The present invention relates to a rigid polyurethane foam formulation comprising a polyisocyanate, one or more polyols, one or more blowing agents and optionally one or more additives. This invention also relates to a rigid polyurethane foam and a method to produce a rigid polyurethane foam.

W02020/076529A1 and W02020/076539A1 describe rigid polyurethane foam formulations.

Polyurethane foam as used in this application includes among others polyurethane foam (PUR), polyisocyanurate foam (PIR), polyisocyanurate-polyurethane-polyurea foam, urethane modified polyisocyanurate foam and polyurethane-polyurea foam. The present invention thus relates to rigid PUR foams, rigid PIR foams and other rigid polyurethane foams. Rigid polyurethane foams are typically closed-cell foams.

Additives which are often used in the production of polyurethane foams are one or more catalysts to start and/or to speed up the reaction between polyisocyanate and polyol, one or more surfactants, etc. The one or more blowing agents produce a cellular structure via a foaming process and/or fill up the cells.

Rigid polyurethane foams are used as thermal and/or sound insulating materials in walls, roofs, refrigerators, floor panels etc. These rigid polyurethane foams can for example be part of and/or form insulation panels. Some insulation panels only comprise rigid polyurethane foam, other insulation panels also comprise facers between which the rigid polyurethane foam is located. These facers can comprise aluminum foil, kraft paper, etc. These insulation panels are often provided with a tongue and groove to connect the panels to each other.

Rigid polyurethane foams can also be present in building elements, such as roof elements, wall elements or floor elements. These building elements for example comprise a base element and the rigid polyurethane foam is located against the base element or is (partially) enveloped by the base element. For example the base element can comprise ribs, wherein the rigid polyurethane foam is than located between the ribs. Such building elements can for instance be designed as an insulating self-supporting roof element. To this end, reference is made by way of example to EP0450731.

Floor panels which are built-up from different layers, can comprise one or more rigid polyurethane foam layers. To this end, a bottom layer of a floor panel can be a rigid polyurethane foam. Also other layers, such as one or more upper layers, can be rigid polyurethane foam layers. In floor panels, rigid polyurethane layers are mostly used for their sound insulating properties. An example of a floor panel comprising a rigid foam layer, such as a rigid polyurethane foam layer, is described in WO2017/046693.

A problem that arises with existing rigid polyurethane foams, is that the compressive strength can be a limiting factor. For example insulation panels used in construction, often get damaged during construction. Something can fall upon the panels, ladders are placed against the panels, fasteners to connect the panels to other constructing elements damage the panels. Also floor panels comprising a rigid polyurethane layer are sensitive to impact damage.

Another problem that arises with existing rigid polyurethane foams, is that the thermal insulation properties, such as the lambda value can be a limiting factor.

Current polyurethane foams are mainly fossil based and have a certain impact upon the environment.

It is therefore an object to of the invention to provide an alternative rigid polyurethane foam formulation, an alternative rigid polyurethane foam and an alternative method to produce a rigid polyurethane foam, which solves one or more of the abovementioned problems of existing polyurethane foams. The first aspect of the invention is a rigid polyurethane foam formulation comprising or consisting of a polyisocyanate, one or more polyols, one or more blowing agents and optionally one or more additives, wherein the one or more polyols comprise at least 5% by weight of lignin. Preferably the one or more polyols comprise at least 8% by weight of lignin, more preferably at least 10% by weight of lignin, even more preferably at least 12% by weight of lignin and most preferably at least 15% by weight of lignin. It was found that the higher the amount of lignin, the better the compressive strength of rigid foams made with the rigid polyurethane foam formulation. The compressive strength can for example be measured with the test method EN 826.

Since the one or more polyols comprise at least a certain amount of lignin, the compressive strength of the rigid foam obtained with this foam formulation, in comparison with a similar rigid foam obtained with a similar foam formulation not comprising lignin, is improved. Lignin molecules are relatively large molecules with a significant cross-linked structure and lignin molecules comprise aromatic rings. Lignin has free OH-groups that can react with a polyisocyanate to produce foam. During this reaction, the structure of the lignin is at least partly preserved such that the lignin contributes to the compressive strength of the obtained rigid foam. The result is a rigid polyurethane foam with an improved compressive strength in comparison with rigid polyurethane foams obtained with similar polyurethane foam formulations which do not comprise lignin.

