensure that all acidic functional groups are in the acid form (as opposed to a salt) and that any alkali present is neutralized. Excess soluble acid is removed by extensive washing. The total intrinsic acid (due to carboxylate and enediol functional groups in cellulose and lignin, as well as any water-insoluble materials associated with sizes, coatings, additives, etc.) is determined by an iodometric back-titration method. Samples are suspended in a solution of iodate/iodide in the presence of a known amount of thiosulphate. During a 48-hour incubation period, the acid in the sample catalyzes the conversion of iodate to iodine and the thiosulphate reacts with the liberated iodine. Residual thiosulphate is titrated with standard iodine. By subtracting the value obtained in the titration from the amount of thiosulphate originally added, the quantity of acid in the sample can be calculated. The reactions involved are as follows:Pretreatment:
data obtained are extremely accurate, even at low acid values;incubation is carried out at near neutral pH (note: the enediols are in equilibrium with carbonyl groups, which are sensitive to alkali, and so measurement of these groups is accurate only if the pH is kept in the neutral/acidic range during analysis);esterified carboxylates (i.e., lactones) are hydrolyzed during incubation, and the free carboxylate is liberated; consequently it is possible to obtain extremely accurate values for the total number of acidic groups present in the fiber.Disadvantages:
analyses are very time consuming to perform;data do not differentiate among the various acidic functional groups without addition of other analytical procedures (e.g., sodium borohydride reduction of aldehydes and ketones).
- Determination of Alkaline ReserveAlkaline papers are soaked in a standardized solution of weak acid. During this time the alkali in the fibers is neutralized by the acid in solution. After a 16-hour incubation time (to allow for complete neutralization), the excess acid is measured by a back-titration using neutrality as the endpoint (pH 7.0 measured by combination electrode). The quantity of acid that has reacted with the alkali can be used to determine the original amount of MgCO3 or CaCO3 present in the paper sample.Benefits:
procedure is relatively quick and easy to do, and use of a pH meter to determine the titration endpoint eliminates errors associated with the use of color indicators;method gives a good measure of the amount of alkali present (alkaline substances like MgCO3 and CaCO3 are only sparingly soluble in water and cannot be accurately estimated by pH methods as described in (a) or (b));the long incubation time produces a more correct estimation of the total alkaline reserve on the fibers (in comparison to industrial test methods, which use a very short incubation time, ca. 1 hour).Disadvantage:
measured value of alkaline reserve will differ from the value quoted by the manufacturer of the paper because different analysis methods are used.
4.2 DETERMINATION OF FIBER DETERIORATION
- Viscometric Average Degree of Polymerization(Doty and Spurlin 1955)The cellulose is dissolved in a 100% solution of the aqueous solvent cadoxen (Donetzhuber 1960) and diluted to 50% with water, and the intrinsic viscosity is determined using a Canon-Fenske viscometer at 30°C. The calculation of average degree of polymerization (DP¯) from the viscosity data is carried out using the following equation:
The data obtained are estimates of the average polymer length.Benefits:
procedure is extremely sensitive to very small changes in average polymer length;full DP¯ range of paper and textiles can be accurately determined by this method (note: cadoxen is able to dissolve the higher DP¯ paper and textiles, which are difficult to solubilize in other cellulose solvents such as cupriethylene diamine (CED) or cuprammonium hydroxide (cuoxam));monitoring DP¯ changes gives excellent data concerning changes in the general state of degradation of the fiber sample;cadoxen is an odorless, colorless, and relatively stable solvent (in comparison to the other commonly used aqueous cellulose solvents).Disadvantages:
procedure is primarily of use in following changes in the cellulose portion of the fiber: lignin is insoluble in the commonly used cellulose solvents, including cadoxen, and interferes greatly with the calculation of intrinsic viscosity of heavily lignified fibers;very large quantities of cadoxen are required for projects of this scope, and the time and cost of synthesizing the amount needed are significant;cadmium oxide, used in the synthesis of cadoxen, is toxic and must be handled with appropriate precautions.
- Carbonyl Functional Groups(Blair and Cromie 1972, 1977; Ermenlenko and Savastenko 1966)The carbonyl groups arising from ketone and aldehyde functional groups are determined by reaction of the fibers with 2,4-dinitro-phenyl hydrazine. The resulting hydrazone derivative is quantified by colorimetric analysis at 400 nm using two different methods:
measurement of surface color by an integrated sphere reflectance spectrophotometerthe hydrazone is dissolved in 100% cadoxen and diluted to 60% with water and the optical density of the solution determined.Since aldehyde and ketone functional groups are important products of oxidative degradation of cellulosic fibers, the resulting data indicate the degree of oxidation of the fiber.Benefits:
procedure is a direct method of analysis that gives good stoichiometric data;procedure differentiates oxidation on the surface of a substrate from oxidation that is more evenly dispersed throughout;method may be useful in following chemical changes in lignins, as high concentrations of carbonyls are found in lignified samples.Disadvantages:
hydrazine reagent and hydrazone derivatives are suspected carcinogens;procedure is relatively time consuming;surface measurements may be difficult to obtain on very thin paper (e.g., tracing paper) or very thick rough-textured textiles (e.g., some linens).
