Effectiveness of the International Phytosanitary Standard ISPM No. 15 on Reducing Wood Borer Infestation Rates in Wood Packaging Material Entering the United States

Abstract

Numerous bark- and wood-infesting insects have been introduced to new countries by international trade where some have caused severe environmental and economic damage. Wood packaging material (WPM), such as pallets, is one of the high risk pathways for the introduction of wood pests. International recognition of this risk resulted in adoption of International Standards for Phytosanitary Measures No. 15 (ISPM15) in 2002, which provides treatment standards for WPM used in international trade. ISPM15 was originally developed by members of the International Plant Protection Convention to “practically eliminate” the risk of international transport of most bark and wood pests via WPM. The United States (US) implemented ISPM15 in three phases during 2005–2006. We compared pest interception rates of WPM inspected at US ports before and after US implementation of ISPM15 using the US Department of Agriculture AQIM (Agriculture Quarantine Inspection Monitoring) database. Analyses of records from 2003–2009 indicated that WPM infestation rates declined 36–52% following ISPM15 implementation, with results varying in statistical significance depending on the selected starting parameters. Power analyses of the AQIM data indicated there was at least a 95% chance of detecting a statistically significant reduction in infestation rates if they dropped by 90% post-ISPM15, but the probability fell as the impact of ISPM15 lessened. We discuss several factors that could have reduced the apparent impact of ISPM15 on lowering WPM infestation levels, and suggest ways that ISPM15 could be improved. The paucity of international interception data impeded our ability to conduct more thorough analyses of the impact of ISPM15, and demonstrates the need for well-planned sampling programs before and after implementation of major phytosanitary policies so that their effectiveness can be assessed. We also present summary data for bark- and wood-boring insects intercepted on WPM at US ports during 1984–2008.

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Citation: Haack RA, Britton KO, Brockerhoff EG, Cavey JF, Garrett LJ, Kimberley M, et al. (2014) Effectiveness of the International Phytosanitary Standard ISPM No. 15 on Reducing Wood Borer Infestation Rates in Wood Packaging Material Entering the United States. PLoS ONE 9(5): e96611. doi:10.1371/journal.pone.0096611

Editor: David L. Roberts, University of Kent, United Kingdom

Received: November 23, 2013; Accepted: April 9, 2014; Published: May 14, 2014

This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

Funding: The Working Group “Effects of trade policy on management of non-native forest pests and pathogens” was supported by a grant from The Nature Conservancy to the National Center for Ecological Analysis and Synthesis, which is a Center funded by the National Science Foundation (Grant #EF-0553768), the University of California Santa Barbara, and the State of California. Partial funding was provided by the New Zealand Foundation for Research and Technology through contracts C02X0501 (Better Border Biosecurity) and C04X0302 (Forest Biosecurity and Protection) to the author EGB. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors declare that no competing interests exist; however, the three authors from New Zealand (EGB, MK, JT) state that they are employees of Crown Research Institutes (CRI) in New Zealand (NZ ), which are wholly owned by the NZ Government and are constituted as limited liability companies. CRI employment for EGB, MK, and JT does not alter the authors’ adherence to PLOS ONE policies on sharing data and materials.

Introduction

International trade has been responsible for the inadvertent introduction of many exotic (nonnative) insect pests and plant pathogens, of which several have become highly invasive and caused serious environmental and economic impacts to multiple habitats worldwide [1]–[7]. In recent years, introductions of several particularly damaging wood-infesting insects and pathogens in the United States (US) have focused public and regulatory attention on the pathways that transport these pests [8]–[12].

Wood-feeding insects are commonly associated with wood packaging material (WPM), which includes items such as pallets, crates, and dunnage (wood used to brace cargo). Packaging for overseas shipments is commonly constructed from wood because it is relatively inexpensive, generally abundant, renewable, and easily manufactured and repaired. Unfortunately, wood used to construct WPM can be infested with a wide variety of bark and wood pests and thereby serve as a pathway for pest movement. Wood-feeding insects can also be transported in logs, lumber, fuelwood, live plants, and various manufactured wood articles [12]–[16].

As international trade volumes soared in recent decades, many countries became concerned about repeated introductions of invasive forest insects and disease organisms, such as Asian longhorned beetle, Anoplophora glabripennis (Motschulsky) (Coleoptera: Cerambycidae), and pinewood nematode, Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle (Nematoda: Aphelenchoididae), as well as the WPM pathway that often vectors these pests. In response, members of the International Plant Protection Convention (IPPC) developed and adopted International Standards for Phytosanitary Measures No. 15 (ISPM15) in 2002, which provided details on approved phytosanitary treatments for WPM used in international trade [17]. A core value of these international standards is the harmonization of national regulations, which facilitates trade. The original stated goal of ISPM15 in 2002 was to “practically eliminate the risk for most quarantine pests and significantly reduce the risk from a number of other pests” by means of either heat treatment or methyl bromide fumigation of WPM [17]. ISPM15 was slightly revised in 2006 [18], and in 2009 the IPPC adopted several important changes such as lengthening the fumigation exposure time, requiring WPM to be made from debarked wood, requiring debarking prior to fumigation, and specifying tolerance limits on the maximum allowable size for individual patches of residual bark [19].  In addition, the goal of ISPM15 was reworded in 2009 to read as follows “to reduce significantly the risk of introduction and spread of most quarantine pests” associated with WPM [19]. The next version of ISPM15 was published in 2011 but consisted simply of changes in text formatting [20]. The newest version of ISPM15 was approved in 2013 and formally adopted heat treatment using dielectric heating (e.g. microwave) along with the corresponding treatment code DH [21]. More than 78 countries (considering the European Union as 27 countries) have implemented ISPM 15 through October 2013. It is important to recognize that the ISPM15 standards can be applied to wood from any tree species, including tropical and boreal species, as well as softwoods (conifers) and hardwoods (angiosperms).