A further advantage of such a foam formulation is that the cell size of the obtained rigid foam is reduced, such that not only the compressive strength is enhanced, but also the thermal insulation properties are improved.

It was also found that the foam structure obtained by using lignin in the foam formulation, is well able to hold blowing agents and to keep blowing agents, such as pentane, in the foam structure. The long-term insulation properties are good.

Another advantage is that lignin is a bio-based material. For example, lignin can be a waste product from the paper industry. The use of lignin to produce polyurethane foams, contributes to sustainability, as rigid polyurethane-based polyols are currently made from fossil-based polyols. For sustainability reasons, the higher the amount of lignin, the better. By using lignin to form rigid polyurethane foams, (a certain amount of) fossilbased polyols are replaced with bio-based polyols. Lignin is also not toxic.

A preferred embodiment is characterized in that the lignin comprises or is a technical lignin. Such lignin is readily available.

The polyisocyanate can comprise a diisocyanate such as methylene diphenyl diisocyanate (MDI). The polyisocyanate can comprise one type of polyisocyanate or can be a blend of different polyisocyanates.

Preferably, the polyisocyanate comprises or consists of pMDI (polymeric methylene diphenyl diisocyanate.

Preferred embodiments of the invention are characterized in that in the formulation the ratio by weight of polyisocyanate to the total of the one or more polyols is higher than 1.8, preferably higher than 2, more preferably higher than 2.5, even more preferably higher than 2.8. The higher values are preferred, as foam panels can be made with such formulations that have higher compressive strength and better flame retardancy.

The isocyanate index of the polyisocyanate is preferably between 100 and 500.

The one or more additives for example comprise one or more catalysts, one or more surfactants, one or more flame retardants, one or more nucleating agents, etc.

The one or more catalysts can comprise one or more of the following catalysts: tertiary amine-based catalysts, catalysts comprising potassium acetate, potassium-based metal catalysts, etc.

The surfactants are for example silicone surfactants or non-silicone surfactants. A preferred embodiment is characterized in that the foam formulation comprises one or more additives, wherein the one or more additives comprise a cyclic siloxane. Such embodiments have the benefit that the rigid foam made with such formulations has a reduced lambda, meaning that the rigid foam has better thermal insulation performance.

A preferred embodiments is characterized in that the formulation comprises a flame retardant, wherein the flame retardant comprises phosphor. Preferably, the flame retardant is selected from the list tris (1 -chi oro-2 -propyl) phosphate (TCPP) and tri ethyl phosphate (TEP).

Preferably the foam formulation comprises between 5 and 20 weight parts of flame retardant per 100 weight parts of the one or more polyols.

It is a benefit of the invention that thanks to the use of lignin in the foam formulation less flame retardant additive needs to be added for a same flame retardance behavior of a rigid foam made with such formulation compared to rigid foams made with the similar foam formulation but not comprising lignin.

It is a further benefit that the presence of the flame retardant additives reduces the viscosity of the foam formulation, which improves the processability of the foam formulation.

The blowing agents can be one or more pentanes and/or water. The pentanes can comprise one or more of the following: n-pentane, i-pentane, cyclopentane, c-pentane, etc.

The lignin is preferably an unmodified lignin. With an unmodified lignin can be meant a technical lignin. Technical lignins are generated as byproducts from the industry, for example from the pulping or cellulosic ethanol production. An example of an unmodified lignin is kraft lignin, which is obtained from the paper industry. The lignin is a waste product of the kraft pulping process. By using an unmodified lignin, no modification/alteration/depolymerization is needed, such that there is no or less additional energy and/or additional reaction steps needed and also no chemical modification is needed, to obtain lignin that can be used to produce rigid polyurethane foam. An unmodified lignin is thus preferably a lignin present in a commercially available lignin source, such as kraft lignin or soda lignin, which has not undergone additional modification/alteration/depolymerization. The lignin can for example be provided in powder form, for example a lignin powder comprising at least 90 % by weight of lignin, preferably at least 93% by weight of lignin, more preferably at least 95% by weight of lignin, and most preferably at least 98% by weight of lignin. The powder can also contain small amounts of water and/or ashes. For example, one could use commercially available lignin powder, and optionally dry the lignin powder, to have a lignin powder with the desired purity.

