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Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 53, No. 2

[ Article ]
Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 53, No. 2, pp.15-23
Abbreviation: J. Korea TAPPI
ISSN: 0253-3200 (Print)
Print publication date 30 Apr 2021
Received 20 Jan 2021 Revised 27 Mar 2021 Accepted 02 Apr 2021

Study on the Manufacturing and Applicability of Natural Glues for Woodcrafts Using Shark Skin
Koang-Chul Wi1,
1Department of Cultural Heritage Conservation, Hanseo University, Seosan, 31962, Professor, Republic of Korea

Correspondence to : †E-mail: (Address: Department of Cultural Heritage Conservation, Hanseo University, Seosan, 31962, Republic of Korea)

Funding Information ▼


In this study, we manufactured an eco-friendly wood adhesive fromgelatin extracted from shark skin. Extracted gelatin demonstrates stronger adhesion ability (321.82 kgf/cm2) than conventional fish glues (Japanese fish skin glue, croaker air bladder glue, sturgeon air bladder glue) that show good adhesive strength even under typical usage scenarios, such as heating in boiling water. In order to overcome the shortcomings related to conventional use of these glues, such as heating in boiling water or submersion in water, we manufactured a modernized adhesive that has low viscosity and high adhesion strength because of the modifying property of shark gelatin-using urea. Through the foregoing, this study, with a simple solubilization, was able to solve problems such as decomposition of gelatin protein by the water used as solvent, gelation that continues even after the elapse of time, change in properties etc. Shark gelatin-modified adhesive manufactured in this study showed an adhesion strength of 312.65 kgf/cm2 at a low viscosity of 3,100 cPs, demonstrating a very short drying duration of 25 min. As compared to that of a modified adhesive made from the air bladder of Japanese croaker with the same viscosity, our glue showed more than triple the adhesion strength. Further, constancy was expected after adhesion because the adhesive had deteriorated less even after being exposed to ultraviolet irradiation.

Keywords: Fish glue, shark skin adhesive, wood crafts, natural adhesive, traditional adhesive

1. Introduction

In today’s context, adhesives are being used in various fields, such as wood-making, architecture, medicine, electrical engineering, electronics and semiconductors construction, automobile production, metal-work, textile production, plastic production, leather-making, book-printing and publication, cultural assets preservation, and aerospace engineering, among many other fields and industries. Such adhesives are produced after adjusting their composition, shapes, and conditions to make them more suitable for their specific use. More than 500 kinds of adhesive are commercially available, even if ones with slightly different properties are included with the same labeling. Although adhesion strength is considered the most important property of adhesives, performance duration, penetrating power, flexibility, and adhesive property applicability are considered important factors. In addition to the aforementioned factors, other conditions that are now considered important include the usage convenience, storage stability, antimicrobial activity, and non-toxicity to humans and environment, thereby necessitating further research on natural adhesives.

Adhesives are created from natural materials, such as animal glue, fish glue, egg white, casein, animal blood, and albumin, in addition to existing vegetable adhesives, such as rice flour, wheat flour, starch, natural resin, seaweed, soybean glue, and varnish lacquer. It is known that both animal adhesives, such as glue, fish glue, and egg white, and vegetable adhesives, such as rice flour, flour, starch, seaweed, soybean glue, and lacquer, have been conventionally used since a long time throughout Korea.1,2) Of these adhesives, fish glue, including animal glues with polymeric properties, show greater adhesive strength than that of other adhesives;3) thus, they were considered superior than other natural adhesives.

It has been reported that the characteristics of fish gelatin, a main ingredient of fish glue extracted from tropical fish and cold-water fish, differs by habitat environment.4) A difference in the gelatin recovery rate by extraction temperature and distribution of molecular weight of peptide by part was also reported.5) Particularly, although there is a difference on an individual basis, the gelatin extracted from the skin portion from where gelatin is mainly extracted differed in individuals and as per pH, resulting in a difference in the elasticity and viscosity coefficient of the gel; thus, gelatin differs as per fish species, habitat environment, and extracted portion.6) Fish gelatin can be integrated with and modified easily by egg protein7) or seaweed polysaccharide;8-11) thus, it can be thus used in multiple ways and can be particularly used or modified as an adhesive. The use of beef glue, hare glue, fish glue, and starch glue is widely cited in the ancient literature of Korea,12-15) with the use of extracted croaker air bladder gelatin as an adhesive demonstrating particular advantages in terms of flexible adhesive properties, owing to which, it is widely used for adhesion in bows and musical instruments that require vibration and supple bending.16,17) However for croaker air bladders, fish glue using this same material’s production has decreased over time due to the price becoming too high in Korea. As a substitute for it, fish glues from other countries used for restoration using synthetic resins are being carried out for the preservation of cultural assets of Korea. Fish glue is required, but the reason why it cannot be used is that it is not usable.