The United States implemented ISPM15 in three phases over a 10-month period from 16 September 2005 to 5 July 2006. On 16 September 2005 the United States implemented Phase 1, which consisted of officially informing importers and the appropriate National Plant Protection Organization of the exporting country if live pests were found in WPM or if the WPM was not marked in compliance with ISPM15. Phase 2 began on 1 February 2006 and required that all WPM entering the United States (except from Canada) meet ISPM15 treatment standards and be marked accordingly. As part of Phase 2, noncompliant shipments and WPM could be denied entry to the United States, or if feasible, the noncompliant WPM would be removed from the shipment and exported at the expense of the importer, and thereby allow the imported products to enter the United States. Phase 3 began on 5 July 2006 and continues to the present and requires that noncompliant WPM and the associated commodities be immediately exported, usually returning it to the country of origin [22].

The objective of the present paper was to compare pre- and post-ISPM15 infestation rates of WPM associated with imports entering the United States. In this paper, we use the term “infestation rate” to refer to the percentage of consignments with WPM in which live pests were found in WPM when the imported consignments were inspected on arrival at US ports. We expected that if the data from the pre- and post-ISPM15 surveys were comparable then we could estimate the effect that ISPM15 had on WPM infestation rates. Further, we anticipated that implementation of ISPM15 would substantially reduce the number and frequency of live pests in WPM because the supporting documents that accompanied the early drafts of ISPM15 indicated that the proposed treatments for WPM were highly effective against many wood-associated insects and fungal pathogens [23]–[24]. The use of interception data for this purpose seemed acceptable because interception records are among the few datasets available that provide insights into the identity and relative infestation rate of pests associated with traded commodities and WPM [8], [15], [25]. We were able to find one large US dataset with interception data that had been collected in a standardized manner both pre- and post-ISPM15, which upon analysis indicated a moderate decline in pest interceptions on WPM after ISPM15 implementation.

Documenting the actual level of effectiveness of an international policy such as ISPM15 and evaluating the suitability of existing data for such an analysis is important for at least three reasons. First, it is important for determining the level of phytosanitary risk still associated with WPM and whether further revisions to ISPM15 are needed, or if individual countries may wish to require additional measures based on a pest risk assessment. Second, it is essential for estimating and understanding the economic costs and benefits of the implemented policy. And, third, it provides insights into the types of data that should be collected in advance of future international standards. For example, the recent approval of ISPM 36 in 2012 [26], which deals with plants for planting, provided such an opportunity.

ISPM15 Standards

To fulfill the requirements of ISPM15, WPM used in international trade must be marked (stamped) in a specific way to indicate that the WPM was subjected to an approved phytosanitary treatment [21]. The official mark includes the IPPC logo, a 2-letter country code indicating in which country the wood was treated, a producer code to indicate the treatment provider, and a treatment code to specify the treatment used, such as HT for heat treatment or MB for methyl bromide fumigation [21]. Each version of ISPM15 has provided more details on how the wood treatments should be conducted, and even more details were added to the 2013 version [21]. After research showed that bark- and wood-infesting insects, both primary and secondary colonizers, could infest and develop in wood after treatment, especially when bark was present [27]–[28], a debarking requirement for WPM was added in 2009. The tolerance limits for residual bark specified that pieces of bark could remain on WPM after debarking if individually they were either less than 3 cm in width (regardless of their length) or if they were greater than 3 cm wide but less than 50 square centimeters in total surface area [19]–[21]. The debarking requirement was not yet in place during the period of time analyzed in the present study.

Pests Commonly Associated with WPM

The principal bark- and wood-boring insects of quarantine concern for the United States include insects in the beetle (Coleoptera) families Buprestidae, Cerambycidae, Curculionidae (including Platypodinae and Scolytinae); the woodwasp family Siricidae (Hymenoptera), and the moth (Lepidoptera) families Cossidae and Sesiidae. Elsewhere in the world there are many other wood pests of concern to specific countries, including species of powderpost beetles (Bostrichidae, including Lyctinae), wood-boring flies (Diptera), termites (Isoptera), as well as wood-decay fungi and nematodes [29]–[32]. It is important to note that many powderpost beetles and termites are secondary colonizers of treated wood, and therefore are rarely the target pests when ISPM15 treatments are applied to newly constructed WPM.

International Pest Interception Databases

Several countries maintain databases of plant pests that are intercepted at their ports of entry, including maritime ports, airports, and international border crossings. For example, long-term pest interception databases have been maintained by governments and plant protection organizations in Australia, Canada, Chile, Europe and North Africa (by the European and Mediterranean Plant Protection Organization, EPPO), Mexico, New Zealand, and the United States. Typically, inspectors target high-risk products or pathways, rather than conduct random surveys. In addition, interception records are usually included in a country’s database only when pests are found although there are exceptions (such as the AQIM database used in the present study).