- Color Measurement(TAPPI 1972, 1977a, 1981a; Grum 1981)Reflectance measurements are made using a spectrophotometer fitted with an integrated sphere. The values are determined relative to an international white ceramic standard obtained from the National Research Council of Canada (Budde et al. 1982). The wavelengths used are 457 nm (Tappi brightness standard; TAPPI 1977a) and 416 nm (in order that data can be correlated to earlier work carried out in this laboratory). The wavelengths chosen are useful in following bleaching (% reflectance increases) or yellowing (% reflectance decreases) of samples. As fibers degrade, they tend to produce chromophores that cause yellowing or darkening of samples. Bleaching is also indicative of some chemical change taking place in the sample. Although color change may be observed and thus a change in the chemical structure of the fibers implied, a direct link between the quantities of functional groups and color has not been established.Benefits:
method is non-destructive to sample;procedure is relatively quick and easy to perform;data relate to a physical property easily understood by non-technical personnel;integrated sphere facilitates accurate measurement of moderately rough materials (e.g., some linens);accurate data can be obtained for fairly inhomogeneous paper providing large areas of paper are covered in the measurements.Disadvantage:
measuring samples that are extremely rough (e.g., some jutes) or crumpled is difficult.
- Physical Strength Testing(TAPPI 1981b; ASTM 1982)Preconditioned samples (ASTM 1979; TAPPI 1970) as individual yarns (textiles) or one in wide strips of paper, are tested for tensile strength and percent elongation (TAPPI 1981b; ASTM 1982) using an Instron “constant rate of elongation” apparatus located in an environmentally controlled area. Tensile strength is the maximum strength of a material subjected to tensile loading. Percent elongation is a measure of sample ductility expressed as gauge length. These properties are related to the physical strength and durability of the sample.Benefits:
results give easily understood information concerning the effects caused by the physical stress concurrent with handling and display of artifacts;after the physical measurements are completed, sample material can be used for other analytical procedures (e.g., acid measurements, etc.).Disadvantages:
good data are difficult to obtain on very weak naturally aged materials (procedure is not sensitive in lower range of fiber strength, where it is still possible to get good data from many chemical methods of analysis);many replicates are necessary, because naturally aged material is difficult to measure due to inhomogeneities of substrate;data do not predict the behavior of materials subjected to sudden or repeated loading.
4.3 DETERMINATION OF FUMIGANT RESIDUES
- Gas Chromatography/Mass Spectroscopy (GC/MS)(Hewlett Packard 1986)Air samples are withdrawn from sealed polyethylene bags containing shipping control and fumigated samples (separate) through a packed sampling tube for preconcentration. (Shipping control samples were not fumigated and were shipped along with the fumigated samples so that the only variable was fumigation.) Adsorbed gases are thermally desorbed and passed through a capillary gas chromatograph. A mass selective detector is used to recognize any molecular fragments characteristic of sulphuryl fluoride. The presence of fragments that can be related to the fumigant is a good indication that Vikane residues were present in air taken from the sealed samples.Benefits:
procedure detects trace levels of gases in the original air samples;method is specific to the fumigant because ion fragments of any molecular sulphuryl fluoride as well as ion fragments of fumigant decomposition products will be detected (i.e., SO2F2, SO2F, and SOF).Disadvantage:
fumigant residues that are adsorbed onto the samples cannot be detected.
- Fluoride Ion-Specific Electrode(Levi et al. 1986; Speaker 1976)Samples are cut into small pieces suspended in pure water at room temperature buffered to pH 5 to 5.5 by the addition of TISAB and incubated for a predetermined length of time. (TISAB, a “total ionic strength adjustment buffer,” the Fisher Scientific Company (#SO-B-175), provides a constant ionic strength, decomplexes fluoride from iron and aluminum, and adjusts solution pH to between 5.0–5.5) The fluoride content is determined by ion-specific electrode. Since fluoride is a product of the breakdown of sulphuryl fluoride, the quantities of fluoride observed in the aqueous extract indicate the relative amount of fumigant residues present in the samples.Benefits:
observed fluoride directly relates to fumigant residues because the natural background levels for fluoride are very low;solutions prepared for the measurement of cold extracted pH can also be used for fluoride determination (with the addition of suitable buffers after the pH measurement is completed);procedure is relatively quick and easy to perform.Disadvantages:
fumigant residues that are not in the form of fluoride (F−) are not quantified;data values will be low if all the fluoride is not decomplexed from the sample substrate.