Abstract
Both Edgeworthia and Wikstroemia have been used traditionally in hand papermaking in China. As plants from the Thymelaeaceae family, these species have been mentioned periodically and have been mixed or substituted with each other. This study reviewed their papermaking background and status, and selected one of the Edgeworthia and three of the Wikstroemia raw materials that are still in use. Pulp was cooked under the same conditions in the laboratory, and handsheets of pure and mixed materials were obtained using a Rapid-Köthen sheet former. A series of physical properties, such as color, structure, and mechanical properties were tested, followed by a comparison and discussion of the raw material properties. The results show that Wikstroemia fibers are generally thinner and shorter, but there are variations between the different genera of raw materials. Thus, compared with Edgeworthia, Wikstroemia has some advantages in tensile strength, although it shows some disadvantages when it comes to tearing strength. The performance of a mixed-ingredient paper sheet falls between the performance of two pure-component papers. For cost and performance reasons, adding a small amount of Wikstroemia (e.g., <20%) to Edgeworthia is preferable.
The bast of woody plants has been widely used for handmade paper in China. Among all the raw materials, the plants of the Edgeworthia and the Wikstroemia genera stand out.
The original use of Thymelaeaceae plants for paper is ambiguous. This kind of material was used in Tibetan paper long time ago, but the difficulty of distinguishing the bast fibers of the family prevented researchers from determining the raw material. Edgeworthia may be one of the plants used
The Ming dynasty may have been a period when Edgeworthia flourished as one of the raw materials used to make paper in many places. First, Edgeworthia was a traditional ornamental plant in China called Jiexiang, which resembled Ruixiang (Daphne) without fragrance
From the late 19th century, Edgeworthia and Wikstroemia were increasingly mentioned in local chronicles, survey reports, and plant resource records as papermaking raw materials. In these documents, they had different names. The former was called Shanyapi, Sanyapi (Mitsumata), Xuehuapi, and Menghuapi, and the latter was called Shanmianpi, Yanpi (Gampi), and Shanyanpi. They were usually used to make everyday paper (i.e., Mian or Xuehuapi paper) or wax paper in fireworks, packages, umbrellas, or oil printing.
The handmade papers of Edgeworthia and Wikstroemia are always described as delicate and strong. During our investigations, craftsmen recalled that Edgeworthia was easier to cultivate artificially; that is, it was more abundant and cheaper as a raw material than Wikstroemia. Consequently, the more expensive pulp is added in small quantities to the cheaper one to make Mian paper. Furthermore, the craftsmen mentioned that this mixed-material handmade paper was better than the original one, that is, the paper was more delicate and stronger. Owing to this, when there was a shortage of Wikstroemia for wax paper, Edgeworthia was chosen as a great substitute.
To better understand the properties of Edgeworthia and Wikstroemia bast in the process of hand papermaking and to verify the statement of mixtures or substitutions, the experimental method by Zhao et al
In Hu'nan Province, Edgeworthia chrysantha bast has been traditionally used to make handmade paper called Xuehuapi (snowflake bark) paper, whereas Wikstroemia, such as Wikstroemia delavayi and Wikstroemia lichiangensis, were recorded to make Dongba paper in Yunnan Province (the former was used in this study). Zhejiang's Mian paper or wax paper was made from other species of Wikstroemia, such as Wikstroemia indica, Wikstroemia ligustrina, and Wikstroemia monnula. Nowadays, the lack of local Wikstroemia bast leads to the import of Philippine species (Wikstroemia lanceolate according to Yi
| Raw material | Origin | Year | Handmade paper |
|---|---|---|---|
| Edgeworthia | Hu'nan Province | 2019 | Xuehuapi paper |
| Wikstroemia | Yunnan Province | 2018 | Dongba paper |
| Wikstroemia | Zhejiang Province | 2020 | Mian paper |
| Wikstroemia | The Philippines | 2020 | Yanpi Paper |
It should be mentioned that the bark may be hard to peel but is relatively thicker in winter. According to papermakers, branches cut down in fall and winter that have been growing for two to three years are a good choice of papermaking raw material.
The cooking agent was sodium carbonate (Na2CO3) anhydrous from Dahe Chemicals Co., Ltd. (Shanghai, China).
The bark of the trees was dipped in water, and the outer black bark was scraped off. Then, the inner white bast was dried and prepared for use. Before heating, the bast was cut into 2-3 cm segments and soaked. Next, 80 g of bast, 64 g of cooking agent (Na2CO3), and 1.6 L of water were placed in a spherical reactor. The reactor was heated rapidly until boiling. Afterwards, the temperature was maintained at 100 ℃ for 5 h at atmospheric pressure. After heating, the bast was washed in a gauze bag with running water until the solution reached a pH value of 7.