Earlier Surveys for WPM-Associated Pests

A comprehensive review of the literature, involving online literature searches as well as direct contacts with several plant protection organizations worldwide, provided a limited number of estimates of WPM infestation rates from before implementation of ISPM15 [33]–[35] and after [28], [36] (Table 1). In general, the pre-ISPM15 surveys were expressed on a consignment basis, such as all WPM in a single shipping container. In contrast, the sampling units used in the two post-ISPM15 surveys were individual WPM items such as a single pallet or a single piece of dunnage. Therefore, the results of these pre- and post-ISPM15 surveys were not directly comparable. Nevertheless, in the pre-ISPM15 surveys, WPM infestation rates ranged from a high of 4.3% of containerized maritime consignments [33] to a low of 0.06% for air cargo consignments [35]. By contrast, in the two post-ISPM15 surveys that involved mostly maritime containerized cargo, infestation rates of individual WPM items ranged from 0.1% [28] to 0.5% [36] (Table 1).

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Table 1. Summary data for the incidence of live insects found in association with WPM during surveys of imported goods that were conducted before or after implementation of ISPM 15 in various countries.

http://dx.doi.org/10.1371/journal.pone.0096611.t001

We found only one publication, a master’s thesis [37], which compared interception data that had been collected in a similar manner both before and after implementation of ISPM15. In this study, the author summarized the insect interceptions on WPM that were associated with 10,870 consignments that arrived at the maritime port of San Antonio, Chile during the 18 months immediately before (7733 consignments) and 12 months immediately after (3137) implementation of ISPM15 in Chile. The interception data were expressed on a consignment basis, and included live bark- and wood-infesting insects that were intercepted in WPM. Overall, data from Sánchez-Salinas [37], indicated that the infestation rate of WPM entering Chile fell 47% after ISPM15 was implemented (Table 1).

USDA Pest Interception Databases

The US Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) maintains two major databases for records of pest interceptions on imported goods at US ports: AQIM (Agriculture Quarantine Inspection Monitoring) and PestID (Pest Interception Database, which was formerly called Port Information Network or PIN). AQIM is a statistically based inspection program based on random sampling of imported shipments at selected US ports. AQIM was designed to monitor the approach rate of agricultural risks on different pathways, and consists of daily or weekly random sampling of international cargo, mail, vehicles, and passenger baggage [38]. WPM was first targeted for inspection in AQIM in 2003 and usually consisted of sampling two containers per week at each of more than 40 participating US ports. Sample selection occurs randomly among commodities known to have associated WPM using a statistically robust stratified sampling plan. Infestation data for WPM are recorded on a consignment basis based on the number of distinct consignments within each of the sampled shipping containers. For each pest interception in AQIM, information is recorded on all pests found to the lowest taxonomic level possible (usually family, genus or species), as well as on the type of cargo inspected, type of WPM present, compliance with ISPM15 marking, and the presence or absence of bark on the WPM. For WPM, all plant pests found are recorded in AQIM, including both bark- and wood-infesting insects as well as those that inadvertently contaminated or “hitchhiked” with the shipment. Negative data, where no pests are found, are also recorded by consignment in AQIM, which allows the calculation of infestation rates (contrary to other interception data where negative inspections are typically not documented).

PestID includes interceptions records of all classes of plant pests intercepted at over 300 ports of entry in the United States, including bark- and wood-infesting insects found in association with WPM. As of January 2014, there were more than 2.5 million interception records in PestID that were recorded since 1984 when what is now known as PestID started as a computerized database. PestID records include information on the identity of intercepted pests, the commodity involved and its country of origin, date and place of interception, and many other details associated with the shipment and inspection such as whether the intercepted pest was associated with WPM. Unlike AQIM, however, PestID does not include information on shipments where no pests were found, and the inspections are not random, but are targeted at specific products, pathways, or countries.

There are other challenges when attempting to interpret PestID data. For example, although APHIS issues inspection guidelines for certain commodities [39], much work prioritization is left to the discretion of experienced, local personnel at the individual ports. As a result, for some commodities and items like WPM, the percentage of arriving shipments inspected can vary over time and among ports. Inspectors may target shipments based on a perceived risk of infestation for certain commodities from particular countries of origin and shippers. Additionally, priority inspection targets vary among ports due to the profile of work to be performed. For example, port inspectors who must clear large volumes of perishable fruits, vegetables, or cut flowers will likely spend less effort inspecting WPM associated with machine parts or quarry products than inspectors at ports that do not receive many perishables. Other limitations on the utility of PestID include: 1) that the data cannot provide an estimate of the number of pests arriving because not all shipments are inspected and inspectors may stop looking at a particular consignment once the first quarantine pest is found, 2) data on many intercepted pests that were classified as “non-quarantine significant pest ” taxa (e.g., cosmopolitan species or species that were regarded to be of low risk) were not included in PestID until March 2009, and 3) variation over time in the numbers of inspectors and their focus likely affected the numbers and kinds of pests that were intercepted.

Although the sampling protocols used in PestID are not random, PestID data are still useful in identifying the most common types of pests arriving in the United States, their countries of origin, and the commodities and pathways they were most often associated with [13], [16], [40].We report various PestID summary statistics below.