Preferably the lignin is added in the foam formulation via a lignin source, for example a lignin powder, wherein the lignin source preferably comprises at least 90% by weight of lignin. The purity of the lignin source is then at least 90%, preferably at least 93%, more preferably at least 95% and most preferably at least 98%. The purer the lignin source, less substances that could possibly hinder the reaction are present, thus the better the lignin will react with the other components in the foam formulation and the better the properties of the obtained rigid polyurethane foam.

More preferably, the lignin source comprises at most 5% by weight of water, preferably at most 3% by weight of water, more preferably at most 2% by weight of water, most preferably at most 1% by weight of water.

The one or more polyols can also comprise a fossil-based polyol such as a polyether polyol and/or a polyester polyol. The polyether polyols can be aliphatic polyether polyols. The polyester polyols can be aromatic polyester polyols. Preferably the viscosity of the fossil-based polyols is below 10000 m Pa*s at 25°C, for example between 5000 and 7500 m Pa*s at 25°C or for example below 3500 or below 3000 m Pa*s at 25°C. The one or more polyols of the foam formulation can for example comprise at most 95% by weight of fossil-based polyols, preferably at most 90%, more preferably at most 85%. This invention for example relates to a foam formulation comprising lignin and one or more polyester polyols. It was found that the combination of lignin and polyester polyols, provides rigid polyurethane foams with improved properties. A polyester polyol means a polyol having an ester functionality in its backbone. Preferably this invention relates to a polyurethane foam formulation, wherein the one or more polyols comprise lignin and polyester polyols, and wherein the one or more polyols comprise at least 90% by weight of the combination of lignin and one or more polyester polyols, preferably at least 95% by weight of the combination of lignin and one or more polyester polyols. Even more preferably the one or more polyols consist of lignin and polyester polyols. The polyester polyols can comprise aromatic polyester polyols.

In a specific embodiment the lignin comprises or consists of a kraft lignin. Preferably the lignin is a kraft lignin, more preferably a kraft soft wood lignin. Kraft lignin is a waste product of the paper industry and is not a source of food/feed. By using such a lignin, a waste product gets upgraded to a useful raw material.

The lignin can also be or comprise a soda lignin, an acid lignin, an organosolv lignin, a lignosulfonate, hydrolysis lignin, and/or a modified lignin.

The foam formulation can comprise one type of lignin, for example a kraft lignin obtained from one supplier. The foam formulation can also comprise two or more types of lignin, for example a kraft lignin and another lignin such as a soda lignin. The foam formulation can also comprise a first kraft lignin of a first supplier and a second kraft lignin from the same or another supplier. The lignin can thus be a lignin blend.

Preferably the average molecular weight according to mass of the lignin is at most 10000 g/mol, preferably at most 8000 g/mol and more preferably at most 7000 g/mol.

Preferably, the average molecular weight according to mass of the lignin is higher than 4000 g/mol, more preferably higher than 5000 g/mol. For example the average molecular weight according to mass can be approximately between 5000 and 8000 g/mol, such as 6000 g/mol. The average molecular weight according to mass can also be between 2000 and 4000 g/mol, or lower. Lignin is a highly heterogeneous polymer. Also, as mentioned before, the lignin can be a lignin blend. However, the best results were obtained when the average molecular weight according to mass of the lignin is not too high. The compressive strength of the foams obtained with this foam formulation is significantly high and this while the reactivity of the lignin is sufficiently high; therefore the lignin can react well with polyisocyanate.

