In-Process Protection of Wood Composites: An Industry Perspective

Chapter 27

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In-Process Protection of Wood Composites: An Industry Perspective 1

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Glenn M. Larkin , Paul Merrick , Marek J. Gnatowski, and Peter E. Laks 1

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School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI 49931 Level Research Center, Weyerhaeuser Company, P.O. Box 8449, Boise, ID 83707 Polymer Engineering Company, Ltd., Burnaby, British Columbia V5B 3A6, Canada 2

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Wood-based composites are commonly used as construction materials. In many of these applications, there is a potential for fungal and insect attack (e.g. millwork, sheathing, exterior siding, and decking). Traditional pressure, spray, and dip treatments suitable for solid lumber typically cannot be employed to treat wood-based composites. In-process application of preservative formulations, however, has proven to be a feasible option. Inorganic borate systems, particularly zinc borate, have an established commercial track record. Their success as an in-process preservative system can be attributed to meeting five basic attributes: they are relatively safe to use, have minimal environmental impact, have regulatory acceptance, are compatible with most wood composite manufacturing processes, and are economical. This chapter discusses the use of in-process preservative systems from a North American industrial perspective, and is aimed at formulation chemists who are developing biocides for inprocess treatment by the wood composite industry. This chapter is based on the oral presentation, "In-Process Protection of Wood Composites: A n Industrial Perspective" by authors Merrick and Gnatowski at the A C S 229 National Meeting In San Diego, C A , March 16, 2005. th

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© 2008 American Chemical Society Schultz et al.; Development of Commercial Wood Preservatives ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


459 Over the last thirty years there has been an increase in the use of woodbased composite building materials. They increasingly substitute for solid lumber in applications exposed to a risk of fungal and insect attack such as millwork, home framing products in termite prone areas, decking, and exterior siding (7). Wood-based composites used in these exposures commonly contain a wood preservative system. These uses of preservatives have sparked interest in new chemistries and technologies (5, 4). The Use Category System is a useful tool developed by the American Wood-Preserver's Association that correlates biodeterioration hazard to product application. Examples of the above applications include (2): U C 2 - sill plates; UC3 A - exterior siding, and coated millwork; Solid wood products used in U C 2 and U C 3 A exposures are frequently pressure treated with a waterborne preservative system, but most wood-based composites are not suited to this treatment approach due to permeability and dimensional instability issues. Gardner and Walinder (5) discussed the various strategies for manufacturing wood-based composites that are resistant to attack by fungi and insects. These strategies include in-process and post-process treatment, use of durable wood species and recycled preservative treated wood, and the use of chemically modified wood. In-process treatment is the current, commercialized approach discussed by Gardner and Walinder (5). It is commercially used to treat particle-based composites including oriented strand board (OSB), laminated strand lumber (LSL) and medium density fiberboard (MDF). Figure 1 shows a typical OSB manufacturing process and the points that are most practical for preservative addition. During in-process treatment, a preservative can be applied to either wet or, more commonly, dry furnish, or the combined wood, adhesive, and water repellent that constitute the composite matrix. Wet flakes are treated after stranding before they are dried to allow time for preservative to diffuse into and fix (chemically bond) with the wood. The dry furnish is treated in the blender via biocide addition to either the wood, and/or adhesive, or combined with the water repellent. The preservative is distributed throughout the furnish during the blending process. This chapter summarizes the selection criteria for biocides intended for wood composite use from the perspective of a wood composite manufacturer. Zinc borate is used to illustrate how these criteria can be met.

Key Attributes of an In-Process Biocide Additive The key attributes summarized as follows:

of an in-process preservative additive can be

Schultz et al.; Development of Commercial Wood Preservatives ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


Figure 1. A schematic of the OSB manufacturing process showing the most practical stage for in-process wood preservative treatment. Adding the preservative to the flakes either prior to drying or to the furnish in the blender makes the most sense (gray shaded area). A detailed description of the OSB manufacturing process may be found elsewhere (6).


461 Is Safe Low Environmental Impact Meets Regulatory Requirements Good Technical Performance Favorable Economics


These attributes were first summarized by Laks (7), and are expanded upon here based on the authors' (Merrick and Gnatowski) industrial experience.

Safety The toxicological profiles of the preservative system and of the treated composite are critical considerations for the manufacturer. Exposure of plant workers, supply chain personnel, secondary processors, and the end user must be considered. The manufacturer must: •

Plan for safe transportation, delivery, and storage of the preservative system;

•

Provide for safe blending, dilution, or mixing of preservative systems by plant personnel. For this reason, a ready-to-use preservative formulation is usually preferred;

•

Properly handle volatile off-gassing during the manufacturing process; and

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Confine dangerous process operations (when practical) to specific areas within the plant.