Following the ISO standard (ISO 5269-2:2004), the bast was processed into pulp using the fiber disintegrator (PTI, Austria), and the handsheets were produced using the Rapid-Köthen Sheet Former RK-2A (PTI, Austria). To make it similar to the handmade paper, the sheets were prepared with grammage of approximately 22 g/
| Sample | Weight ratio of raw materials % | |
|---|---|---|
| Hu'nan Edgeworthia | Yunnan Wikstroemia | |
| E-HN | 100 | 0 |
| E8W2 | 80 | 20 |
| E6W4 | 60 | 40 |
| E4W6 | 40 | 60 |
| E2W8 | 20 | 80 |
| W-YN | 0 | 100 |
| W-ZJ | 100% Zhejiang Wikstroemia | |
| W-Ph | 100% Philippine Wikstroemia | |
The experiment was carried out according to ISO 5351:2010 using the viscosity method to determine the DP
| (1) |
where h is the viscometer constant (0.095013). Because different values of the viscosity ratio corresponding to the value of [η]·ρ can be find from the Appendix B in the international standard (ISO 5351:2010), the solution density (ρ) can be calculated by
| (2) |
where m is the absolute dry weight of the pulp, while V1 and V2 are the volume of CED and water respectively. With the value of [η]·ρ, the intrinsic viscosity [η] can be obtained. And the was calculated with
| (3) |
Four types of bast fiber were observed under the optical microscope. The Herzberg stain
In this study, the colors of handmade paper are described by the color model proposed by the Commission Internationale De L'Eclairage in 1976, the CIE-Lab. Following ISO/CIE 11664-4:2019, an CS-411 instrument from Color Spectrum Technology Co., Ltd. (Hangzhou, China) with a D65/10° light source and a D/8 structured integrating sphere was selected in the SCE measurement mode.
There were no characters or other contents in the four samples. Thus, based on the K-M optical principle, the evenness of the paper is represented by measuring the transmittance of visible light passing through it
Fig. 1 Test of evenness
It should be emphasized that this method is not universal and only applicable to comparing paper samples of relatively close thickness under the same conditions, such as the handsheet samples with close grammage in this study.
Refers to ISO 534:2011, the thickness was tested by MC-5633 Paper Thickness Gauge from Hechuang Precision Instruments Factory (Xiamen, China). The paper density (d) can be calculated using
| (4) |
where g is the grammage of the samples, and δ is the thickness of the samples.
There were 10 replications in this experiment.
The test was carried out with reference to ISO 1974:2012. The samples were cut into pieces with a size of (63±0.5) mm × (50±0.2) mm. Four layers of sheets were stacked together for one measurement. There were three replications in this experiment.
Tensile strength refers to ISO 1924-2:2008, and it was tested using the ZB-L vertical computerized tensile strength tester produced by Zhibang Automation Technology Co., Ltd. (Hangzhou, China). The samples were cut into pieces with a width of (15±0.1) mm. The test length was 10 mm, and the tensile speed was (10±5) mm/min. There were eight replications in this experiment. The tensile index can be calculated by
| (5) |
where F is tensile strength, and g is the grammage of the samples.
Refers to ISO 15361:2000, the Zero-span tensile strength was tested by YT-ZL zero-span tensile tester by Yante Scientific Instruments Co., Ltd. (Zhejiang, China). The samples were cut into pieces with a width of (15±0.1) mm. There were eight replications in this experiment.
The zero-span tensile strength is considered to be representative of the strength of the fiber itself. Paper-bonding strength index (PBSI) describes the bonding strength between the fibers, which can be calculated using Page's formulation (
| (6) |
where T is the tensile index in
| (7) |
where F′ is zero-span tensile strength, and g is the grammage of the samples.
Cellulose is a polymer made up of repeating units, which is the main chemical component of plant fibers; its DP differs in variable plants. During the cooking and pulping process, DP can be used to reflect the extent which the cellulose has been broken down.
Fig. 2 DP of samples
Since excessive DP suggests inadequate cooking, these results indicate that the Edgeworthia and Yunnan Wikstroemia may require longer cooking time than the other two during the pulping process. However, the different plant species may have provided natural differences in the DP of these raw materials. In other words, the molecular chains of Edgeworthia may be originally longer than those of the two Wikstroemia species. After the cooking process, longer cellulose molecule chains provide advantages to the paper's physical properties, and the DP declines with aging time. Therefore, the Wikstroemia from Zhejiang Province and the Philippine may show a disadvantage in durability.