Lignin is normally derived from a handful of precursor lignols that crosslink in diverse ways. The lignols that crosslink can be of three main types: coniferyl alcohol (4-hydroxy- 3-methoxyphenylpropane), sinapyl alcohol (3, 5 -dimethoxy -4-hydroxyphenylpropane), and paracoumaryl alcohol (4-hydroxyphenylpropane). The lignin of the foam formulation is preferably derived from a significant amount of coniferyl alcohol. Such lignin can be derived from the paper industry.

In a very preferred embodiment, the one or more additives comprise one or more viscosity reducing agents. With the aid of viscosity reducing agents, it was found that higher amounts of lignin can be used in the foam formulation. This means that with the aid of the viscosity reducing agents, the one or more polyols can comprise at least 10% by weight of lignin, and preferably at least 15% by weight of lignin, and this as the foam can be made in a good manner and in an industrial manner. With the aid of viscosity reducing agents, high amounts of kraft lignin can be used for the foam formulation. In industry, polyisocyanate is mixed with a polyol blend comprising the one or more polyols, the one or more blowing agents and optionally the one or more additives. A problem that occurred when using higher amounts of lignin, is that it became difficult to mix the polyol blend and the polyisocyanate with standard industrial machines. To contact the polyol blend with the polyisocyanate, these standard industrial machines can comprise a spray apparatus, a mix head with or without a static mixer, a spray head, etc. It was found that with the aid of viscosity reducing agents, even when high amounts of lignin are used, standard industrial machines can be used to form the rigid polyurethane foam and the polyols react in a good manner with the polyisocyanate. The viscosity reducing agents can also have additional interesting properties for the foam formulation. For example the viscosity reducing agents can also have flame retardant and/or blowing/foaming properties.

Surprisingly it was also found that by using viscosity reducing agents, normal amounts of catalysts can be used. Rigid polyurethane foam formulations often comprise one or more catalysts, for example a weight ratio of 1 to 5 upon 100 for catalysts upon polyols. Lignin has a lower reactivity than the fossil-based polyols which are normally used to produce rigid polyurethane foams. One could therefore assume that higher amounts of catalyst are required, when using lignin. However, with the aid of viscosity reducing agents, it is not necessary to use higher amounts of catalyst and the amounts described above can be used. However, one could choose to use higher amounts of catalyst.

Preferably the weight ratio of the one or more viscosity reducing agents upon the polyols is between 0.05 and 0.25, preferably between 0.1 and 0.2. This way, the viscosity reducing agents are not too numerous, such that they do not hinder the reaction between polyisocyanate and the polyols, but are still sufficiently present. There can be for example 100 parts by weight of polyols and between 10 and 20 parts by weight of viscosity reducing agents, preferably between 14 and 16 parts by weight of viscosity reducing agents.

Preferably the one or more viscosity reducing agents comprise a cyclic organic viscosity reducing agent, more preferably selected from the group consisting of propylene carbonate, ethylene carbonate, methyl-2-pyrrolidinone and caprolactone. Since the viscosity reducing agent comprises a cyclic molecule, this contributes to a higher compressive strength of rigid polyurethane foams made with such polyurethane foam formulation. These viscosity reducing agents have no negative influence upon the foam production. One could use between 1 and 10 parts by weight of a cyclic organic viscosity reducing agent, for example 3 or 5 parts by weight, and 100 parts by weight of polyols.

A preferred embodiment is characterized in that the foam formulation comprises a viscosity reducing agent, wherein the viscosity reducing agent is selected from one or more than one of the list monoglycidyl ether (MGE), mono ethylene glycol (MEG) - preferably bio-MEG - , DEG (diethylene glycol), poly ethylene glycol or 5-(tetra deyloxy)-2furoic acid (TOFA).

