Preparation and Testing of Plant Seed Meal-based Wood Adhesives

Summary

To facilitate the effort in seeking more economic and environment-friendly formulations of natural product-based wood adhesives, this work demonstrates the preparation and testing of plant seed-based wood adhesives. This protocol allows one to assess plant seed-based agricultural products as suitable candidates for the substitution of synthetic-based wood adhesives.

Abstract

Recently, the interest in plant seed meal-based products as wood adhesives has steadily increased, as these plant raw materials are considered renewable and environment-friendly. These natural products may serve as alternatives to petroleum-based adhesives to ease environmental and sustainability concerns. This work demonstrates the preparation and testing of the plant seed-based wood adhesives using cottonseed and soy meal as raw materials. In addition to untreated meals, water washed meals and protein isolates are prepared and tested. Adhesive slurries are prepared by mixing a freeze-dried meal product with deionized water (3:25 w/w) for 2 hr. Each adhesive preparation is applied to one end of 2 wood veneer strips using a brush. The tacky adhesive coated areas of the wood veneer strips are lapped and glued by hot-pressing. Adhesive strength is reported as the shear strength of the bonded wood specimen at break. Water resistance of the adhesives is measured by the change in shear strength of the bonded wood specimens at break after water soaking. This protocol allows one to assess plant seed-based agricultural products as suitable candidates for substitution of synthetic-based wood adhesives. Adjustments to the adhesive formulation with or without additives and bonding conditions could optimize their adhesive properties for various practical applications.

Introduction

Adhesive bonding of wood plays an increasing role in the forest product industry and is a key factor for efficiently utilizing timber resources¹. Interest in the use of natural product-based adhesives for wood increased steadily from the 1930s to reach a peak around 1960². After this period, the price of petroleum-based adhesives became so low that they displaced protein adhesives from several traditional markets. In the past two decades, this trend has reversed with renewed interest in the use of materials that are renewable, biodegradable, and more environmentally acceptable. These natural resources include, but are not limited to, soy protein^3-5, cottonseed protein⁶, rice bran⁷, wheat gluten⁸, distillers grain protein⁹, canola protein and oil^10-12, lignin from sorghum and sugar cane bagasse^13,14, and polysaccharides derived from shrimp shells¹⁵.

Whereas seed protein isolates have been widely evaluated as potential wood adhesives, the isolation procedure involves corrosive alkaline and acidic reagents and it makes isolate-based adhesives relatively expensive and less environment-friendly¹⁶. Thus, some defatted seed meals (flours) with or without treatment have also been tested for the adhesive purpose, even though the adhesive properties of these meals do not perform as well as protein isolates^17-19. We have sequentially fractionated cottonseed meal (CM) into different fractions, and examined their adhesive strength in bonding wood veneers^20,21. The water-insoluble solid fraction (hereinafter washed cottonseed meal-WCM) could be used as wood adhesives, comparable to cottonseed protein isolate (CSPI), and would be less costly to prepare than CSPI.

Adhesive strength and water resistance are two critical parameters in evaluating the performance of a potential adhesive material. Here, the adhesive strength is reported as the shear strength at break of the lap bond of each wood specimen. Water resistance of the adhesive is measured by the change in lap shear strength of the bonded wood specimen at break due to water soaking. Using defatted cottonseed and soy meals as raw materials, this protocol provides a simple and straightforward way to prepare and test plant seed-based products as wood adhesives. This protocol would be helpful in facilitating the effort in seeking more economic and environment-friendly formulations of natural product-based wood adhesives.

Protocol

1. Cottonseed and Soy Meal-based Products (Figure 1)

1. Obtain the raw materials, defatted cottonseed and soy meals, from commercially available sources.
2. Obtain the working meal by grinding the solid defatted meal in a cyclone sample mill to pass a 0.5 mm steel screen¹⁶.
3. Prepare water washed meals from the working meals after water extraction (25 g meal:200 ml water) to separate water soluble components in the meals²¹.
4. Prepare protein isolates from the working meals by alkali extraction and acid precipitation¹⁶.