Fiber cells can be considered the skeletons of paper and an important identifying feature of the type of paper material. Thymelaeaceae bast fiber can be distinguished from other bast handmade paper by its color in Herzberg stain and some morphological characteristics.
The fibers of Edgeworthia and Wikstroemia are similar. Under the optical microscope, the stained fibers showed a yellow to green color. They resemble most of the Thymelaeaceae bast fibers. Edgeworthia and Wikstroemia can be characterized by the numerous dislocations, the board central portions, and the variety of their lumens (
Fig. 3 (a) Board central portions of Wikstroemia bast fiber; (b) bifurcated and irregularly shaped ends of Edgeworthia bast fiber under optical microscope
The fibers of all samples are relatively short and thin. As indicated in
Fig. 4 (a) Fiber length and (b) fiber width of the samples
By dividing the length by width, the smaller width gives Zhejiang and Philippine Wikstroemia a higher aspect ratio (
| Sample | Aspect ratio |
|---|---|
| E-HN | 379.6 |
| W-YN | 267.5 |
| W-ZJ | 439.0 |
| W-Ph | 490.5 |
As the fiber length has an obvious effect on the paper physical properties, Hu'nan Edgeworthia and Yunnan Wikstroemia with significant differences in length were mixed to make handsheet samples.
To the naked eye, the sheets of Hu'nan Edgeworthia and Yunnan Wikstroemia were whiter. The results of CIE-Lab clearly show that the sheets of Zhejiang and Philippine Wikstroemia had a lower
Fig. 5 CIE-Lab results of the samples
The mixed sheet samples showed little difference because Hu'nan Edgeworthia and Yunnan Wikstroemia resembled each other in color. The latter may, however, be brighter; thus, the mixed samples showed a slightly increasing trend in
The fiber varies in different types of plants, resulting in a different paper structure. If the samples in this study are simply seen as a network of interwoven linear fibers, their density and evenness are decisive for the mechanical properties
Fig. 6 Density of samples
However, the addition of Wikstroemia to Edgeworthia did not result in a significant density increase. The density of mixed sheets was much closer to the density of 100% Edgeworthia sheets than that of 100% Wikstroemia.
A higher standard deviation indicates greater variation in grayscale value.
Fig. 7 Evenness of samples
Fig. 8 Microscopic photo of Philippine Wikstroemia sheet
Since the lower DP implied a relatively full cooking of Philippine Wikstroemia pulp, these fiber bundles may be impurities that need to be removed separately during the papermaking process. Thin and short fibers may be the key to a tight and even paper under ideal conditions, as the sample of Yunnan Wikstroemia showed. However, the Hu'nan Edgeworthia with the longest and widest fiber was not the worst in terms of density and evenness. Thus, there must be other factors affecting the structure of the sample except the size of fibers.
The evenness of mixed-ingredient sheets also showed an improved tendency with increasing amounts of Wikstroemia pulp because the latter had the best evenness.
Fiber properties and bonding strength are considered sources of the mechanical properties of paper. Many physical indicators are standardized for testing such properties. Tearing and tensile strength were tested first.
Tearing strength is considered to be sensitive to fiber length.
Fig. 9 Tearing strength of samples
The addition of Wikstroemia causes the tearing strength of the Edgeworthia sample to decrease. In terms of tearing strength alone, the mixed-ingredient sheets performed even worse owing to the weakness of the Wikstroemia sample. Fortunately, this downward trend was relativity slight when a small amount of Wikstroemia was added. It was not until the weight of Wikstroemia exceeded that of Edgeworthia that a rapid decrease in tearing strength occurred.
Tensile strength may be the most common physical property to describe the strength of paper. Papermakers preferred to check the strength of paper by pulling it, telling the investigators that the fibers of raw materials were great. According to Page's theory
Fig. 10 (a) Tensile strength and (b) zero-span tensile strength and PBSI of samples
Among the pure ingredient samples, Yunnan Wikstroemia sheet had the best tensile strength. The result was opposite to the tearing strength, which was the worst. Compared with the Edgeworthia sample, the Zhejiang Wikstroemia sheet had a lower tensile index, whereas the Philippine Wikstroemia sheet had a higher value. Conversely, the result of zero-span tensile strength showed that the fiber strengths of Zhejiang and Philippine Wikstroemia sheets were similar but lower than that of Yunnan Wikstroemia and the Edgeworthia samples. Hence, there was a great advantage of the bonding strength between the fibers in the samples of Philippine and Yunnan Wikstroemia sheets, which led to the higher value of tensile strength of these two samples. Furthermore, a lower bonding strength led to the lower tensile strength of Zhejiang Wikstroemia sheets, even though its fiber strength was a little bit higher than the Philippine sample. Notably, except Yunnan Wikstroemia, although the mechanical properties of the remaining two Wikstroemia sheets differed from those of the Edgeworthia sheets, these differences were not vast.