Methods

AQIM Data Analyses

We analyzed AQIM records where WPM was recorded for a 6-year period from October 2003 through September 2009. This period was chosen because it begins when APHIS started inspecting WPM as part of the AQIM program and ended in 2009, which was the year when several changes were made to ISPM15 [19]. Therefore, the data analyzed during the post-ISPM15 period in the present paper were collected during a period with consistent regulations. We excluded Canadian shipments from our analysis because the United States did not require Canadian WPM to meet ISPM15 standards during the sampling period. The policy of limited inspection on shipments from Canada is largely because most bark- and wood-infesting insects native to Canada are also native to the United States and because the long shared and largely forested border between the two countries presents no barriers to the migration of native or non-native insects. For example, about 97% of bark and ambrosia beetle species (Scolytinae) native to Canada are also native to the United States [41]. Similarly, we excluded all Chinese imports from our AQIM analysis because as of 17 December 1998, which was nearly six years prior to US implementation of ISPM15, the United States began regulating WPM from China in response to the rapidly increasing frequency of pest interceptions on Chinese WPM in the 1990s and the discoveries of Asian longhorned beetle infestations in New York in 1996 and Illinois in 1998 [8], [42]–[43]. This 1998 regulation on WPM [43] only affected exports from China to the United States. During the period from 1999 until US implementation of ISPM15, noncompliant Chinese shipments were typically fumigated at US ports, whereas after US implementation of ISPM15 most noncompliant shipments were sent back China. In addition, given that Mexico was the origin of more AQIM records than any other country (34% of all AQIM records during the 6-year study period, and 41% of the dataset after removal of the Canadian and Chinese records), we analyzed the remaining data both with and without the records from Mexico. The large number of Mexican consignments in the AQIM database was because several US-Mexico border crossings participated in AQIM program.

In our analyses, we tested separately the initial dates of Phase 1 (16 September 2005) and Phase 3 (5 July 2006) as the division points between pre- and post-implementation of ISPM15. For each date, we tested two scenarios: 1) exclusion of all data related to Canadian and Chinese imports, and 2) exclusion of all data related to Canadian, Chinese, as well as Mexican imports (for reasons explained above). We constructed a 2×2 contingency table for each scenario, comparing pre- and post-ISPM15 infestation rates of WPM, and analyzed each for statistical significance using Fisher’s exact test (right-sided probability, PROC FREQ) [44]. We used a significance level of α = 0.1 because infestation rates of WPM are typically low and we wished to reduce the likelihood of committing a Type II error (i.e., a false negative). We also calculated the power of our analysis to detect large reductions in pest infestation rates using presumed treatment effectiveness levels for ISPM15 of 50%, 70% and 90% mortality of the WPM-associated quarantine pests (PROC POWER) [44]. These results, tested with α = 0.05 and 0.1, would indicate the probability of detecting a 50%, 70% and 90% change in infestation rate had one occurred. When calculating the post-ISPM15 infestation rates in the above analyses, we only used data for those consignments in which the WPM was apparently compliant with ISPM15, i.e., stamped with the ISPM15 mark. We also calculated on an annual basis the percent of inspected consignments in which the WPM had the proper ISPM15 mark after US implementation of ISPM15 (2005–2009), and analyzed the data with nonlinear regression (PROC NLIN) [44]. In addition, we used methods similar to those described above to compare the pre- and post-ISPM15 infestation rates of WPM from the single country of Italy, which was the country of origin for the most borer interceptions on WPM that entered the United States during 1985–2000 [8].

PestID Data Analyses

PestID data cannot be used to statistically analyze for the effects of ISPM15 on interception rates because the data are collected in a nonrandom manner and the number of inspections where no pests are found is not recorded. Nevertheless, we did extract all interceptions of bark- and wood-boring insects in PestID from the 25-year period 1984 through 2008 to demonstrate changes over time in the types of borers being intercepted, the countries of origin, and the imported commodities most often associated with wood pests. As noted earlier, we recognize that the PestID data can be influenced by many factors such as changes in interception policies, staffing, etc. We restricted the dataset to those families of wood borers that were consistently targeted during port inspections over the 25-year period: Buprestidae, Cerambycidae, Cossidae, Curculionidae (including Platypodinae and Scolytinae), Sesiidae, and Siricidae. For records where the imported commodity was reported, we assigned the commodity to one of several trade sectors according to the Global Trade Analysis Project (GTAP) [45]–[46]. For example, some of the common GTAP sectors that we used included fabricated metal products (e.g., ironware, metalware, tubes, and wire), primary metals (e.g., aluminum, iron, and steel), machinery and equipment, quarry products (e.g., granite, marble, and slate), and fruit and vegetables.

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Wood preservatives. Determination of the protective effectiveness against marine borers

BS EN 275:1992

Wood preservatives. Determination of the protective effectiveness against marine borers

Status : Current, Project Underway   Published : November 1992

Standard Number	BS EN 275:1992
Title	Wood preservatives. Determination of the protective effectiveness against marine borers
Status	Current, Project Underway
Publication Date	15 November 1992
Cross References	BS 5666:Part 1, ISO 3130, Health and Safety at Work etc. Act 1974
International Relationships	EN 275:1992
Draft Superseded By	90/50033 DC
Descriptors	Wood, Wood preservation, Wood preservatives, Wood-boring organisms, Limnoria, Impregnated materials, Test specimens, Specimen preparation, Dimensions, Marking, Marine environment tests, X-ray analysis, Testing conditions, Test equipment, Test duration, Preservatives, Visual inspection (testing), Performance testing, Comparative tests, Grading (quality), Crustacea, Mollusca, Teredo, Reports
ICS	71.100.50
Title in French	Produits de préservation du bois. Détermination de l'efficacité protectrice vis-à-vis des organismes térébrants marins
Title in German	Holzschutzmittel. Bestimmung der Schutzwirkung gegenueber marinen Organismen
Committee	B/515
ISBN	0 580 21047 2
Publisher	BSI
Format	A4
Delivery	Yes
Pages	24
File Size	550.9 KB
Price	£158.00

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Novel lignocellulosic hybrid particleboard composites made from rice straws and coir fibers

ScienceDirect 

Materials & Design

March 2014, Vol.55:19–26, doi:10.1016/j.matdes.2013.09.066

• Li Zhang
• Yingcheng Hu ^,

• Key Laboratory of Bio-based Material Science and Technology of Ministry of Education of China, College of Material Science and Engineering, Northeast Forestry University, Harbin 150040, China

Received 8 May 2013. Accepted 27 September 2013. Available online 10 October 2013.