2. Preparation of Wood Veneer Strips

1. Cut wood veneers (1.59 mm thick) obtained from a commercially available source into strips 25.4 mm wide by 88.9 mm long.
2. Pencil mark a line across the wood grain at 25.4 mm (1.0") length from one end of each strip. Label these strips appropriately with testing treatments or numbers. 5 - 10 wood pairs are prepared for each testing variable.

3. Preparation of Adhesive Slurries

1. Calculate the amount of water washed meal needed per the wood specimens for testing, by application rate (e.g., 4 mg dry content cm^-2) x total bonding area (e.g., 581 cm² of 90 wood strips with 2.54 x 2.54 cm bonding area each) plus about 30% extra for enoughness (i.e., 4 x 581 x 130% 3 g of water washed meal for the example).
2. Mix water washed meal with deionized water (3:25 w/w), and stir with a magnetic stir bar for 2 hr in a beaker sealed with Parafilm.

4. Preparation of Bonded Wood Specimens

1. Brush adhesive slurry onto one end of 2 wood veneer strips covering 25.4 mm (1.0") length. Air-dry for 10 - 15 min or until tacky.
2. Brush a second layer of adhesive slurry on top of the first layer and air-dry again. The amount of dry adhesive preparation applied is about 4.5 mg dry solid per cm² of bonding are of each wood strip.
3. Overlap the tacky adhesive coated area (25.4 x 25.4 mm or 1.0" x 1.0") of 2 wood veneer strips. Hot-press using a Benchtop Heated Press at 100 °C for 20 min at a pressure of 400 psi (2.8 MPa). Note the pressure is the force applied by the press divided by the overlapped area of the wood samples. These bonding parameters may be changed as needed for each testing variable.
4. Cool and condition the bonded wood specimens for 48 hr in a conditioning room or an incubator with humidity control (temperature of 22 - 23 °C and relative humidity of 50 - 60%; Figure 2).

5. Water Resistance Experiments

1. Immerse the bonded wood specimens, after initial conditioning, in tap water for 48 hr in a plastic tray at RT (22 - 23 °C). The wet specimens after soaking are tested immediately for the shear strength at break and reported as wet strength. Excess water on the veneer surface may be removed by gently patting with paper tissue prior to measurements.
2. Immerse another set of bonded wood specimens, after initial conditioning, in a water bath at 63 ºC for 4 hr, then dry at room conditions (temperature of 22 - 23 °C and relative humidity of 50 - 60%) O/N (18 - 20 hr). Repeat the immersion-drying cycle once with a 48 hr drying time. The dried specimens are then tested for the shear strength at break and reported as soaked adhesive strength.

6. Lap Shear Strength Measurements

1. Fit a bonded wood specimen into the 32 x 40 mm fishscale gridded wedge grips on a Materials Tester with a gripping pressure of 7 MPa, and set the crosshead speed at 1 mm min^-1.
2. Measure and record the shear strength at break for each bonded wood specimen. The results of multiple measurements are averaged for each adhesive formulation and test variable.

Results

Each adhesive formulation’s performance is determined by the shear strength of the bonded wood specimen at break and the values vary depending on the dimensions of the wood veneer used. For example, in Table 1, the dry and soaked adhesive strength values of the bonded specimens are lower when thinner and narrower maple strips are used (see Cottonseed-1), as opposed to the thicker and wider strips of Cottonseed-2 recommended in the protocol, using the same cottonseed-based adhesive formulation. Also obser...

Discussion

This paper presents a basic procedure to prepare and test plant seed-based products as wood adhesives. The adhesive slurries exampled in this protocol are simply the defatted seed meal product and water. Various adhesive formulations can be reached by addition of testing reagents (such as sodium dodecyl sulfate, sodium bisulfite or tung oil)^5,6,23 and/or changes in mixing conditions (such as pH, ratio of solid and water)^3,24,25. Adjustment of the adhesive formulation is also needed if the rheologica...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Defatted cottonseed meal	Kentwood Co-op		Kentwood, LA, USA
Defatted soy meal	Kentwood Co-op		Kentwood, LA, USA
Wood veneers	Certainly Wood, Inc.		East Aurora, NY, USA
Cyclone sample mill (model 3010-014)	UDY Corporation		Fort Collins, CO, USA
Benchtop heated press (model 3856)	Carver, Inc.		Wabash, IN, USA
Materials tester	Zwick GmbH & Co.		Ulm, Germany