Preferably the one or more viscosity reducing agents comprise a phosphate, such as triethyl phosphate (TEP). Triethyl phosphate is a non-halogenated phosphate that also has other properties, such as flame-retardant properties. Therefore not only the viscosity is brought to the desired level, also other desired properties are given to the foam formulation. Other phosphates that can be used are for example tris (1 -chi oro-2 -propyl) phosphate (TCPP). One could use between 5 and 15 parts by weight of TEP and 100 parts by weight of polyols. One could use between 10 and 20 parts by weight of TCPP and 100 parts by weight of polyols. One could also use a two or more phosphates

More preferably, the weight ratio of the cyclic organic viscosity reducing agent upon the phosphate is between 0.2 and 0.7.

In an embodiment, the foam formulation comprises NaOH and/or KOH. With the aid of NaOH and/or KOH, the dispersion of the lignin in the polyol blend is very good. The polyol blend comprises the one or more polyols, the one or more blowing agents and the one or more additives, wherein the additives than comprise at least NaOH and/or KOH.

In a specific embodiment the one or more polyols comprise a polyester polyol, wherein the polyester polyol preferably has a hydroxyl number between 150 and 800 mg KOH/g, more preferably between 250 and 600 mg KOH/g, more preferably below 400 mg KOH/g, even more preferably below 300 mg KOH/g. Preferably the polyester polyol is an aromatic polyester polyol. Together with the lignin, this ensures a rigid polyurethane foam with a good compressive strength. The one or more polyols can comprise a mixture of different polyester polyols.

Preferably, the one or more polyols comprise at least 50 percent by weight of polyester polyol. A preferred embodiment is characterized in that the one or more polyols comprise polyester polyol, wherein the polyester polyol is an aromatic polyester polyol - preferably wherein the one or more polyols comprise at least 50 weight percent of aromatic polyester polyol wherein the polyester polyol preferably has a hydroxyl number between 150 and 800 mg KOH/g, more preferably below 400 mg KOH/g, even more preferably below 300 mg KOH/g.

The use of aromatic polyester polyols provides rigid insulation foams that have better flame retardancy properties and higher compressive strength.

A preferred embodiment is characterized in that the one or more polyols comprise one or more fossil based polyols, such as a polyester polyol and/or a polyether polyol, preferably an aromatic polyester polyol, wherein the polyester polyol preferably has a hydroxyl number between 150 and 800 mg KOH/g, more preferably below 400 mg KOH/g, even more preferably below 300 mg KOH/g.

In embodiments wherein polyester polyols - e.g. aromatic polyester polyols - are used, preferably that the viscosity of the polyester polyols is below 10000 mPa*s at 25°C. Such embodiments provide better processability of the foam formulation.

In a preferred embodiment the foam formulation comprises water, wherein the foam formulation preferably comprises at most 6 parts of water per 100 parts of polyol, more preferably at most 5 parts of water per 100 parts of polyol, and even more preferably at most 3 parts of water per 100 parts of polyol.

Water in a rigid foam formulation can be used as a blowing agent. Therefore small amounts of water as such are often added to rigid foam formulations. For example between 0.1 and 1 part by weight of water is added upon 100 parts by weight of one or more polyols. If lignin is added to the rigid foam formulation with the aid of a lignin source comprising water, such as lignin powder comprising at most 10% by weight of physical water, preferably at most 8% weight of water, then less additional water as such can be added to the foam formulation or no additional water could be added. The water present in the lignin source can thus act as a blowing agent. Too much water can hinder the foam formation, such that the lignin source preferably is as pure as possible. Lignin sources with a purity of at least 95%, preferably at least 98%, are preferred.

The weight ratio of polyisocyanate upon the total of the one or more polyols is preferably between 1.4 and 3.2, more preferably between 1.9 and 3.2 or between 1.5 and 1.8; more preferably between 1.4 and 2.4, more preferably between 1.6 and 2.2. This way the optimal ratio between the polyisocyanate and the polyols is present.

If the weight percentage of lignin upon the total weight of one or more polyols is higher than 10% and/or the purity of the lignin source is higher than 95%, the weight ratio of polyisocyanate upon the polyols is preferably higher than 1.7 and more preferably higher than 1.8.