Highlights

• •
  Coir fibers were used as mixture materials for production of straw particleboard.
• •
  Alkali treatment was an effective method for improving wettability of rice straw.
• •
  The MOR and MOE of RC particleboards decreased with coir fibers content increased.
• •
  The addition of coir fibers significantly improved the IB and TS of RC boards.
• •
  PF-bonded boards with optimal R/C ratio can be used for load-bearing application.

Abstract

Novel lignocellulosic hybrid particleboard composites with low cost and high performance using the mixture of rice straws and coir fibers were developed in this work. NaOH (sodium hydroxide) aqueous solution was used to remove the wax and silica layer of rice straw surface. The effects of rice straws/coir fibers (R/C) mass ratios on the physical (thickness swelling) and mechanical (modulus of rupture, modulus of elasticity and internal bond strength) properties of particleboard composites were investigated. The results show that NaOH treatment was an effective method for improving wettability of rice straw surface with smaller contact angles and larger diffusivity–permeability constant. The SEM (scanning electron microscope) observation also gave some evidences such as more rough surface and less number of silica cells after NaOH treatment for improving wettability of rice straw surface. The coir fibers content had a significant negative linear effect on the bending properties and thickness swelling, but a significant positive linear effect on the internal bonding strength due to the lower wax and holocellulose content of coir fiber. When no diisocyanate resin applied, the particleboard composites made with only phenol formaldehyde resin at the optimal R/C ratio satisfied the requirements for load-bearing boards used in dry conditions based on Chinese Standard, indicated that the mixture of rice straws and coir fibers to make high quality particleboard composites was a cost-effective and environment friendly approach.

Keywords

• Rice straw
• Coir fiber
• Particleboard composite
• Physical property
• Mechanical property
• Scanning electron microscopy

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• ⁎ 
  Corresponding author. Tel.: +86 451 82190394; fax: +86 451 82191748.

Copyright © 2013 Elsevier Ltd. All rights reserved.

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Combined effect of thermoplastic and thermosetting adhesives on properties of particleboard with rice husk core

Mat. Res. vol.17 no.5 São Carlos Sept./Oct. 2014  Epub Sep 09, 2014

Jin Heon Kwon^I,  *; Nadir Ayrilmis^II; Tae Hyung Han^I
IDepartment of Forest Biomaterials Engineering, College of Forest and Environmental Sciences, Kangwon National University, 200-701 Chuncheon city, Republic of Korea 
^IIDepartment of Wood Mechanics and Technology, Forestry Faculty, Istanbul University, Bahcekoy, Sariyer, 34473, Istanbul, Turkey

Keywords: dimensional stability, mechanical properties, phenol formaldehyde, urea-formaldehyde, thermoplastic adhesive, particleboard, rice husk.

ABSTRACT

This study investigated the combined effect of adhesive type and content on the dimensional stability and mechanical properties of three-layer particleboards made from a mixture of wood particles (face layer: 30 wt %) and rice husk particles (core layer: 70 wt %). Two types of thermosetting adhesives, liquid urea-formaldehyde (UF) and phenol-formaldehyde (PF), and thermoplastic adhesive (low density polyethylene: LDPE) powder were used as binder in the experiments. Thickness swelling and water absorption of the particleboards significantly decreased with increasing content of the LDPE powder. The incorporation of LDPE powder into the core layer of particleboard greatly improved the internal bond strength.

1. INTRODUCTION

The rate of global deforestation and its impact on the environment has led particleboard manufacturers to search for alternative feedstock, especially in countries where wood is less available compared to other cellulosic natural products. The use of renewable resources such as agricultural residues, is gaining increased interest in production of particleboard. Rice husks are important by-product of the rice milling process, which are available in fairly large quantities in certian agricultural areas. It is reported that about 0.23 tons of the rice husk are generated per ton of rice produced¹. The main components of rice husk are cellulose (25 to 35%), hemicellulose (18 to 21%), lignin (26 to 31%), silica (15 to 17%), solubles (2 to 5%), and moisture content of 5-10%². The reasons behind the use of rice husk in particleboard industry are its high availability, low bulk density (90-150 kg/m³), toughness, abrasive in nature, resistance to weathering and unique composition³. Although previous studies reported that rice husk particleboard could be used in the manufcture of furniture and interior fitments, the physical and mechanical properties of the particeboards were lower than those of the particleboards made from wood particles^4-6. The main reasons for lower physical and mechanical properties of the rice husk particleboards are low aspect ratio and waxy/silica layer of the rice husk particles.

Polyethylene adhesives are milky white, translucent substances derived from ethylene (CH₂₉CH₂). Low density polyethylene (LDPE) typically has long side-chain branching off the main molecular chain and therefore is a more amorphous polymer those branched polyethylene plastics, having a standard density of 0.91-0.92 g/cm³. LDPE is the most widely used of all plastics, because it is inexpensive, chemical-resistant, very resistant to fungal attack, and have good dimensional stability when exposed to moisture⁷. Instead of urea-formaldehyde (UF) adhesive, LDPE offers many environmental and technological benefits when used as a binder for wood particles in the core layer of rice husk particleboard, such as no formaldehyde emission, higher water and fungal resistance.

