Anticorrosive wood quality inspection and control

1. Classification of antiseptic wood

01. Use classification

According to my country's current anticorrosive wood standard GB/T 27651-2011 "Classification and Requirements for the Use of Anticorrosive Wood", anticorrosive wood and its products are classified into categories Cl to C5 based on the final use environment and exposure conditions, as well as the degree of harm of biological spoilage factors under different environmental conditions. Among them, C3 and C4 are each divided into 2 categories. See Table 1 for details.

02. Preservative Category

Currently, the most widely used water-borne preservatives in my country are traditionally those containing chromium and arsenic, such as copper chromium arsenic (CCA). Newer water-borne preservatives include ammonia-soluble alkylamine copper (ACQ), micronized quaternary copper (MCQ), copper azole (CuAz), and boron-containing preservatives. Due to their environmentally friendly properties, these agents have gradually increased their market share in recent years. Organic preservatives, such as tebuconazole and oxadiazole, are also available. Wood treated with different preservatives requires different loading levels, and therefore different quality control requirements during quality testing.

2. Quality requirements and quality control points of antiseptic wood

01. Drug loading

This refers to the amount of active preservative ingredients remaining in treated wood, typically expressed in kg/m³. Drug loading is the most critical factor in determining the antiseptic properties of treated wood. Under appropriate treatment processes, the concentration of the preservative working solution directly impacts the acceptable drug loading.

Table 2 shows the drug loading requirements for antiseptic wood and its products used under various conditions.

02. Penetration

For sapwood, the sapwood penetration rate is expressed as the ratio of the depth of penetration of the preservative's active ingredients into the sapwood to the total depth of the sapwood (on the same side), expressed as a percentage. For surface heartwood, the penetration rate is expressed as the penetration depth (unit: mm). The moisture content and permeability of the wood before treatment are the direct factors affecting the penetration rate. Wood permeability is determined by the wood's properties. For the same species, the most significant factor affecting preservative penetration is the moisture content of the wood. Therefore, drying the wood to reduce its moisture content before treatment is crucial.

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

03. Other requirements

1) There should be no visible preservative deposits on the surface of the treated wood;

2) Before the wood is treated with preservatives, it should be processed to the final size as much as possible to avoid mechanical processing such as sawing and drilling of the preservative wood;

3) If the treated wood is machined, the newly exposed wood surface should be coated with a concentrate of the original preservative to seal the newly exposed wood surface.

3. Anticorrosive wood quality testing methods and precautions

01. Drug loading detection method

Drug loading testing is conducted in accordance with GB/T 23229-2009, "Analysis Methods for Waterborne Wood Preservatives." The main steps include sample pretreatment to obtain the test solution, determining the solution concentration using appropriate analytical methods, and calculating the drug loading on the wood. Copper, chromium, and arsenic content in preservative wood is determined using atomic absorption spectrometry; DDAC and BAC can be determined using titration or high-performance liquid chromatography; tebuconazole and propiconazole are determined using high-performance liquid chromatography; and boron is determined using plasma atomic emission spectrometry (ICP). Detailed testing procedures and conditions are available in the standard.

02. Penetration detection method

Penetration testing involves sampling with a 5 mm or 10 mm diameter hollow drill or growth cone. A color developer is applied or sprayed onto the copper-containing core of CCA, ACQ, or CA treated wood. The core of wood treated with boron will display a dark blue color, while the core of wood treated with CCA, ACQ, or CA treated wood will display a red color. Alternatively, the developer can be sprayed onto a freshly sawn cross-section. The wood will display a dark blue or red color. The depth of the color development determines the penetration of the preservative into the wood.

03. Notes on sample delivery and testing

1) When submitting samples for testing the drug loading and penetration of preservative wood, 5 samples shall be taken for each batch of wood with a treated volume of less than 5m³. 10 samples shall be taken for each batch of wood with a treated volume of 5m³ or more.

2) Each sample should be from a different wood. Samples should be taken from the middle of the wood's length, avoiding knots, cracks, and stressed wood. Each sample should be no less than 5 cm long and have the same width and thickness as the board being tested.

3) When submitting samples, you must provide information on the wood species and preservative type, which is essential for calculating the drug loading.

Zhang Jingpeng, Conservation Office, Wood Industry Research Institute, Chinese Academy of Forestry