The treated wood composite will eventually be handled by construction workers and homeowners who may not use the same level of personal protective equipment. For this reason, the finished product must be safe for the secondary processors to machine and end users to handle.

Environmental Impact Preservative systems used in wood-composite production may negatively impact existing manufacturing facility permits. Their use may also affect the manner in which waste streams are handled. A biocide that does not have a negative impact on existing permits will be more favorably received. A l l plants generate waste streams that may be partially recycled. This may include re-use in the manufacturing process as furnish, by-product sale for other uses, and burning as boiler fuel. Some waste may be sold to other wood-



462 composite manufacturers. For example, short strands from L S L production may be sold to OSB manufacturers. Secondary processor and end user waste disposal also needs to be considered. For example, window manufacturing plants may sell their sawdust as animal bedding, and construction site waste may be used as mulch. Waste is most frequently burned as fuel in the plant boiler. Therefore, the preservative system must not interfere with pollution control devices installed on the furnace, nor with ash disposal. The preservative system's contribution to air emissions must be considered against the current permitted levels.

Regulatory Acceptance The safety and environmental impact of a wood preservative determine its ability to comply with legally required registrations and permits. There may also be commercially desirable listings for the preservative system and/or treated wood composite product.

Legal Wood preservatives which claim pesticide attributes must have regulatory approval. Registrations are necessary with the appropriate agencies at the Federal, State, and local levels. In the United States the EPA, and in Canada the P M R A , manage the registration of pesticides at the Federal level. A partial list of preservative treatments for wood composites manufactured in the U S A has been published by Laks (7). In addition, i f there is any emission related to biocide use, the manufacturing plant itself will have to obtain a permit at the state level for emission and waste disposal. For this reason, such emissions are not desirable and emission free biocides will receive preferential consideration.

Commercial Secondary manufacturers and distributors of the treated composite may request that the preservative system and/or treated wood composite product be listed with various standards, codes or trade association bodies. This generally involves a substantial cost and effort in the evaluation of a preservative treated wood-based composite's efficacy and suitability for a particular end use. Pre existing listings and support from the biocide supplier are required. Examples of trade association, standards bodies, and code agencies that recognize preservative actives and treated wood-based composite are given below:



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American Wood Preservers' Association (AWPA) - A respected association that writes treatment and performance standards for wood preservatives and treated wood products. A W P A Standards are referenced by the model building code (2).

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International Code Commission Evaluation Service (ICC-ES) - ICC-ES establishes standards for the model Building Codes. ICC Wood preservative listing requirements for the ICC are outlined in Acceptance Criteria (AC) 326 (i).

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Window and Door Manufacturer's Association ( W D M A ) - A well established trade association that writes performance specifications for manufacturers of doors, skylights, and windows in North America.

Technical Performance The technical performance of a preservative for in-process incorporation into wood and wood-based composites ultimately needs to be evaluated by each manufacturer for their process, product, and intended end uses. There are, however, common elements for such assessments. Although each case is unique, a typical checklist might include the following:

Chemical Performance •

Is the preservative (formulation) stable while stored at the plant?

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Is the preservative compatible with other components of the composite, especially the adhesive? For example, amine and metal ions have the potential to catalyze the reaction of isocyanate adhesives which results in poor bonding between wood particles (9). Some borates react with phenols in phenol formaldehyde resins (PF) to preventi complete adhesive curing (70). Biocides may decompose in the strongly alkaline environment of the PF glue line rendering them ineffective (77).

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Does the preservative "bond" to the wood particle? Is there chemical fixation? Adsorption? Biocide fixation in the glue line may interfere with biological activity.

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Is the preservative corrosive to plant equipment?

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W i l l the preservative produce unsafe volatile by-products?

Manufacturing Performance •

Can the preservative be used with existing plant equipment? Is it easy to use? Schultz et al.; Development of Commercial Wood Preservatives ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


464 •

Will it arrive ready-to-use or require on-site tank mixing and/or dilution?

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Is it an emulsion, powder, or solution? What safety precautions are needed?

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Will the preservative tolerate the manufacturing process? How stable is it with respect to press temperatures, which may be in excess of 200°C?

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Is the preservative able to provide protection to multiple wood species?

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Will one formulation of the preservative allow use with two or more adhesives?

End Use Performance •

Does the preservative afford adequate protection for the intended end use?

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Is there an analytical method that allows for quick preservative assays in the plant setting? Are there clearly defined Q A / Q C criteria for the preservative and for the finished composite material?