The UF adhesive is one of the most common adhesives used in wood-based panel industry. Its low price and good strength properties of glue lines under dry conditions result in its being widely applied despite its low water resistance⁸. In order to increase water resistance of UF adhesives, they are commonly modified with melamine⁹and diisocyanate¹⁰. Investigations conducted in this respect showed that the application of melamine and isocyanates in the UF adhesive improved the glue-line strength to a considerable degree and increased water resistance of the UF adhesive^9,10. However, modifiers are still expensive and increase the cost of UF adhesive. Polyethylene matrix is extensively used in the production of lignocellulosic filled thermoplastic composites because it is high performance binder for lignocellulosics^11-13. For this reason, LDPE could play an important role in the production of particleboard having a rice husk core.

When a particeboard is used in moist areas, it absorbs water. The core layer of particleboard is mainly responsible for thickness swelling (TS) and water absorption (WA) due to its high shell ratio. If the voids and spaces among the rice husk particles are filled by the melted LDPE, the dimensional stability of the particleboard can be improved. Internal bond strength of the particleboard having a rice husk core can be improved as the rice husk particles are encapsulated in the hydrophobic LDPE matrix. The objective of this study was to investigate the combined effect of liquid thermosetting adhesives (UF or phenol-formaldehyde (PF) adhesives) and powder thermoplastic adhesive (LDPE) on the dimensional stability and mechanical properties of three-layer particleboard. The particleboards were made from a mixture of rice husk particles (core layer: 70 wt%) and wood particles (face layer: 30 wt%). The wood particles used in the top and bottom layers were bonded with UF or PF adhesive while the core layer consisting of rice husk particles were bonded with a mixture of the thermosetting adhesives (UF or PF adhesive) and LDPE powder.

2. EXPERIMENTAL
2.1. Materials
The rice husk particles were obtained from a rice mill in Chuncheon, capital city of Gangwon Province, South Korea. The average moisture content of rice husk particles prior to the production of the particleboards was 5% based on the oven-dry weight of the rice husk particles. The average length, width, and thickness of rice husk particles used in the experiments were 6.79±0.35 mm, 2.96±0.26 mm, 0.17±0.02 mm, respectively (these values were an average of 30 the rice husks and standard deviation). The wood particles having a moisture content of 4-5% were obtained from a commercial particleboard company located in South Korea. The average length, width, and thickness of wood particles were 13.53±3.72 mm, 1.95±0.74 mm, 0.97±0.40 mm, respectively.
A commercial E1 (urea/formaldehyde ratio: 1/0.8, viscosity: 180 cps) grade liquid UF adhesive with a solid content of 56 wt% and liquid PF adhesive (viscosity: 195 cps) with a solid content of 59.4% were used in the production of the particleboards. The UF and PF adhesives were supplied by Hansolhomedeco company in Iksan city, South Korea. As a hardener 1% of ammonium chloride (NH₄Cl) solution with 20 wt% solids content based on the UF adhesive solids content was added in to the UF adhesive solution. This study did not include the addition of any external wax or water-repellent chemicals to the wood and rice husk particles.
The LDPE powder (particle size: 50 mesh, melting temperature = 105 °C, density = 0.926 g/cm³, MFI (melt flow index) = 24 g/10 min) was supplied by M.J Powder company in Ulsan city, South Korea.


2.2. Production of experimental particleboards
Three-layer particleboards consisting of a central layer (core) and two outer layers (faces) were manufactured under laboratory conditions (Figure 1 Both surfaces were made from the fine wood particles while the core layer was made from rice husk particles. The surface and core particles were separately placed in a drum blender. Then the UF adhesive was applied with an air-atomized metered spray system for 5 min to obtain a homogenized mixture. This procedure was also performed for the PF adhesive application. The LDPE powder was applied to the core particles with UF adhesive or PF adhesive. In the first phase, six levels of the LDPE powder (5-30 wt %) based on the composition by weight, were mixed with the core particles (rice husk) with 8 wt % UF adhesive. In the second phase, the LDPE powder content was kept constant at 10 wt% in all the treatments and the UF adhesive or PF adhesive contents applied to the core layer was decreased gradually from 8 to 4 wt%. The experimental design was presented in Table 1.

The layer construction of the particleboards based on the oven-dried weight ratio of the wood particles was 15:70:15 (face/core/face). The surface and core particles for three-layer boards were separately weighed and distributed evenly by hand into a 400 mm x 400 mm forming box. Release agent was used to avoid direct contact of the wood particles with the steel caul plates during heating and pressing. To reduce the mat height and to densify the mats, they were subjected to a cold-press. Particleboard mats having 10% moisture content were subjected to hot-press, using a manually controlled, electrically heated press. The hot press temperature, maximum pressure, and total press cycle were 180 °C, 2.5 N/mm², and 5 min, respectively. The particleboards were then trimmed to a final size of 380 mm x 380 mm x 10 mm after the cooling process. A total of 72 particleboards, three for each type of formulation and control, were produced (Table 1).The average density values of the particleboards varied from 805 to 825 kg/m³.

Prior to the physical and mechanical tests all the specimens prepared from the particleboard the specimens were conditioned in a climatized room at 20°C and 65% relative humidity. Duration of the conditioning process was determined by regular weighing of the specimens until no changes in the weights were detected.