For the mixed-ingredient samples,
Recalling the experimental results of DP in the previous section, the high value of DP gives the plant fiber itself a strong tensile strength but does not exactly contribute to a better bonding strength between the fibers (Edgeworthia). Perhaps, the thinner and shorter fibers offer a tight structure that makes the paper stronger in some aspects (Yunnan Wikstroemia). Moreover, the unevenness of the fiber webs may lead to a weakness point when paper sample is being pulled. However, it must be noted that the fibers are flexible to a different extent owing to their chemical ingredient. Being similar in size does not only simply give the paper sheet a similar structure. Therefore, the mechanical strength can still differ (Zhejiang and Philippine Wikstroemia).
Finally, in this study, a disintegrator was used in the laboratory papermaking process without beating or pulping. Hence, the bonding strength came from the properties of the fibers themselves. In the actual papermaking process, many techniques can be controlled to change the properties of the fibers. For example, by cutting the raw material, the fiber length can be decreased to prevent the unevenness of the paper. Furthermore, beating the pulp is a step to disintegrate fibers better and improve bonding strength.
Edgeworthia and Wikstroemia bast fibers are the raw materials used for Chinese handmade paper. The production is small, but the paper is considerably distinctive. Wikstroemia plants are more difficult to cultivate artificially than Edgeworthia plants; thus, they are more expensive. Their fibers are relatively thin and short in the bast category of papermaking, with an average length of <4 mm and width of <20 μm, but the exact size depends on the origin and species. As a basic component of the existing intangible cultural heritage and ancient paper raw materials, both for inheritance and conservation, the characteristics of this category of raw materials need to be explored more deeply to regulate the papermaking process.
Experiments indicate that the Yunnan Wikstroemia pulp with shorter fiber has better tensile properties but worse tearing properties than the Edgeworthia pulp. The Zhejiang Wikstroemia pulp with slightly longer fiber has lower tensile but better tearing properties than the Edgeworthia pulp; the Philippines Wikstroemia pulp, which has an average fiber length similar to Edgeworthia, has better tensile and tearing properties.
When the two types of pulp with different properties were mixed, the properties of the mixed-ingredient sheets obtained were between those of the two pure ingredient sheets. However, the trend of each property did not change consistently with the pulp mass ratio. The experiments showed that the addition of Yunnan Wikstroemia pulp with shorter fibers to the slightly longer Edgeworthia pulp improved the tensile strength of the paper but reduced the tearing strength. The tensile strength increased steadily with a small proportion of Wikstroemia (0-40%), but the decrease in tearing was not significant. Until the proportion of Wikstroemia was sufficiently large (more than 50%), the tensile strength increased slowly and the decrease in tearing was more pronounced. Therefore, the small amount of Wikstroemia was added, for example, 20% of the mass, indeed at a lower cost to obtain stronger paper. Although the magnitude of the modification is limited, the papermakers' claim was confirmed to some extent. Further, it can be assumed that if the Wikstroemia pulp with better tensile and tearing strength, is added to the Edgeworthia pulp, both properties will be improved. Of course, the mechanical properties of Edgeworthia differ from those of Zhejiang and Philippines Wikstroemia, but they are close; thus, with a suitable making process, it is also feasible to use the lower-cost Edgeworthia instead of Wikstroemia.
The experiments in this study did not involve cutting, pulping, and the addition of many chemicals, and the resulting differences in papers are a more visual reflection of the characteristics of the fibers themselves. From the experimental results, it can be assumed that the size of the fibers and the binding force has a significant effect on the structure of the paper sheet and thus on the mechanical strength of the paper. Therefore, in the case of Edgeworthia and Wikstroemia handmade paper, it is extremely important to control the evenness and density of the paper sheet to obtain a compact sheet. Appropriate shortening of excessively long fibers, or the addition of slightly shorter fibers, may be able to improve the performance of handmade paper. Taking cost, process, and demand into account, the proper raw material is selected and matched with the right papermaking process to obtain the ideal paper.
Acknowledgements
This research is supported by the National Key R&D Program of China (2019YFC1520300).
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