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Do the preservative formulation and the treated wood composite have an acceptable environmental profile?

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W i l l the preservative supplier provide ongoing technical support?

These questions, while not exhaustive, provide a framework for the types of questions that need be asked when assessing the suitability of a preservative formulation for use within a wood composite manufacturing process. Wood composite manufacturers expect that preservative formulation suppliers actively assist in answering these questions.

Economics The choice of which preservative system to incorporate into a wood composite is determined by whether the cost plus delivered performance is recognized by the customer as having value. Does the cost of the treated wood composite compare favorably with alternative building materials, such as treated lumber or steel? In almost all cases, the treated wood composite will be more expensive to manufacture than the untreated version. Will the treatment enable the product to more effectively perform over the expected service life of the application? Will the addition of a preservative treatment enable the product to enter new markets, thus expanding the overall volume of product produced and perhaps lowering the manufacturing cost structure? These questions must be answered before any serious developmental work on new biocides or in-process treatment processes occurs.


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Zinc Borate as an In-Process Composite Wood Preservative


Two forms of zinc borate are commonly used commercially (Table 1). One has been approved by the E P A for use as a wood preservative, and finds use as a biocide in the treatment of wood composites. Both are used as fire retardants in plastics. The number designations refer to chemical structure.

Table 1. Examples Commercially Available Zinc Borate Compounds Commercial Reference Chemical Formula ZnO(%) B0(%) H0(%) Water Solubility (%) 13.6 0.28 ZB2335 48.2 2ΖηΟ·3Β0·3.5Η0 38.2 0.04 19.2 ZB223 43.5 34.2 2ΖηΟ·2Β0·3Η0 2

3

2

3

2 3

2

0

2 3

2

a

AWPA Standard P18-04, Nonpressure Preservatives (Allows In-Proccess Treatment of Wood Composites)

b

223 & 2335 forms used as a fire retardant & smoke suppressant in plastics industry

Zinc Borate Safety, Regulatory Issues, and Environmental Impact Zinc borate is currently the most commonly used in-process preservative for the protection of wood-based composites and has been in use for over a decade. It (ZB) is commercially available in several grades, and from several manufacturers in the United States. Type ZB2335 is registered with the United States Environmental Protection Agency (EPA) and listed in A W P A Standard Ρ18-04, Non Pressure Preservatives (2) for use in wood composites. Commercial products differ including particle size (Figure 2) which may affect handling during processing. For example, fine particle size may cause excessive dust which would not be desirable. Pure zinc borate is considered safe and environmentally friendly. It generally has a very low worker exposure hazard and relatively low environmental impact. However, all commercial preservative formulations, including zinc borate, may contain undesirable contaminants. A n example of contaminants found in commercial samples of zinc borate is shown in Table 2.

Zinc Borate Technical Performance The technical performance properties of zinc borate are well established and its use as a preservative for in-process treatment of wood composites is supported in the literature (9, 12). Zinc borate arrives at the manufacturing plant as an easy to store and use powder, most commonly in palletized super sacks. At its current use rate, Z B is



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Figure 2. Electron photomicrographs of 4 types of zinc borate (5000x). Note the differences in particle geometry and size. Photos from Marek Gnatowski.

Table 2. Select Elemental Analysis in four Commercial Samples of ZB2335

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Element Antimony Arsenic Barium Cadmium Calcium Chromium Copper Lead Sodium Strontium a

A

— — — 0.5 1060 0.6 12 3 95 0.5

Zinc Borate (ppm) Β C

— — —

0.7 1230 2.3 6 4

— —

D

—

1660 16 2 5 1330

6 234 9 1030

8 45 689 19

7 434 143 34

—

—

Detection Limit 0.2 0.2 0.2 0.04 10 0.2 0.2 0.2 10 0.2

Data from Marek Gnatowski


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compatible with other wood composite components including water repellants and adhesives such as polymeric diphenylmethyl diisocyanate (pMDI) and phenol formaldehyde (PF). Discrete zinc borate particles (in OSB) physically adhere to the wood-adhesive interface during panel manufacture. It is considered a relatively "leach resistant" wood preservative with low water solubility. When exposed to moisture, a slow hydrolysis reaction follows (Equation 1), yielding zinc hydroxide and boric acid (13). Boric acid can then diffuse away from the glue line and into the wood fiber where it provides protection. 2 Z n 0 3 B 0 3 . 5 H 0 + 7.5H 0 - » 2Zn(OH) | •+ 6 H B 0 2

3

2

2

2

3

3

(1)

