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Qualitative and quantitative fiber analysis in recycled raw materials for packaging.

Abstract

Understanding the influence that heterogeneous recycled raw materials have on packaging-grade paper performance offers great potential value to the corrugated board and packaging industry. Fifty-seven linerboards and corrugating medium were selected to represent all the variety of paper grades available on the Spanish market at the moment for the production of corrugated board. The origin of softwood, hardwood, and nonwood fibers and their percentages by weight were determined with light microscopy and standard fiber analysis techniques.

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Environmental pressure and associated regulations have led to a significant increase of recycled paper as the main fibrous component of corrugated board the last several years (European Commission 1994, Young 1997, CEPI 2003). Packaging-grade papers, linerboards, and corrugating medium contain 60 to 100 percent recycled softwood, hardwood, and nonwood fibers that have been produced with a variety of pulping methods. Corrugating packaging industry is facing the challenge to enhance products derived from recycled pulp and to ensure a satisfactory strength of packages. Qualitative and quantitative data on the different fiber types of the recycled raw materials are needed for the evaluation of packaging fiber supply sources, which are becoming more heterogeneous, numerous and smaller, as well as for the utilization of the available resources in an optimal manner.

Materials and methods

Thirty-nine linerboards (15 kraft-liners, 8 test-liners, 16 recycled-liners) and 18 corrugating medium (11 semi-chemicals and 7 recycled-medium) were provided by representative Spanish paper manufacturers to cover the variety of grades available on the market for the production of corrugated board. Kraft-liner is mainly made from virgin softwood and hardwood fibers, but also includes some recycled fibers. Test-liner consists mainly of selected recycled fibers that increase the quality of the sheet. Recycled-liner is made from recycled fibers and of lower quality than test-liner. Semi-chemical is a virgin-based corrugating medium and has been increasingly used for special purposes (e.g., in humid conditions). The main virgin fiber used in its manufacture is semi-chemical hardwood, but lately recycled fibers have also been used. Recycled-medium is the multipurpose medium most frequently used and represents the lowest grade, which has been heavily recycled.

For each of the papers, microscope slides were prepared with fibers as for usual fiber analysis. In order to obtain a better resolution of structural details, staining of the fibers was performed by adding a drop of aqueous safranin and then covering with a cover glass. The procedure produced a total of about 300 fibers per slide.

The microslides were observed under a light microscope to a magnification range of 100x to 800x. In identifying the various components of pulps, attention should be focused on the structural characteristics of one or two cell types with the combined assistance of various keys and textbooks with illustrations (Carpenter and Leney 1952, Strelis and Kennedy 1967, Cote 1980, Panshin and de Zeeuw 1980, Parham and Gray 1990, Ilvessalo-Pfaffli 1995). The identification of softwoods was mainly performed on the basis of the anatomical characteristics of the thin-walled earlywood tracheids (cross-field pitting, height of cross-field areas, pits to ray tracheids, intertracheid pitting, spiral thickenings, and width). Differentiation of hardwoods was based on the features of vessel elements (size and shape; types of perforation; presence of spiral or reticulate thickenings; type of intervessel pitting; size, shape, and arrangement of pits leading to ray parenchyma; and presence of pits leading to vascular or vasicentric tracheids). Information on nonwood components (grass, bast, leaf and fruit fibers) was obtained from the presence of parenchyma cells, epidermal cells, vessel elements, and rings from annular vessels, from the general shape of fibers including width and length, and from the shape of fiber ends.

[FIGURE 1 OMITTED]

The calculation of weight percentages of the fiber components was done in accordance with ISO 9184-1 (ISO 1990). Counting of fibers was made in more than two microslides per paper at a magnification of 80x. In order to simplify the analysis, fibers were classed into three categories: softwood, hardwood, and nonwood. The weight factors used to convert the fiber counts into percentages by weight were predetermined literature values and taken largely from Einspahr and Hankey (1978). For the three categories, average weight factors were assigned to each paper so as to represent the combination of all the genera and species identified. The total fiber count of each category was multiplied by its respective weight factor and then their percentages by weight of the total weight were calculated.

Results

All papers and especially the recycled based liners and medium exhibited significant variability comprising a large number of different components. Semi-chemical proved the least variable grade. Figure 1 illustrates some of the diagnostic features used for the recognition of the fiber components of papers. The few positive anatomical characteristics available, degradation (cutting and shortening, tearing, fibrillation, etc.) of fibers due to processing and presence of similar species in the furnishes, severely limited the identification of individual species and, thus, identification was made mainly to genera or subgroups of genera.

