Quality Inspection and Control of Preservative-treated Wood

I. Classification of Preservative-Treated Timber

01. Use of Classification

According to my country's current standard for preservative-treated wood, GB/T 27651-2011 "Classification and Requirements for the Use of Preservative-Treated Wood", preservative-treated wood and its products are classified into categories C1 to C5 based on the final use environment, exposure conditions, and the degree of harm from biological decay factors under different environmental conditions. Among them, C3 and C4 are each divided into two categories, as detailed in Table 1.

02. Preservative Category

Currently, the most widely used preservatives in my country are water-based preservatives, traditionally chromium- and arsenic-containing preservatives such as copper chromium arsenic (CCA). Newer water-based preservatives include ammoniacal alkylamine copper (ACQ), micro-quaternary ammonium copper (MCQ), copper azole (CuAz), and boron-containing preservatives. Due to their environmental friendliness, these agents have seen a gradual increase in market share in recent years. In addition, there are organic preservatives, such as tebuconazole and oxadiazon. Different preservative treatments for wood have different required loading levels, and therefore different quality control requirements during testing.

II. Quality Requirements and Key Points for Preservative-Treated Wood

01. Drug Loading

This refers to the amount of effective preservative components retained in the wood after preservative treatment, commonly expressed in kg/m³. The preservative loading is the most critical factor determining the preservative performance of treated wood; under appropriate treatment processes, the concentration of the preservative working solution directly affects whether the preservative loading is up to standard.

Table 2 shows the required drug loading of preservative-treated wood and its products under various conditions.

02. Penetration

For sapwood, penetration is expressed as sapwood penetration rate, which is the ratio of the depth to which the effective component of the preservative penetrates into the sapwood to the total depth of the sapwood (on the same side), expressed as a percentage. For surface heartwood, penetration depth is expressed as depth (unit: mm). The moisture content and permeability of the wood before preservative treatment are direct factors affecting penetration. Wood permeability is determined by the properties of the wood. For the same tree species, the moisture content of the wood has the greatest impact on preservative penetration; therefore, drying the wood before preservative treatment to reduce its moisture content is a crucial step.

Table 3 shows the input rate of preservatives in the sapwood of timber.

03. Other requirements

1) The surface of the preservative-treated wood should be free of visible preservative deposits;

2) Before preservative treatment, the wood should be processed to its final dimensions as much as possible to avoid sawing and drilling.

3) If the preservative-treated wood has been mechanically processed, a concentrated solution of the original preservative should be applied to the newly exposed wood surface to seal it.

III. Quality Testing Methods and Precautions for Preservative-Treated Wood

01. Drug Loading Detection Method

The determination of preservative loading was conducted according to GB/T 23229-2009, "Analytical Methods for Waterborne Wood Preservatives". The main steps included sample pretreatment to obtain the preservative solution, determining the solution concentration using appropriate analytical methods, and calculating the preservative loading on the wood. Specifically, atomic absorption spectrometry was used to determine the copper, chromium, and arsenic content in the preservative-treated wood; titration or high-performance liquid chromatography (HPLC) was used to determine DDAC and BAC; high-performance liquid chromatography (HPLC) was used to determine tebuconazole and propiconazole; and inductively coupled plasma atomic emission spectrometry (ICP) was used to determine boron. Detailed determination procedures and conditions can be found in the standard.

02. Penetration Testing Method

The penetration test method involves taking samples using a 5 mm or 10 mm diameter hollow drill or growth cone. For CCA, ACQ, and CA preservative-treated wood, the copper-containing core will show a deep blue color after being dripped or sprayed with a coloring agent, while the boron-containing core will show a red color. Alternatively, the coloring agent can be sprayed onto a freshly sawn cross-section of the wood; the wood will show a deep blue or red color, and the depth of the colored area indicates the penetration degree of the preservative into the wood.

03. Precautions for Sample Submission and Testing

1) When testing the amount of preservative-treated wood and its penetration, if the treated wood volume is less than 5m³, 5 samples should be taken. If the treated wood volume is greater than or equal to 5m³, 10 samples should be taken.

2) Each sample should be from a different type of wood. The sample should be taken from the middle of the wood along its length, avoiding knots, cracks, and stressed wood when determining the specific location. Each sample should be at least 5 cm long, and its width and thickness should be consistent with the board to be tested.

3) When submitting samples, information on the tree species and type of preservative must be provided, as this is essential for calculating the drug loading.

Contributed by Zhang Jingpeng, Protection Laboratory, Research Institute of Wood Industry, Chinese Academy of Forestry