Animal glues, including fish glue, had to be used after liquefaction at high temperature on being heated in boiling water and adjusting the concentration by adding some water. However, there were additional problems that arose during this process that led to the disuse of the residue; therefore, it has been replaced by synthetic resins. In other words, the problems of fish glue can be traced back to not only to the problem of its pricing, but also to its lack of usage convenience.18,19) Air bladder glue from Japanese cod, pike eel, and sturgeon air bladder glue that are mass produced in Europe, cod, shark and flat fish skin glue20,21) which are used in Japan and North America, are also used after heating in boiling water or the direction for use has not been quantified yet, and problems of irregularities in their shape and property as well as the storage difficulty because of their corruptible property have persisted for some time.

Thus, we manufactured a fish glue from Korean shark skin and compared it with the glue manufactured from commercialized Japanese fish skin. While simultaneously working to heighten its purity, we aimed to manufacture modernized fish glue that is convenient to use and has antimicrobial activity and persistency suitable for woodcraft by inducing peptide bonds in the extracted fish gelatin.

2. Materials and Methods
2.1 Materials for experiment

This study divided a Korean shark into its head, bones, fat, meat and skin, and removed the salt from the skin by repeating deposition and washing it with distilled water. This study used materials upon the completion of natural drying and processing until the salinity became 0%, as measured using a salinometer (PAL-03S, ATAGO, Japan) (Fig. 1). With respect to Japanese fish glue, the comparison material, this study used commercial fish glue manufactured for mass production using fish skin.

Fig. 1. 
Pre-treated shark skin (A: outer surface of shark skin, B: inner surface of shark skin).

2.2 Adhesive components extraction method

After adding shark skin in the amount of 55.0 g in 2 L distilled water and heating it at >100℃ in a stirring and heating mantle without a distiller, we continued to heat the temperature of the mixture for 8 h while adding distilled water in the amount of 500 mL at two different times to prevent the viscosity from rising due to evaporation. While continuing to heat the mixture for 8 h, we removed the fire source when the remnants, including shark skin, measured at 400 mL. We then lowered the temperature to room temperature, removed the impurities using decompression filtration with filter paper of 2.7 μm pore size, and pulverized after drying for 72 h in a drying machine at 70℃ (Fig. 2).

Fig. 2. 
Fish glue powder (A: Japanese fish glue, B: Korean croaker air bladder glue, C: European sturgeon air bladder glue, D: manufactured shark skin extract).

2.3 Adhesive manufacturing method

For the improvement of the functionality and usability of adhesive components extracted from the shark skin, we checked the optimum mixture ratio at each concentration by mixing urea (Samchun Chemicals, purity 99.0%) with H2O2 (Samchun Chemicals, purity 34.5%). To identify the manufacturing status as per the concentration of the extracted adhesive components, we stirred the mixture after adding distilled water in the amounts of 25.0 mL, 30.0 mL, and 35.0 mL and shark skin extract in the amounts of 15.0 g, 20.0 g, and 25.0 g. Thereafter, 3 g urea and 2 g H2O2 were added in that order. Stirring was continued at 80℃, the minimal temperature at which urea can undergo a reaction without the addition of a catalyst. We manufactured an adhesive from shark skin (hereinafter termed “shark skin adhesive”) and from Japanese fish glue (hereinafter termed “fish skin adhesive”) using the same method.

2.4 Property experiment

For the tensile adhesion strength, we measured the adhesion strength based on the KS M 3705 (General test method for adhesives) using woods where the grain of wood is uniformly distributed in 2.5×4.5×1.0 cm pieces of the Quercus acutissima Carruth specimen type. After applying 0.3 g of manufactured adhesive on the connecting pieces of wood specimens on the section where they overlapped by 1.5 cm, we used the adhesion specimen to fix and dry the pieces by using a clamp at room temperature (Fig. 3). For the measurement of the adhesion strength, we used a universal material tester (AG-X plus 5 kN, Shimadzu, Japan), measuring at 2 mm/min tension speed.