The occurrence of identified softwoods, hardwoods, and nonwood fibers was visually estimated in proportion to their relative amounts on the examined microslides and is shown in Table 1. The most plentiful fiber types on a microslide were characterized as "abundant" and fiber types encountered rarely (one to three fibers per microslide) as "infrequent." Fiber types found in moderate or small numbers were classified as "frequent" or "less frequent," respectively. Fibers of Pinus sylvestris, Pinus pinaster, Pinus radiata followed by the genera Larix or Picea were found in abundance in almost all packaging-grade papers. Pinus nigra as well as southern pines were present in small amounts in some papers. Genera with minor importance were Abies and Pseudotsuga and fibers of Pinus halepensis. All papers contained Betula, Eucalyptus, and Populus in their hardwood mix. Fagus sylvatica and Tilia were frequently observed in the papers and in some of them they were amongst the major hardwood components. The infrequent presence of a number of hardwood species and genera (Alnus, Acer, Castanea sativa, Quercus, Carpinus, Magnolia acuminata, and Magnolia grandiflora) with the exception of Liquidambar styraciflua, Lyriodendron tulipifera, and Nyssa sylvatica in some recycled-based papers was attributed to the recycling process. Nonwood fibers, mainly grasses and bast and leaf fibers were found in all packaging grades also as a result of the recycling process. In most of the papers, more than one grass species was present, for example, cereal straws, reed (Phragmites communis), sabai (Eulaliopsis binata), rice (Oryza sativa), albatrine (Lygeum spartum), esparto (Stipa tenacissima), sugar cane (Saccharum officinarum), corn (Zea mays), and bamboo (Dendrocalamus strictus). The limited availability of diagnostic features (associated cells) for bast and leaf fibers did not allow species identification. In some papers, cotton fibers (linters) were observed in insignificant numbers.

Table 2 presents the weight percentages (minimum and maximum values) of fiber categories in the paper grades together with the number of fibers counted for each category. Except for some kraft-liners that contained greater softwood content (up to a weight of 69%) than hardwood, in all packaging-grade papers hardwoods were found to be the main fiber component (up to 94% per weight in the semi-chemicals). Nonwood fibers entering the manufacturing process through recycling comprised a significant fiber component in most of the grades and their weight percentage varied between 2 and 9 percent. The results on the quantitative fiber composition reflected the differences in quality between the paper grades. For example, the recycled-liners contained generally lower softwood content (25% to 37%) than the stiffer kraft-liners, and test-liners represent medium qualities of linerboard. Also, in the corrugating medium grades, the recycled-medium values were more variable than the semi-chemical values due to the use of higher proportions of recycled fibers in their production.

Conclusion

Fiber analysis techniques were applied to evaluate the fibrous materials used in packaging. Packaging-grade papers were found to be highly variable, containing a large number of different types of softwood, hardwood, and nonwood fibers. Information both on the structure and quality of the paper grades was obtained by the quantification of the identified fiber components. Despite the large heterogeneity, the combined qualitative and quantitative results provide fundamental knowledge of the recycled raw materials, which is essential for a sustainable packaging industry.

In order for paper manufacturers to be able to make an end product of consistent and acceptable quality, they should always know which species and species groups are included and how much of a certain fiber type or group of fiber types they use. The usefulness of fiber composition could be further explored to assist in the quality control of paper manufacturing for packaging.

Literature cited

Carpenter, C.H. and T. Leney. 1952. 91 papermaking fibers. SUNY College of Forestry, Syracuse, NY.

Confederation of European Paper Industries (CEPI). 2003. CEPI annual report. CEPI, Brussels, Belgium.

Cote, W.A. 1980. Papermaking Fibers. A Photomicrographic Atlas. Syracuse Univ. Press, Syracuse, NY.

Einspahr, D.W. and J.D. Hankey. 1978. Improved weight factors for fiber analysis. Tappi J. 61(12):86-87.

European Commission. 1994. European Parliament and Council Directive 94/62/EC of 20 December 1994 on packaging and packaging waste. Official J. L365:10-23.

Ilvessalo-Pfaffli, M.-S. 1995. Fibre Atlas: Identification of Papermaking Fibres. Springer Verlag, Berlin.