2.3. Determination of dimensional stability
The TS and WA tests were carried out according to EN 317 (1993). Ten replicate specimens, 50 mm x 50 mm x 10 mm, from each type of particleboard were used for the TS and WA properties. At the end of 1-day of submersion, the specimens were taken out from the water and all surface water was removed with a clean dry cloth. The specimens were weighed to the nearest 0.01 g and measured to the nearest 0.001 mm immediately. The specimen thickness was determined by taking a measurement at a specific location, the diagonal crosspoint, on the specimen. The densities of specimens were evaluated according to the test method specified in EN 323 (1993).

2.4. Determination of mechanical properties
The bending strength (MOR) and modulus of elasticity (MOE) of the specimens were performed according to EN 310 (1993). A total of nine replicate specimens with dimensions of 250 mm x 50 mm x 10 mm were tested for each type of particleboard. The bending tests were conducted in accordance with the third point loading method at a span-to-depth ratio of 20:1. The crosshead speed was adjusted so that the failure would occur within an average of 60 s ± 10. The specimens were tested on Instron testing machine (Model: 4482) equipped with a load cell with a capacity of 10 kN. The internal bond (IB) strength tests were conducted on the specimens cut from the particleboards according to EN 319 (1993). Ten replicate specimens with dimensions of 50 mm x 50 mm x 10 mm from each type of paricleboard were used to determine the IB strength.

2.5. Statistical analysis
An analysis of variance, ANOVA, was conducted (p< 0.01) to evaluate the effect of adhesive type and adhesive/LDPE content on the physical and mechanical properties of the particleboards. Significant differences between the average values of types of the particleboards were determined using Duncan's multiple range test.

3. RESULTS AND DISCUSSION

3.1. Dimensional stability
The TS and WA values of the particleboards are presented in Table 2. The dimensional stability of the specimens was greatly improved by increasing the LDPE content. Significant (p<0.01) differences were observed between the specimens with and without LDPE. The control specimens had much higher TS and WA values than the specimens containing the LDPE (Table 2). For example, the TS values of UF-bonded particeboards decreased from 102.9 to 58.6% as 5 wt% LDPE powder was incorporated into the core layer, and then decreased to 13.2% as the LDPE content increased up to 30 wt%. Similar results were found for the PF-bonded particleboards. It is well known that the PF adhesives are resistant to wet conditions whereas UF adhesives are not. As the LDPE is a good barrier to water due to its hydrophobic character, by replacing UF by LDPE, swelling thickness and water absorption are significatively reduced. The positive effect of LDPE on the TS and WA of particleboards was not significant between 25 and 30 wt% LDPE contents (Table 2).

The UF-bonded particleboards swelled more than two times higher than the PF-bonded particleboards at all loading levels of LDPE (Table 2).  Hydroxymethyl phenols will crosslink on heating to around 120 °C to form methylene and methyl ether bridges through eliminating water molecules. At this point the PF adhesive is a 3-dimensional rigid network, which is typical of polymerised phenolic¹⁴. This makes the PF adhesive very resistant to water. However, the UF adhesive has lower water and weather resistance than the PF adhesive because aminomethylene linkages are susceptible to hydrolysis. Therefore, the UF adhesive is not stable at high humidity and temperature. Polycondensation of urea with formaldehyde usually results in hydrolytically sensitive bonds and low cross-link density which invariably increases sensitivity to losses in stress-bearing applications⁸.

The silica and waxy water repellent cuticle cover almost the entire outer layer of the rice husk¹⁵. The outer waxy layer of the rice husk particles has lower wettability. The silica in the husk is about 12% as ash content. The structure of silica was amorphous with a purity of 99.7% SiO₂^[16]. The adhesive could not efficienty wet and penetrate to the cellular structure of rice husk due to its waxy cuticle. This results in the poor interfacial adhesion between the rice husk particles, which increases thickness swelling and water absorption of the particleboards.

As the amounts of UF and PF adhesives decreased from 8 to 4 wt% at the same LDPE content (10 wt%), the TS and WA vaules of the particeboards increased. However, the TS and WA values of the particleboard containing LDPE were significantly lower than the control specimens (Table 2). For example, the TS and WA values of particleboards bonded with 4 wt% UF adhesive and 10 wt% LDPE were 49.4% and 78.2% while they were found to be 102.9% and 95.5% for the control particleboards bonded with 10 wt% UF adhesive, respectively. Similar results were observed for the PF-bonded particleboards. The decrease in the amount of UF adhesive or PF adhesive resulted in higher TS values because swelling induced stresses caused the separation of the rice husk particles within the core layer.

3.2. Mechanical properties

The incorporation of LDPE powder into the core layer of particleboard greatly improved the bonding between the rice husk particles. The IB values of particleboards increased by 366% and 280% as 30 wt% LDPE powder was incorporated into the core layer of UF- and PF-bonded particleboards, respectively. As compared to the control particleboards, the particeboards containing the LDPE powder, except for the particleboards containing 5 wt% LDPE, showed significant increases in the IB strength. Significant differences (p<0.01) in the IB values are presented in Table 2, as letters. The increase in the amount of the LDPE powder considerably increased the IB strength because the LDPE powder led to better linkage between the rice husk particles. The polymer matrix used in the core layer acted as an adhesive to bond the rice husk particles together mechanically. The melted LDPE also improved the interfacial adhesion between core layer (rice husk particles) and face layers (wood particles) of the particleboard. Another explanation of this phenomenon was the uniform distribution of the melted LDPE over the rice husk particles. Similar results were found in previous studies regarding lignocelluosic filled thermoplastic composite panels^17-19.

