Gilsonite resin is a commercially proven thermoplastic binder for fiberboard products made by the wet/dry process. High purity, naturally occurring Gilsonite resin is a hard, non tacky binder that produces strong, water resistant fiberboard products, often at faster process rates and with special cost reduction features. Gilsonite has outstanding dimensional stability, stiffness, and strength when compared with other binders, such as phenol/formaldehyde, urea/formaldehyde, bitumen, wood rosin derivatives, and linseed oil. Unique binder systems with special properties are possible by combining Gilsonite with bitumen, tall oil products, or thermosetting resins. Either alone or in combination with other binders, Gilsonite offers the fiberboard manufacturer a means to:
  • Avoid formaldehyde emissions
  • Produce thinner, stronger MDF products
  • More efficient use of existing plant facilities.
Research suggests that Gilsonite binder can also be used effectively in wet/wet and dry/dry processing. The following report provides detail on Gilsonite’s commercial use in wet/dry processing and discusses other uses of Gilsonite as suggested by our current research studies.  

Gilsonite  Resin as a Binder in Wood Products


The principal use of Gilsonite resin as a binder for wood products is in the production of medium density fiberboard siding. A major U.S. manufacturer used Gilsonite to replace linseed oil and Vinsol/HMA wood rosin derivative in their wet/dry process. The manufacturer first used linseed oil, but because of excessive fires during drying and hot pressing operations, the plant converted to Vinsol/HMA resin. Subsequently, Gilsonite replaced Vinsol due to cost and availability considerations. Gilsonite also provided a 10% increase in modulus of rupture in the final product. An average modulus of rupture of 2500 psi (175 kg/cm²) was achieved using 13% Gilsonite binder. In the opinion of knowledgeable wood products manufacturers, the Gilsonite-based siding had premium quality with respect to strength and stability.

In this wet-dry process, aspen wood chips were converted to fibers in single and double-disk mills. The defibration was done on pre-softened wood chips. After defibrating, the fibers were slurried to about a 1% solids content. Alum was used at a rate of about 0.9% of the fiber formulation weight. Slack wax containing 9-10% oil was emulsified and added to the fiber slurry at 1.3% on a dry-weight basis.

A Bepex Pulvacron PC38 pulverizer was used to prepare the Gilsonite for the process. Gilsonite was fed through a weightometer and pulverized to -100 mesh material. If the product coarsened to 5% plus 100, the mill settings were corrected.

After pulverizing, the Gilsonite was introduced to an agitated slurry tank. A 10% Gilsonite slurry was prepared using Texo LP 583 dispersant to wet the Gilsonite.

The compounded furnish was pumped through a fan pump to a double-cylinder forming machine. This machine had counter-rotating, screen-covered cylinders on which the mat was formed. Mat from each cylinder was overlayed to produce the final mat thickness required.

Pinch rolls with vacuum dewatering chambers were used to bring the mat to thickness. Thickness gages were used to control mat thickness prior to drying. Mat was dried for 3½ hours in a 350 foot-long oven. The temperature was controlled in three zones in the oven – first stage, 243°C; second stage, 210°C; final stage 141°C. The board left the dryer at 4% to 5% moisture.

After cooling, the board was roll coated with a pre-press sealer on both top and bottom. A spray system applied additional sealer to the top of the board. Pre-press sealer was applied at a rate of 10 kg per 92.9 cm². The pre-press sealer consisted of a Rohm and Haas thermoset acrylic resin, talc, and iron oxide for color.

Prior to hot pressing, the boards were dried for 1 to 1½ hours at 149°C. Moisture content after drying was 1 to 1½%. Some residual moisture was desirable in the board since it improved surface finish and reduced sticking.

Board pressing was done on hydraulic presses fitted with either chrome-plated, embossed plates or Vicalloy smooth press plates. The press plates were treated with Quaker Oats Iso-strip to reduce sticking. Platen temperature was 238°C in either steam or oil heated presses. The press time was 5½ minutes from cycle start to decompression. Eight minutes were required from press load to press load. The press cycle for ½ inch MDS board was 8 minutes long from start to decompression.

After pressing, the board was cooled for about 10 minutes prior to being humidified to 2 to 3% moisture content by means of water sprays.

Other Production Experience

At another wet-dry process medium density siding plant, linseed oil was used as the binder. The process was slightly different from the commercial use described above in that the wood fiber was pressure-refined Southern Yellow pine. The forming machine was a more conventional horizontal screen, Foudrinier type, papermaking machine.

On a dry wood basis, 4½% linseed oil was used. Two and one half percent wax was incorporated into the formulation to reduce the water absorption and linear expansion of the linseed oil-based board. Approximately 1½% ferric chloride was used for pH control and to act as a drier for the linseed oil.

