PROCESS FOR BINDING LIGNOCELLULOSIC MATERIALS USING POLYISOCYANATE COMPOSITIONS

Abstract

A process for binding lignocellulosic material comprising the steps of a) bringing lignocellulosic material into contact with a methylene bridged polyphenyl polyisocyanate composition and b) subsequently allowing said material to bind wherein said polyisocyanate composition has a surface tension below or equal to 46 mN/m.

Claims

1. A process for binding lignocellulosic material comprising: a) bringing the lignocellulosic material into contact with a methylene bridged polyphenyl polyisocyanate composition having a surface tension below or equal to 46 mN/m; and b) subsequently allowing said material to bind.

2. The process according to claim 1, wherein the polar contribution of the surface tension is at least 6 mN/m.

3. The process according to claim 1, wherein said polyisocyanate composition comprises methylene bridged polyphenyl polyisocyanate modified by adding monol or diols, branched polyols, amines, wetting agents and/or surfactants.

4. The process according to claim 1, wherein monoalkyl ethers of polyethylene glycols are added to the methylene bridged polyphenyl polyisocyanate in an amount of at least 10 pbw.

5. The process according to claim 1, wherein the lignocellulosic material is primarily wood based and fibrous in nature.

6. The process according to claim 5, wherein the wood fibers are single wood fibers and/or bundles of such fibers.

7. The process according to claim 6, wherein the fibers have lengths of 7 mm or below and width/thickness of 0.005 to 0.2 mm and the fiber bundles are less than 2 cm long and less than 1 mm wide/thick.

8. The process according to claim 1, wherein the polyisocyanate composition is applied in such an amount as to give a weight ratio of polyisocyanate to lignocellulosic material in the range 0.1:99.9 to 20:80.

9. The process according to claim 1, wherein step b) involves pressing the lignocellulosic material at 120 C. to 300 C. and 2 to 6 MPa specific pressure.

10. (canceled)

Description

EXAMPLE 1: SURFACE TENSION OF POLYISOCYANATES

[0083] Four polyisocyanates were evaluated, polymeric MDI (Suprasec 5025), two emulsifiable MDIs, based on polymeric MDI with an increased amount of mono-functional polyol (methoxy polyethylene glycol of MW750) (MoPEG750), and a prepolymer of polymeric MDI and F442 (a polyether polyol having an ethylene oxide content of 73.5%, a functionality of 2.8 and molecular weight of 3500).

[0084] As depicted in Table 1 below the surface tension of polymeric MDI is the result of only dispersive contributions and the addition of MoPEG750 generates a polar component that increases with its loading. The total surface tension is also decreasing slightly with the increase of the polyol concentration.

TABLE-US-00001 TABLE 1 Measured values of surface tension and its dispersive and polar contribution for different isocyanates Surface Tension Dispersive Polar part Isocyanate Composition (mN/m) Part (mN/m) (mN/m) Suprasec 5025 Polymeric MDI 48 48 0 Suprasec 1042 Polymeric MDI + 47 46 1 3% MoPEG750 eMDI10 Polymeric MDI + 46 40 6 10% MoPEG750 Prepolymer Polymeric MDI + 44 38 6 10% F442

EXAMPLE 2: WETTABILITY BETWEEN WOOD FIBERS AND POLYISOCYANATE

[0085] Wood fibers were obtained from the Wood Institute of Dresden where, with the use of a pilot scale blow line, fibers could be made in a controlled manner. The fibers were produced at 140 m grinding plate distance, 3-4 minutes cooking time, 9 bar pressure (180 C.). Pine wood (Pinus Sylvestris) freshly cut (no more than one week before) was used.