International Organization for Standardization (ISO). 1990. Paper, board and pulps. Fibre furnish analysis. Part 1: General method. ISO Standard 9184-1. ISO, Geneva, Switzerland.

Panshin, A.J. and C. de Zeeuw. 1980. Textbook of Wood Technology, 4th ed. McGraw-Hill, New York.

Parham, R.A. and R.L. Gray. 1990. The Practical Identification of Wood Pulp Fibers. 2nd ed. Tappi Press, Atlanta, GA.

Strelis, I. and R.W. Kennedy. 1967. Identification of North American Commercial Pulpwoods and Pulp Fibers. Univ. of Toronto Press. Toronto, Canada.

Young, R.A. 1997. Processing of agro-based resources into pulp and paper. In: Paper and Composites from Agro-Based Resources. R.M. Rowell, R.A. Young, and J.K. Rowell, eds. CRC Lewis Publishers. Boca Raton, FL. pp. 137-245.

The authors are, respectively, Former Postdoctoral Research Fellow, Dept. of Pulp, Paper and Packaging, AIDIMA (Wood, Furniture and Packaging Technology Institute), Valencia, Spain, and currently Research Teaching Staff, Dept. of Forestry and Management of the Environment and Natural Resources, Democritus Univ. of Thrace, Orestiada, Greece (sadamop@fmenr.duth.gr); and Head of the Wood Technology and Biotechnology Dept., AIDIMA (joliver@aidima.es). Part of research carried out within the framework of a EU Marie Curie Postdoctoral Fellowship contract HPMD-CT-2000-00043. This paper was received for publication in May 2005. Article No. 10048.
Table 1. -- Results of the qualitative analysis of packaging-grade
papers.

Fiber category
and occurrence (a) Genera, groups of genera and species

Softwoods
 Abundant Pinus sylvestris, Pinus pinaster, Pinus
 radiata
 Frequent occurrence Larix or Picea
 Less frequent occurrence Pinus nigra, southern pines (resembling
 Pinus taeda, P. echinata, P. elliottii, P.
 palustris)
 Infrequent occurrence Abies. Pseudotsuga, Pinus halepensis
Hardwoods
 Abundant Eucalyptus, Betula, Populus
 Frequent occurrence Fagus sylvatica, Tilia
 Less frequent occurrence Liquidambar styraciflua, Liriodendron
 tulipifera, Nyssa sylvatica
 Infrequent occurrence Acer, Alnus, Quercus, Castanea sativa,
 Carpinus, Magnolia acuminata, Magnolia
 grandiflora
Nonwood fibers
 Frequent occurrence Grasses, bast and leaf fibers
 Infrequent occurrence Fruit fibers (cotton)

(a) The occurrence was based on visual estimation of the relative
amounts of earlywood tracheids with different morphological
characteristics for softwoods, different vessel elements for hardwoods,
and fibers and associated cells for nonwood components. The occurrence
of the different fiber types encountered in high (prominent fiber types
on microslides), moderate, small, and very small (rare presence of one
to three fibers per microslide) numbers was categorized as abundant,
frequent, less frequent, and infrequent, respectively.

Table 2. -- Weight percentages of fiber components in packaging-grade
papers.

 Weight (a)
 Linerboards
Fiber category Kraft-liner Test-liner Recycled-liner
 (%)

Softwoods 34 .... 69 39 .... 44 25 .... 37
 (190 to 329) (194 to 279) (92 to 248)
Hardwoods 29 .... 58 51 .... 58 58 .... 66
 (281 to 596) (421 to 730) (491 to 867)
Nonwood fibers 2 .... 8 2 .... 5 2 .... 9
 (20 to 68) (16 to 33) (14 to 53)
Total fiber count 630 to 854 648 to 1,025 629 to 1,129

 Weight (a)
 Corrugating medium
Fiber category Semi-chemical Recycled-medium

Softwoods 6 .... 35 25 .... 40
 (36 to 132) (99 to 181)
Hardwoods 56 .... 94 53 .... 70
 (419 to 791) (410 to 478)
Nonwood fibers < 2 .... 9 4 .... 7
 (5 to 54) (15 to 41)
Total fiber count 605 to 836 602 to 653

(a) Minimum and maximum values (range of fiber counts in parentheses).
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Author:Adamopoulos, Stergios; Oliver, Jose-Vicente
Publication:Forest Products Journal
Geographic Code:1USA
Date:Feb 1, 2006
Words:2047
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