Fig. 3. 
Tensile shear adhesive strength wood specimen.

With respect to the drying time, we applied adhesive to the wood specimens and dried them at room temperature by referring to the KS M 5000 (the test method for paint and relevant raw materials), confirming with tack-free time that fingerprints were not left on the membrane surface when we gently pressed the portion of the membrane surface making contact with the fingertip, measuring at 1.5 cm.

While measuring the hydrogen ion concentration of the manufactured adhesive with a pH measuring instrument (Testo 206, Testo, Germany), we also measured the viscosity using a rotary type viscometer (V100002, Fungilab, Spain) and SP: L4. For the artificial degradation experiment that measures the UV irradiation condition, we measured the variation using a spectrum colorimeter (CM-2600d, Minolta, Japan) after irradiating with UV for 96 h at room temperature using a UV tester (Ecposure to Man-made Ultraviolet Light Test Chamber, Korea).22)

3. Results and Discussions

Yield as per the part of shark was as follows: head 37.5%, bone 6.4%, fat 10.1%, and shark skin 12.5%. We extracted a form of gelatin from the protoplasmic collagen existing in each part via extraction and heating. Thus, we manufactured gelatin from each part of the shark to form a 30% solution and compared its adhesion strength to that of commercial Japanese fish glue, Korean croaker air bladder glue, and European sturgeon air bladder glue. Adhesion strength as per the portion of shark was as follows: meat 90.3 kgf/cm2, head 112 kgf/cm2, bone 122.4 kgf/cm2, and skin 321.8 kgf/cm2. Thus, we confirmed that skin had the highest adhesive strength.

The comparison with commercial fish glue showed that the strength of the Japanese fish glue was 152.4 kgf/cm2, that of croaker air bladder glue was 195.6 kgf/cm2, and that of sturgeon air bladder glue was 294.1 kgf/cm2. Thus, shark skin extract was selected as an adhesion material with adhesion strength superior to that of commercial fish glues (Table 1).

Table 1. 
Adhesion strength of existing fish glue adhesives and Korean shark skin extract (kgf/cm2)
Fish glue Adhesion
Shark meat 90.3
Shark head 112.0
Shark bone 122.4
Japanese fish glue 152.4
Croaker bladder glue 195.6
European sturgeon air bladder glue 294.1
Korean shark skin 321.8

With respect to gelatin, simple solubilization was insufficient to solve the decomposition and gelation of gelatin protein caused by moisture, resulting in problems with usability, persistency, and preservability. To prevent such problems, we compared the manufacturing of an adhesive by inducing peptide bonds using urea and compared its properties to that of other adhesives. For the adhesive manufactured in this study, distilled water, gelatin, urea and oxygenated water were used; their compositions are shown in Table 2 (shark skin adhesive) and Table 3 (fish skin adhesive). For shark skin adhesive, we added oxygenated water as a response initiating oxidizing agent during the course of inducing the peptide bonds of urea and gelatin; however, there was no great difference in their variation and adhesion strength as it responded easily at a relatively high temperature of 80℃. The adhesion strength lowered due to postoxidation because of the added hydrogen peroxide. Thus, the use of hydrogen peroxide during the course of synthesizing is not expected to significantly affect the synthesis initiation factor or the antimicrobial activity of shark skin adhesive; thus, it is considered advantageous to not use it as an additive for synthesis. As shown in Table 2, the adhesive property of adhesive that is distilled water, shark skin gelatin and urea are mixed at the rate of 25.0 g, 25.0 g, 5.0 g, and synthesized at 80℃ resulted in the highest quality, showing an adhesive strength of 312.7 kgf/cm2 even at a low viscosity of 3,100 cPs and when dried for 25 minutes, a very short period time. When comparing the adhesion strength (180 kgf/cm2) of commercialized resin adhesive (PVAc adhesive) for woodcraft, which is used the most widely, and the adhesion strength of Japanese commercial glue solution (185 kgf/cm2) from glue, these are considered to be excellent results. However, the use of hydrogen peroxide during the manufacturing of fish glue adhesive seemed to promote some reactions. The composition of distilled water, fish glue, urea, and hydrogen peroxide solution was 30.0 g, 20.0 g, 3.0 g, and 4.0 g, exceeding the adhesive strength of 30% aqueous solution using only fish glue (152.4 kgf/cm2).