A preferred rigid polyurethane foam formulation is characterized in that the one or more polyols comprise polyols derived from vegetable oil (e.g. derived from rape seed oil); more preferably wherein the one or more polyols comprise at least 3 weight percent - and preferably less than 15 weight percent - polyols derived from vegetable oils. Such embodiments have the benefit that the amount of bio-based material in the rigid polyurethane foam formulation is further increased.

For example the one or more polyols can comprise at least 5% by weight of unmodified lignin, preferably at least 10% by weight of unmodified lignin, more preferably at least 15% by weight of unmodified lignin, and further can comprise at least 5% by weight of modified lignin, preferably at least 10% by weight of modified lignin, more preferably at least 15% by weight of modified lignin. Here the one or more polyols can comprise more than 30% by weight of the combination of unmodified lignin and modified lignin, preferably more than 50% by weight of the combination of unmodified lignin and modified lignin.

A preferred rigid polyurethane foam formulation is characterized in that the one or more polyols comprise modified lignin, preferably wherein the one or more polyols comprises at least 1% by weight of modified lignin, more preferably at least 5% by weight of modified lignin, more preferably at least 10% by weight of modified lignin, more preferably at least 15% by weight of modified lignin, more preferably at least 20% by weight of modified lignin, more preferably at least 25% percent by weight of modified lignin, more preferably at least 30% by weight of modified lignin. The addition of modified lignin allows to increase the amount of lignin products used compared to when unmodified lignin is used. The amount of lignin based polyols is even more increased when using a combination of unmodified lignin and modified lignin.

A preferred rigid polyurethane foam formulation is characterized in that the one or more polyols substantially do not comprise unmodified lignin, but do comprise modified lignin.

A preferred rigid polyurethane foam formulation is characterized in that the one or more polyols comprise

- at least 1 percent by weight, and preferably at least 5 percent by weight, and preferably at least 10 percent by weight of unmodified lignin, e.g. Kraft lignin or soda lignin; and

- at least 1 percent by weight, and at least 5 percent by weight, and preferably at least 10 percent by weight, more preferably at least 10 percent by weight, more preferably at least 15 percent by weight, more preferably at least 20 percent by weight, more preferably at least 25 percent by weight, more preferably at least 30 percent by weight of modified lignin.

The amount of lignin based raw material - and thus of bio-based raw material - can be increased when using a combination of modified lignin and unmodified lignin.

Preferably, the percentage by weight of modified lignin is higher than the percentage by weight of unmodified lignin. Such embodiments provide foam formulations that can be processed more easily, as the viscosity of the total of the one or more polyols is lower.

A preferred rigid polyurethane foam formulation in which a combination of unmodified lignin and modified lignin is used, is characterized in that the one or more polyols comprise at least 50 percent by weight of polyester polyols, preferably of aromatic polyester polyols. Such embodiments are preferred because such embodiments allow to achieve optimum rigid foam properties.

A preferred embodiment in which modified lignin is used is characterized in that the modified lignin is a functionalized lignin. Functionalized lignin allow to achieve better properties of the rigid foam made with the rigid foam formulation.

In a preferred example, the modified lignin comprises aliphatic chains comprising a terminating hydroxyl group (which is called an aliphatic hydroxyl group), more preferably wherein substantially all lignin phenolic hydroxyl groups have been transformed into aliphatic hydroxyl groups. The aliphatic hydroxyl group are more easily accessible and reactive towards the isocyanate, resulting in better reactivity of the modified lignin, which is beneficial for the properties of rigid foam made with such rigid foam formulation.

The functionalized lignin can comprise or consist of a lignin functionalized by means of a reaction of lignin with a cyclic carbonate. Preferably, the cyclic carbonate is selected from the list of ethylene carbonate, propylene carbonate, vinyl ethylene carbonate, glycerol carbonate. Such functionalization of lignin provides hydroxyl groups which are more easily accessible and reactive towards the isocyanate, resulting in better reactivity of the modified lignin, which is beneficial for the properties of rigid foam made with such rigid foam formulation.

