Ambient ageing and medium density fibreboard
Zhenhua Gao of the Northeast Forestry University, China, discusses the ambient ageing of wood fibres and its effect on medium density fibreboard panels.
Medium-density fibreboard (MDF) is an important bio-based composite material that is widely used in furniture manufacturing, interior decorating, floor production and elsewhere. The wood fibres for such panels are commonly prepared by refining wood chips under thermomechanical conditions. The resin to bind the fibres together is usually urea-formaldehyde (UF).
It is well known that the properties of MDF panels generally depend on the resin and the fibre characteristics, as well as the process of bonding. Many studies have been devoted to investigating the contributions of various factors to MDF properties, including resin curing rate, catalyst content, fibre refining conditions, wood chemical characteristics, fibre treatment and hot-pressing. However, no study has reported on the effects of fibre ambient ageing.
Ambient ageing occurs during long-term storage of wood fibres under ambient conditions – at a temperature of about 15-35°C, with relative humidity (25-70%), no sunshine and ventilation.
The adsorption theory states that adhesion results from intimate intermolecular contact between two materials and involves surface forces that develop between the atoms in the two surfaces. Therefore, adhesion strength arises in the event of intimate contact between the adhesive and the adherend through a better wetting of the two. The adsorption theory implies that the adhesion properties in MDF will change if the amounts and species of polar groups on the fibre surfaces change after ambient ageing.
Long-term ambient ageing of wood fibres in the presence of oxygen and under moisture variations can also lead to changes in the fibre’s characteristics, such as the type and amount of chemical groups on the fibre surface, resin wettability to fibre, or the compatibility between wood fibre and resin. The molecular weight of wood components will also alter, resulting in different bonding properties.
Research at Northeast Forestry University, Harbin, China, has confirmed the effects of fibre ageing on the properties of MDF panels. One experience that enlightened this work came from two batches of MDF manufactured in the laboratory using the same fibres, UF resin and hot-pressing process, but manufactured about six months apart. Comparing the mechanical properties of the two batches showed the latter was much worse than the former.
Another two batches of MDF panels were manufactured using the same UF resin and hotpressing process, but one batch was made with fresh poplar wood fibres, supported by a local MDF plant, without spraying resin, while the other used ambient-aged fibres. The fresh fibre was kept at room temperature without sunlight, but with ventilation, for six months.
Modern techniques of fourier transform infrared spectroscopy, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), in combination with evaluation of panel properties, have been employed to investigate the effect of fibre-ambient ageing on the mechanical properties of MDF panels.
The main properties of MDF panels are presented in the chart overleaf. The modulus of rupture, modulus of elasticity and the internal bond strength of MDF made with ambient-aged wood fibres are much worse than those panels made with fresh wood fibre, and these properties are reduced, on average, by 29.7%, 22.0%, and 31.3%, respectively.
Interfibre bonds created via the adsorption and secondary attraction forces (penetration and/or interlocking) of UF resin with fibre surfaces provide good mechanical properties. Therefore, the surface properties of the fibres used will determine the mechanical properties of the final MDF.
In general, MDF panels prepared with the same fibres, UF resin and hot-pressing process will have similar properties if their densities and moisture contents are similar. The basic properties and DSC characteristics indicate that the UF resins used in the two batches of MDF panels are similar to each other, so the effects of the resin can be excluded in the variations in the mechanical properties indicated on the chart.
Young and old
The SEM observations (pictured below 1a and 1b) imply that the UF resin shows a better wettability on fresh fibres than on aged ones. The resin is uniformly distributed on the surface of fresh wood fibres as an even covering (1a). On the other hand, on the surface of aged fibres, it is agglomerated (1b).
Due to the good wettability of UF on fresh fibres, breakage of the bonded fibres in the MDF generally leads to many cohesive fibre failures, indicating that both the cohesive strengths of the cured resin and the resin-fibre interface strength is higher than the cohesive strength of the fibres (2a). However, in the MDF made with ambient-aged fibres, breakage frequently occurs on the resin-fibre interfaces, with few cohesive fibre failures (2b).
Fourier transform infrared spectroscopy analyses of fresh and aged fibre in the chart (see below, left) further confirm that the UF resin has worse wettability with aged fibres due to the chemical changes in the fibres after ambient ageing. Quantitative analysis indicates that the relative concentrations of hydroxyl groups (at about 3,412cm-1) and carbonyl groups (at about 1,733cm-1) in the ambient-aged fibres are three per cent lower and 144% higher, respectively, compared to those in fresh fibres. The obvious increase of carbonyl groups in the fibres confirms the ageing of wood fibres during ambient storage for six months.
However, long-term ambient ageing is a rather complicated process, which involves many physical and chemical changes in wood fibres, such as oxidation reactions, migration of wood extracts to the fibre surfaces and pH value. All these changes might have great effects on the bond strength of MDF panels.
The research also implies that the variations in panel properties resulting from ambient ageing of wood fibres should be taken into account in the cases where MDF panels are manufactured in the laboratory with fibres of different ages. This effect may also exist in other wood composites subjected to ambient ageing.
Northeast Forestry University, 26 Hexing Road, Xiangfang District, Harbin, China