Table 2. 
Physical properties of Korean shark skin glue adhesive
Distilled water
Shark skin extract
pH Adhesion strength
Drying time
35 15 3 2 203.6 6.27 156.7 36
30 20 3 2 930 6.32 228.6 28
25 25 3 2 8,220 6.35 256.9 23
25 25 5 2 14,000 6.39 269.7 23
25 25 7 2 12,000 6.60 237.7 31
25 25 10 2 1,800 6.61 216.4 23
25 25 5 0 3,100 6.59 312.7 25
25 25 5 4 3,100 6.48 250.7 25
25 25 5 6 5,200 6.27 275.1 23

Table 3. 
Physical properties of Japanese fish glue adhesive
Distilled water
Shark skin extract
pH Adhesion strength
Drying time
35 15 3 2 980 6.10 174.5 15
30 20 3 2 15,291 6.07 224.9 17
25 25 3 2 - 6.21 166.3 21
30 20 5 2 65,000 6.62 212.6 27
30 20 7 2 23,700 6.53 166.6 52
30 20 10 2 12,708 6.32 178.9 31
30 20 3 0 14,054 6.26 370.6 57
30 20 3 4 13,797 6.25 397.5 61
30 20 3 6 13,672 6.16 363.8 65

The adhesion strength of peptide bonds that were induced using hydrogen peroxide measured at 397.5 kgf/cm2, showing a result of excessive adhesion strength (370.6 kgf/cm2) without the usage of hydrogen peroxide. Hydrogen peroxide breaks the long polymer chain of the fish glue adhesive, making a strong bond with urea. Thus, the adhesive strength increased without a change in the viscosity.

Shark skin adhesive was confirmed to have adhesion strength more than three times higher than that of fish skin adhesive when the strength was measured by adjusting each adhesive to the same viscosity of 3,100 cPs. When these factors were considered, the high adhesive performance of imported fish skin glues results in high viscosity (Fig. 4). Moreover, in spite of high viscosity, the drying speed was >61 min, and the pH decreased to 6.25 owing to the influence of the addition of oxygenated water (shark skin adhesive pH 6.59).

Fig. 4. 
Adhesive tensile strength results in final composition.

As for the changed value of color degradation after ultraviolet irradiation, fish skin adhesive marked +6.50 in b* value and △E*ab value, representing the overall value of color variation at +6.82, while the b*value of shark skin adhesive was +3.32, and the △E*ab value, representing the overall value of color variation, was very small as +4.13. Thus, it was considered a constant adhesive without changes in property due to its own aging (Table 4).

Table 4. 
Color change after UV irradiation
Yellowness (b*) Variation (△E*ab)
Shark skin adhesive 3.32 4.13
Fish skin adhesive 6.50 6.82

4. Conclusions

This study was conducted with the aims of improving the preservability, usability, and adhesive property of natural adhesives used for the preservation of cultural assets and the manufacturing of woodcrafts as well as to overcome the harmfulness of resin adhesives.

We manufactured an adhesive that is convenient to use, has long-lasting antimicrobial activity, and is harmless to humans by extracting gelatin from Korean shark skin (maximum yield 37.5%). Our study provides the following conclusions:

  • 1) To solve the problem of the decomposition of gelatin and the usability and persistency caused by gelation caused by the moisture of fish glue solution, which is a natural adhesive, we manufactured an adhesive by inducing urea peptide bonds that shows excellent workability without decomposition by preventing gelation that continues after solubilization. Thus, the foundation for the development of preservability-guaranteed adhesive has been established.
  • 2) The shark skin adhesive manufactured in this study shows excellent adhesion strength that is three times that of existing natural adhesives. We were able to manufacture a natural adhesive where the adhesion strength, which is the most important element in adhesives, is improved.
  • 3) We manufactured shark skin adhesive by adjusting the pH to be slightly acidic, close to the neutral value of 6.59, by enabling it to be used for the adhesion of paper after dilution. We manufactured it to be used as a general-purpose adhesive with its durability against ultraviolet rays.

Shark skin adhesive made as per the findings of this study is expected to provide an alternate to existing natural fish glue adhesives, if stability and reliability of preservability and property are secured via long-term monitoring.


This paper was researched by the intramural research support project 2020 of Hanseo University.

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