The functionalized lignin can comprise or consist of a lignin functionalized by oxypropylation or by hydroxy oxypropylation. Such functionalization of lignin provides hydroxyl groups which are more easily accessible and reactive towards the isocyanate, resulting in better reactivity of the modified lignin, which is beneficial for the properties of rigid foam made with such rigid foam formulation.

The functionalized lignin can comprise or consist of a lignin that has been depolymerized, preferably chemically or catalytically depolymerized or depolymerized by means of pyrolysis. Preferably, the average molecular weight according to mass of the depolymerized lignin is lower than 4000 g/mol, more preferably lower than 3000 g/mol, more preferably lower than 2000 g/mol, and preferably higher than 1000 g/mol. Depolymerization of lignin can be performed as described in WO2021/204790. Depolymerization increases the reactivity of the lignin, resulting in improved rigid foam products made with the rigid foam formulations comprising depolymerized lignin. Depolymerized lignin also has the benefit that its viscosity is lower, resulting in improved processability.

The functionalized lignin can comprise or consist of a lignin that has been extracted from a lignin, preferably from Kraft lignin or from soda lignin. Preferably, the average molecular weight according to mass of the extracted lignin is less than 2500, and preferably less than 2000. Extracted lignin is beneficial thanks to its lower average molecular weight according to mass. The lower molecular weight results in better compatibility in the foam formulation, lower viscosity and higher reactivity. The result is improved processability of the foam formulation and improved rigid foam properties.

The functionalized lignin can comprise or consist of a lignin that has been chemically modified and/or epoxidized and/or hydrolyzed and/or hydrolyzed and/or transesterified. The result is a lignin that has improved reactivity to the isocyanate of the rigid polymer foam formulation resulting in better rigid foam properties.

A preferred rigid polyurethane foam formulation comprising modified lignin is characterized in that the modified lignin is a lignin having an average molecular weight according to mass less than 2000 g/mol, preferably less than 1500 g/mol, and preferably more than 800 g/mol. Such modified lignin can e.g. be obtained by means of extraction or depolymerization of lignin, e.g. of kraft lignin, of biorefinery-derived lignin, of organosolv lignin or of hydrolysis lignin.

Such modified lignin with lower average molecular weight according to mass has higher reactivity of the lignin, resulting in improved rigid foam products made with the rigid foam formulations comprising such modified lignin. The lignin with lower molecular weight also has the benefit that its viscosity is lower, resulting in improved processability.

The second aspect of the invention is a rigid polyurethane foam made from a rigid polyurethane formulation according to any embodiment of the first aspect of the invention. Preferably, the polyurethane foam comprises closed cells. Closed cell foams provide optimum thermal insulation properties.

This invention thus also relates to a rigid polyurethane foam formed from the rigid foam formulation described in the first aspect of the invention. The rigid polyurethane foam is thus a lignin based rigid polyurethane foam with a good compressive strength and this while the desired insulation properties are obtained. For example an initial lambda value lower than 0.025 W/m*K is possible. The lambda values can for example be measured with the test method EN 12667 or EN 12939. The initial lambda value of the rigid polyurethane foam can be lower than 0.025 W/m*K, preferably lower than 0.023 W/m*K, more preferably lower than 0.022 W7m*K.

The advantages, preferred embodiments and specific embodiments that are described for the rigid polyurethane foam formulation, also apply to the rigid polyurethane foams.

The compressive strength value of the rigid polyurethane foams is preferably higher than 85 kPa. The compressive strength can for example be higher than 100 kPa or higher than 110 kPa. The test method EN 826 can be used to measure the compressive strength.

The third aspect of the invention is a rigid polyurethane foam, comprising the reaction product of a composition comprising a polyisocyanate, one or more polyols, one or more blowing agents and optionally one or more additives, wherein the foam comprises coniferyl alcohol units. Optionally, the rigid polyurethane foam of the third aspect of the invention is a rigid polyurethane foam according to any embodiment of the second aspect of the invention. With the aid of coniferyl alcohol units, the foam has a good compressive strength and good insulation properties. The presence of coniferyl alcohol units can be obtained with the aid of lignin. For example the one or more polyols can comprise at least 5% by weight of lignin. The one or more polyols, and therefore also the lignin, has reacted with the polyisocyanate to form the foam, such that the foam will have lignin-based structures. We can therefore speak of lignin based rigid polyurethane foams. The lignin-based structure offers the foam the desired compressive strength and/or insulating properties. Such a foam can be made from a rigid polyurethane foam formulation as in any embodiment of the first aspect of the invention.