The IB strength of PF-bonded specimens was significantly higher than that of the UF-bonded specimens. This revealed that the UF adhesive was more affected by the waxy silica layer compared with the PF adhesive. This is because the UF adhesive tends to have higher surface energy than the PF adhesive²⁰. Although the IB strength of particleboards containing 10 wt% LDPE decreased with decreasing UF adhesive content in the core layer, it was higher than that of the control particleboards. For example, the average IB strength of the control particleboards was 0.03 N/mm² while it was found to be 0.04 N/mm² for the particleboards bonded with a mixture of 4 wt% UF adhesive and 10 wt% LDPE. The IB strength values of control and particleboards made with a mixture of 4 wt% UF adhesive and 10 wt% LDPE were found to be 0.05 N/mm² and 0.04 N/mm², respectively. The IB strength of the particleboards containing 10 wt% LDPE powder decreased by 42.9% and 55.5% as the UF and PF adhesive contents decreased from 8 to 4%, respectively.

Different fracture modes were observed during the IB tests. The fracture modes of particleboard samples with and without LDPE were presented in the Figure 2. The weakest area of fracture resistance was found in the core layer of the control samples. As shown in Figure 2, the fracture for the control samples without the LDPE was occured in the core layer while this was observed in the layer between the core layer and face layer of the samples with the LDPE. This result revealed the bonding between the rice husk particles was significantly improved by the melted LDPE. The fracture surfaces of IB test samples with and without LDPE were presented in Figure 2.

The MOR and MOE of the particleboards were significantly improved by the incorporation of LDPE into the core layer (Table 2). The incorporation of the LDPE into the rice husk core layer increased the bonding performance between the rice husk particles. The LDPE decreased the micro voids in the core layer of particleboard, which resulted in increased strength and modulus. As 30 wt% LDPE was incorporated into the core layer, the MOR of the UF- and PF-bonded particleboards increased by 33.9% and 27.9%, respectively. The MOE of UF- and PF-bonded particleboards increased by 17.7% and 18.6% as 30 wt% LDPE was incorporated into the core layer, respectively. The MOR and MOE of the particleboards having rice husk core with LDPE were comparable with particleboards made from wood particles. For example, Nasser²¹ found that the MOR and MOE of three layer particleboards made from Pithecellobium dulce wood particles were 16.1 N/mm² and 2.56 GPa, respectively. In another study, Hiziroglu²² determined that MOR and MOE of UF-bonded particleboard were 11.7 N/mm² and 2.04 GPa, respectively.

The change in the values of MOR and MOE of the particleboards was associated with the characteristics of the outer layer and the core layer. This was because the outer layer was the layer subjected to higher stress levels when the sample was under flexion state. In the samples A to G (or H to N) the amounts of thermoplastic adhesive increased in the core layer while the amount of rice husk decreased. Hence the total content of adhesive (thermosets and thermoplastics) increased. By increasing the content of adhesive, the value of MOE and MOR of boards improved slightly because only the core layer was modified.

The MOR and MOE of the particleboards containing 10 wt% LDPE decreased as the UF or PF adhesive content decreased from 8 to 4 wt% in the core layer (Table 2). This was due to the fact that the adhesives could effectively transfer and uniformly distribute stresses, thereby increasing the strength and stiffness of the particleboard. The MOR and MOE of particleboards containing 10 wt% the LDPE decreased with decreasing the amount of adhesive. However, there were no statistically significant differences in the MOR and MOE values of the particeboards as the UF adhesive content decreased from 8 to 4 wt% in the core layer. The MOR of UF-bonded particleboards containing 10 wt% LDPE decreased from 12.1 to 10.9 N/mm² as the UF adhesive content decreased from 8 to 4 wt% in the core layer. As for the PF-bonded particeboards, the MOR decreased from 15.2 to 13.6 N/mm² as the PF adhesive content decreased from 8 to 4 wt% in the core layer. The IB, MOR, and MOE values of the PF-bonded particleboards were higher than those of the UF-bonded particleboards.
4. CONCLUSIONS

The dimensional stability and mechanical properties of particleboards with rice husk core were significantly improved by the incorporation of LDPE into thecore layer. As the amounts of UF and PF adhesives decreased from 8 to 4 wt% at the same LDPE content (10 wt%), the TS and WA of the particeboards increased, but were significantly lower than those of the control specimens. The reason was that a hydrophilic material (RH) was replaced by a hydrophobic material (LDPE). Although the IB strength of particleboards containing 10 wt% LDPE decreased with decreasing UF adhesive content in the core layer, it was higher than that of the control particleboards. The MOR and MOE of the particleboards containing 10 wt% LDPE decreased with decreasing the amounts of UF and PF adhesives. However, there were no statistically significant differences in the MOR and MOE values of the particeboard types as the adhesive content decreased from 8 to 4 wt% in the core layer. Based on the findings obtained from the present study, it can be said that the rice husk particleboards containing LDPE, in particular above 20 wt% of LDPE, are suitable for use in damp places, such as bathrooms, toilets, kitchens, and laundries.

ACKNOWLEDGEMENTS
This study was supported by 2012 Research Grant from Kangwon National University and by the MSIP (Ministry of Science, Ict & Future Planning). The authors would like to thank them for their financial support.

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