The first plant trial of Gilsonite resin in this facility was run using 10% to 11% Gilsonite and 1½% wax in the formula. The wax level was lower than what had been used previously with linseed oil because the hydrophobic nature of Gilsonite contributed to water retardation. Gilsonite was not pre-wetted in this plant trial. It was found that this resulted in poor binder distribution. The poor distribution resulted in lower than expected board strength and a higher than normal strength loss on humidification.

In a second plant trial, the resin was pre-wetted using surfactant to form a well dispersed slurry prior to adding to the pulp. Board produced in this plant trial showed excellent stability and did not change during humidification at 80% relative humidity and a temperature of 82°C. An average modulus of rupture of 173 kg/cm² after humidification was recorded at press times of both 4½ and 5 minutes using a press platen temperature of 238°C.

Linear expansion measurements were made on 11% Gilsonite resin board as compared to 4½% linseed oil board. The following results were obtained:

% Binder

% Wax

Linear Expansion %

Water Absorption %






Linseed Oil





  Based on population variance statistics, this linear expansion improvement for Gilsonite was significant at the 99% significance level. It can be concluded from industrial experience using Gilsonite as the binder in wet-dry processing that dimensionally-stable, high strength, exterior siding can be produced with cost/performance advantages over other binder systems. WET/WET PROCESSING MDS Board In the laboratory, Gilsonite resin has been evaluated in the wet-wet process for the production of hardboard siding. This test work was done in the Wood Technology Laboratory of a major university. The experimental work involved fiber preparation, pulping, mix additives, mat forming, mat dewatering, hot pressing, and oven curing. White fir wood chips were steamed for 5 minutes at 2.1 kg/cm² pressure to soften the lignin that binds of the cellulose. The steamed chips were refined in a double-disc attrition mill. A 1.5% solids slurry was prepared from the fiber and 15% pulverized Gilsonite that was prewetted with surfactant. 1.5% wax sizing and 1% alum were added to the formulation. A deckle box was used to form the furnish into 5.9 cm by 5.9 cm sized wet mat. Dewatering was accomplished in a press to remove excess water. Hot pressing at 450°F for 30 minutes produced a dry pressed board of 12.7 mm thickness. Oven curing at 350°C completed the processing. Board produced by this wet/wet procedure had the following characteristics:


Internal Bond

Modulus of Rupture

Thickness Swell















  These properties demonstrate the potential for using Gilsonite resin as a binder in high performance fiberboard products made by wet/wet processing. Investigations are presently underway to determine the interaction of phenolic resins and Gilsonite in wet-wet processing for exterior siding products. This concept may significantly reduce press time. The concept is that thermoset resin will polymerize at a relatively low temperature and bond the board structure before Gilsonite is fully activated, thus shortening total press time. Subsequent drying at temperatures high enough to melt Gilsonite should provide the final dimensional stability required for exterior applications.

Sheathing Board

Insulation-type wood fiber products have been produced on a plant trial basis using the wet-wet process. The sheathing contained 6.0% asphalt (as emulsion) and 3% Gilsonite. This fiberboard product had acceptable properties which met the “racking” strength required for intermediate sheathing. Similar tests were conducted in the laboratory using Gilsonite resin as a binder for Southern yellow pine fiber. Conventional wet-wet laboratory procedures were used to produce the insulation board. The test results are shown in the table below.


Control (No Binder)

Gilsonite with Surfactant

Asphalt Emulsion

Binder Content (%)




Density (g/cc)




MOR (kg/cm2)




Calculated MOR at 0.35 g/cc density




2 Hour Water Soak

Thickness Swell (%)




% H20 Absorption




When the data are analyzed on an equal density basis using a least squares statistical method, Gilsonite binder is shown to develop higher strength than board produced with no binder or with a hard asphalt binder. The data suggest that Gilsonite can be used to develop a binder system with utility in both wet-dry and wet-wet fiberboard siding and sheathing applications.

Dry/Dry Processing

Experimental work is in progress on the use of Gilsonite binder in the dry process manufacture of fiberboard and flake board for exterior applications. Press temperatures are known to be adequate for Gilsonite activation. The test work concentrates on materials handling techniques for Gilsonite under dry process conditions and the quantification of pressed board properties.

Modifier for Phenolic Binder Systems

The natural hydrophobic nature of Gilsonite may be a means of improving the dimensional stability of fiberboard made with phenolic binder systems. Gilsonite will be evaluated as an extender and performance improvement aid for phenolic and other thermosetting binders. Both reduced cost and improved exterior performance may be possible.