This limited leaching in contact with environmental moisture is an important attribute of biocides intended for wood composites used in the construction industry. It makes it well suited for many applications which are out of ground contact and protected from direct weathering ( A W P A U C 1 U C 3 A ) (2). Examples include trusses, sill plates, door and window millwork, exterior trim, and siding. Zinc borate is not considered corrosive to standard manufacturing plant equipment, or to metal fasteners used in construction. From a dry-process composite manufacturing perspective, zinc borate is relatively easy to use. Typically, powdered Z B is added to the wood furnish, adhesive, and other additives in a blender on a percent weight basis. Alternatively, it may be suspended in a slurry with wax emulsions and spray applied (14). Zinc borate does not chemically degrade during wood composite manufacturing processes. Manufacturing quality assurance/quality control (QA/QC) is relatively fast, with the potential to assay treated materials in a matter of minutes using X-Ray fluorescence (for zinc, with indirect calculation of boron content).

Economics Zinc borate currently is low cost. It is not a formulated product, so there are no additional costs for co-solvents and diluents. It provides cost effective protection to wood composites against a wide variety of wood decay fungi and insects, including termites. The overall use cost and protection efficiency should be analyzed when considering a new biocide for in-process treatment of a wood composite.

Conclusion In-process preservative treatment of wood-based composites has proven to be commercially viable for more than a decade. In light of the current



468 technologies available, it is an economical approach to the protection of particlebased wood composites including O S B , M D F , L S L and particleboard. Important criteria in evaluating a candidate in-process preservative system are safety, environmental impact, regulatory acceptance, technical performance, and economics. Each manufacturer will place a different value and emphasis on these criteria. With continued research formulation chemists may develop new in-process treatments for wood-based composites which may become successfully commercialized. Zinc borate serves as an example. Zinc borate is currently the most commonly used in-process preservative system for OSB and L S L . Research and commercial use have established that Z B has good efficacy as a wood preservative against wood decay fungi and termites when used in the appropriate applications. Successful commercial use of zinc borate demonstrates it has an acceptable, and minimal impact on production processes and satisfies all of the established criteria for use as an inprocess preservative for wood-based composites outlined in this chapter.

References 1.

Laks, P.E. Protection of Wood-based Composites. Proceedings: American Wood Preservers Association 2004, 100, 78-82. 2. American Wood Preserver's Association Standards 2005: Selma, A L 3. Laks, P.E. In: 33 International Particleboard/Composite Materials Symposium Proceedings; Wolcott, M . P . ; Tichy, R.; Bender, D.F.; Eds; Washington State University: Pullman, W A , 1999, 151-158. 4. Smith, W.R.; Wu, Q. Forest Products J. 2005, 45[2], 8-17. 5. Gardner, D.J.; Wǻlinder, M.E.P. In: Wood Deterioration and Preservation Advances in a Changing World Goodell, B . ; Nicholas, D.D.; Schultz, T., Eds; A C S Symposium Series 845; American Chemical Society: Washington, D C , 2003; 399-419. 6. Yougquist, John Α.; In Wood Handbook: Wood as an Engineering Material; FPR-GTR-113. Madison WI: U S D A Forest Service, Forest Products Laboratory, 1999, 10-13 - 10-14. 7. Laks, P.E.; Palardy, R.D.; Protection of Wood-Based Composites; Preston, A.F.; Ed. Forest Products Society: Madison, W I , 1993, 12-17. 8. ICC-ES -AC326 - Acceptance Criteria for Proprietary Wood Preservative Systems - Common Requirements for Treatment Process, Test Methods and Performance; ICC Evaluation Service, Inc. 2006, Whittier, C A 9. Tiele,L.; Becker, R. In: Adv. In Urethane Science and Technology Frisch,K.C.; Klempner, D., Eds.; Technical Publishing Company: Lancaster, PA, 1993, 59-85 10. Laks, P.E.; Haataja, B . A . ; Palardy, R.D.; Bianchini, R.J. Forest Products J. 1998, 38(11), 23-24 rd



469 11. Collins, P.A.; Kennedy, M . J . ; Vella, R.D., Stability of bifenthrin in a commercial plywood glue, International Research Group on Wood Preservation 2003 Doc. IRG/WP/03-30311 12. Laks, P.E. and M . J . Manning, Proceedings: Second International Conference on Wood Protection with Diffusible Preservatives and Pesticides Forest Products Society: 1997, 62-88. 13. Schubert, D . M . ; Alam, F.; Visi, M . Z . ; Knobler, C.B. Chem. Mater. 2003, 15, 866-871 14. Fookes, D.; Gnatowski, M.J.; Pike, R.L.; Templeton, D.A.; 1999. U S Patent 5,972,266.


In-Process Protection of Wood Composites: An Industry Perspective

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