A cold-set wood adhesive based on soy protein

doi:10.1016/j.ijadhadh.2020.102801

International Journal of Adhesion and Adhesives

Volume 106, April 2021, 102801

https://doi.org/10.1016/j.ijadhadh.2020.102801 Get rights and content

Abstract

In this study, combinations of bisphenol A diglycidyl ether (BPADGE) and polyethylenimine (PEI) were investigated as effective crosslinking agents for soy protein isolate (SPI) at room temperature. Three-ply plywood panels were prepared with the SPI-BPADGE-PEI adhesives via a cold press process and were evaluated for their water resistance and shear strengths in accordance with the requirements of the American National Standard for Hardwood and Decorative Plywood for exterior applications. The effects of BPADGE/(SPI + PEI) weight ratio, SPI/PEI weight ratio, solids content of the adhesive, the rate of adhesive usage, and the press time at room temperature on the water resistance and the shear strengths of plywood panels were investigated. The pot-life of the adhesive was also measured. Plywood panels bonded with the SPI-BPADGE-PEI adhesives met the industrial requirements for exterior applications.

Introduction

A hot press process is commonly used for the production of particleboard and plywood, but is not used for the production of thick wood-based composite panels such as cross-laminated timber (CLT), glulam, or thick plywood because heat cannot be effectively transferred through wood. It takes a long hot-press time to produce thick wood-based composite panels if a thermoset adhesive is used, which is not a desirable way of using expensive hot press equipment. Therefore, thick wood-based composite panels are typically produced with an adhesive that can be cured at room temperature or with a thermoset adhesive that is cured with radio frequency heating [1,2]. Such an adhesive is commonly called a cold-set adhesive. The commonly used cold-set adhesives include resorcinol-formaldehyde (RF), phenol-resorcinol-formaldehyde (PRF), melamine-urea-formaldehyde (MUF), and polyurethanes (PUR) [1,3]. RF, PRF, and MUF are formaldehyde-based adhesives and typically contain residual carcinogenic formaldehyde that is emitted during the production of panels [[4], [5], [6]]. PURs are isocyanate-based adhesives and isocyanates are very toxic. The application of PURs typically requires a good ventilation and good personal protective equipment. Also, the combustion of PURs generates toxic fumes such as hydrogen cyanide [7,8]. It is desirable to develop an isocyanate-free and formaldehyde-free cold-set wood adhesive. In our recent study, we discovered the first effective isocyanate-free and formaldehyde-free cold-set wood adhesive that consists of bisphenol A diglycidyl ether (BPADGE), polyethylenimine (PEI), and a polyamidoamine (PAA) [9]. The plywood panels made with this PAA-BPADGE-PEI adhesive met industrial requirements for exterior applications [9]. However, BPADGE, PEI, and PAA are all expensive, petrochemical-based, and synthetic polymers. In an effort of reducing the cost of this adhesive and reducing the usage of petrochemicals, soy protein isolate (SPI) was investigated as the replacement of PAA in this study.

Soy protein is renewable, abundant, and readily available. Soy protein contains a great amount of polar functional groups such as amino, carboxylic acid, and hydroxyl groups that can form hydrogen bonding with wood components and among themselves. Soy protein itself is a great wood adhesive if the resulting wood-based composite panels are kept dry. In other words, wood-based composite panels made with only soy flour are not water-resistant. For improvements of the water resistance of the resulting wood-based panels, various curing agents have been investigated for crosslinking those polar functional groups in soy protein. The effective curing agents reported so far include metal oxides such as magnesium oxide; polyamines such as PEI, polyamidoamine-epichlorohydrin (PAE), and ammonia-epichlorohydrin adducts; and polyepoxides such as poly (glycidyl methacrylate-co-styrene), triglycidylamine, and glycerol polyglycidyl ether [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. However, heat is required for curing all these adhesives that are based on soy protein with these curing agents.

In this study, combinations of BPADGE and PEI were found to be effective crosslinking agents for SPI at room temperature. The resulting plywood panels made with the SPI-BPADGE-PEI adhesive were able to meet the industrial requirements for exterior applications.

Section snippets

Materials

SPI (protein content on dry basis, 91.4 wt%; moisture content, 5.7 wt%; and pH, 7.6) was obtained from MP-Biomedical LLC (Solon, Ohio, USA). BPADGE was obtained from TCI America (Portland, Oregon, USA). PEI (50 wt% aqueous solution, Mw = 750,000 g/mol) was purchased from Sigma-Aldrich (Milwaukee, WI, USA). The molar ratio of primary/secondary/tertiary amines of the PEI was 1.00/1.07/0.77. Yellow poplar (Liriodendron tulipifera L) veneers with the thickness of 3.5 mm were obtained from Columbia

Results

The mixing order of the ingredients (SPI, BPADGE, and PEI) was found to be critical for the water resistance of the resulting plywood panels. Resulting plywood panels met the water-resistant requirements for exterior applications only when the ingredients were mixed in the following order: SPI and BPADGE were mixed first, followed by the addition of PEI. All results presented in this paper resulted from the adhesives with this mixing order. Our extensive experiments revealed that mixing all the

Discussion

Results from this study revealed that the replacement of petrochemical-based PAA with SPI produced water-resistant plywood panels for exterior applications. The replacement also lowered the overall cost of the adhesive. The prices of the adhesive ingredients generally have the following order: PEI > PAA > BPADGE > SPI. The least expensive but still effective formulation of this SPI-BPADGE-PEI adhesive containing 75.0 wt% SPI, 20.0 wt% BPADGE, and 5.0 wt% PEI is much less expensive than the

Conclusions

The SPI-BPADGE-PEI adhesive was an effective cold-set wood adhesive for the production of exterior plywood. The SPI/PEI weight ratio of 15 or less and the BPADGE/(SPI + PEI) weight ratio in the range of 0.25–1.15 were required. The solids content of the adhesive had to be at least 15 wt% and the adhesive had to be consumed within 120 min after all the ingredients were mixed. The adhesive usage had to be at least 4 mg/cm² on the dry weight basis and the cold-pressing time of plywood panels had

Acknowledgements

Authors are grateful to Dr. Willie E. Rochefort for his assistance on the measurement of the viscosity of adhesives. This project is conducted through the TallWood Design Institute, and funded by the U.S. Department of Agriculture’s Agricultural Research Service (USDA ARS Agreement nos. 58-0202-5 and 58-0204-6), and the royalty fee return from patents of K. Li's group.

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View full text

A cold-set wood adhesive based on soy protein

Abstract

Introduction

Section snippets

Materials

Results

Discussion

Conclusions

Acknowledgements

Fund Appl Toxicol

Int J Adhesion Adhes

Int J Adhesion Adhes

CLT Handbook: cross-laminated timber

Cross laminated timber (CLT): overview and development

Eur J Wood Wood Prod

Shear strength and durability testing of adhesive bonds in cross-laminated timber

J Adhes

Formaldehyde release from wood products: an overview

Release of formaldehyde by wood products

Aldehyde emission from particleboard and medium density fiberboard products

For Prod J

The role of isocyanates in fire toxicity