[0086] The wood fibers were separated with the use of a Sieve Shaker, Analysette (Fritsh) provided with five sieves of different mesh size: 2.36, 1.4, 0.71, 0.355, 0.18 mm. The sieves were collocated on top of each other ordering them by the mesh size with the larger mesh size on the top. Approximately 1 g of wood fibers were dispersed manually on the top sieve and shaken for 5 minutes with an amplitude of 8 on 10 and permanent impulse. The fibers were collected on top of each sieve and the bottom cup yielding 6 fractions diversified by their size: >2.36 (large fiber bundles), 1.4-2.36 (medium fiber bundles), 0.71-1.4 (small fiber bundles), 0.355-.071 (truncated fiber bundles), 0.18-0.355 (single fibers), <0.18 mm (fibres and fines).

[0087] Wood fibers were extracted in a 500 ml glass jar using subsequent extractions in 4 different solvents: dichloromethane, toluene/ethanol (2/1), ethanol and acetone. The jar was filled with 5 g of wood fibers and the selected solvent. After three days the solvent was removed by filtration, the fibers were left to dry for one night at room temperature and then the following solvent was added.

[0088] The fibers were conditioned before analysis in a Weiss Climate Chamber for at least 3 days at a temperature of 22 C. and a relative humidity of 55%, resulting in a theoretical moisture content of the fibers of 10-12%.

[0089] The contact angle of all the six fiber fractions towards water and dimethyl sulfoxide was measured. An average of at least 5 measurements was taken for each point.

[0090] The Washburn method was used to perform contact angle measurements via a Kruss 100 Tensiometer. Measuring the contact angle with two liquids of which the surface tension and its polar and dispersive components are known, and through the combination of the Young equation and the Owens and Wendt equation the total surface free energy of the solid can be derived.

[0091] Based on the Washburn method and according to Young-Owens-Wendt equations the surface free energy of the various wood fiber fractions and its polar and dispersive part was calculated from measurements of contact angles towards water and DMSO. The results are depicted in FIG. 2.

[0092] The total free energy is slightly higher for fiber bundles than for single fibers. Single fibers are dominated by dispersive contribution while fiber bundles by their polar contribution.

[0093] Work of adhesion was used to evaluate the affinity between each wood fiber fraction and isocyanate resins. Higher the value, higher the affinity and hence the wetting.

[0094] The work of adhesion (WA) is the reversible work done in separation of unit area of solid to liquid interface. It can be used to evaluate the affinity between a solid and a liquid. The higher the value the higher the affinity hence the wetting. The general expression for WA can be complicated but Dupre and Fowkes have found an equation to be adequate at least for polymer-liquid systems (Wa=2(sd.Math.ld)+2(sp.Math.lp)1/2). In this equation the dispersive and polar contributions of both solid and liquid are taken into account. WA is surely additive and it can be divided into contributions of different forces of adhesion. Knowing the solid surface free energy, dispersive and polar contributions, and measuring the liquid surface tension, dispersive and polar contributions, the WA can be determined

[0095] The work of adhesion calculated for the various wood fiber fractions and the polyisocyanates of Table 1 is reported in FIG. 3.

[0096] Polymeric MDI (S5025) shows higher work of adhesion, hence affinity and wetting, with single fibers rather than fiber bundles. Its total dispersive character has higher affinity with single fibers because their surface energy is dominated by dispersive contributions. Fiber bundles are instead dominated by polar contributions hence the lower affinity with polymeric MDI.

[0097] With the increase of polarity of the isocyanate, the work of adhesion increases both for single fibers and fiber bundles although it increases more for fiber bundles that have a polar dominated character than the single fibers. This generates a leveling out of the differences in wetting between single fibers and fiber bundles with the use of a higher polar isocyanate as eMDI10.

[0098] This observation clearly shows that a polyisocyanate resin according to the invention having a reduced surface tension can accommodate the intrinsic difference in fiber types during industrial production. Since industrially all the different fiber types are present simultaneously, in different amount depending on the refining conditions, the possibility to use an isocyanate resin that wets all fibers similarly will be beneficial for the stability of the production and improves the board properties.