The fourth aspect of the invention is a building element comprising a rigid polyurethane foam as in any embodiment of the second aspect of the invention and/or as in any embodiment of the third aspect of the invention.

The advantages and preferred and specific embodiments described for the rigid polyurethane foam formulation and the rigid polyurethane foams also apply to this building element. This building element can be an insulation panel, a wall element, a roof element, a floor panel, etc. By using rigid polyurethane foam made from a foam formulation comprising lignin, these building elements have the desired compressive strength and thus the desired impact resistance, have the desired insulation properties and have an improved sustainability.

This building element can for example be an insulation panel comprising only a rigid polyurethane foam as described above or comprising a rigid polyurethane foam as described above and at least one facer and preferably two facers. For the latter the rigid polyurethane foam is preferably embedded between the facers such that the insulation panel preferably comprises consecutively a first facer, the rigid polyurethane foam and a second facer. The first and the second facer can have the same composition or a different composition. Since the compressive strength of lignin based rigid polyurethane foams is good, such insulation panels are very well suited to be provided with tongue and groove to connect these insulation panels with each other. Surprisingly, it was further found, that the insulation panels comprising at least one facer, can be made with relatively high amounts of lignin. For example it can be made with one or more polyols comprising at least 10% by weight of lignin, preferably at least 15% by weight of lignin. The insulation panels comprising at least one facer are thus very suitable for lignin based polyurethane foams.

Surprisingly, it was also found that the insulation panels comprising two facers, with the rigid polyurethane foam embedded between the two facers and connected to the two facers, can have a thickness of more than 5 cm, more than 8 cm and even more than 10 cm, and this while the rigid foam is substantially uniform over the entire thickness. This because the two facers will ensure the uniformity of the rigid foam.

The facers can be one-layer facers or multilayer facers, for example facers comprising, 3, 5, 7 or more layers. The facers can have one or more kraft paper layers, one or more aluminum layers, one or more plastic layers such as PE layers, PET layers, etc.

A preferred building element is characterized in that the building element is an insulation panel comprising two facers in between which the rigid polyurethane foam - preferably the rigid polyurethane foam is a rigid polyisocyanurate foam - is embedded. More preferably the facers comprise or consist of an aluminum sheet, more preferably the aluminum sheet has a thickness of more than 5 micrometer, more preferably the aluminum sheet has a thickness of more than 40 micrometer, more preferably of less than 75 micrometer, even more preferably of less than 60 micrometer. The aluminum sheet provides for gas tightness of the insulation panel, beneficial for the durability of the thermal insulation properties of the insulation panel.

Preferably, the facers comprise or consist of a multi-layered laminate; wherein the multilayered laminate comprises a polymer film. Preferably, the polymer film provides the surface of the insulation panel. Preferably the polymer film is a polyethylene film or a polyvinylidene film. Preferably the multi-layered laminate comprises one or more aluminum layers. The aluminum layers are provided for gas tightness of the insulation panel. Preferably the aluminum layers have a thickness more than 5 micrometer and preferably of less than 20 micrometer. The multi-layered laminate comprises one or more than one paper layers, preferably Kraft paper layers. The presence of the paper layers provides rigidity to the facer.

Insulation panels comprising facers comprising or consisting such multi-layer laminates have optimized properties.

A preferred building element is characterized in that the building element is an insulation panel. The insulation panel is rectangular and has a certain thickness, wherein the insulation panel comprises at least at two opposite edges coupling parts for coupling the insulation panel to another such insulation panel at their corresponding opposite edges comprising the coupling parts. Preferably, the coupling parts comprise at the first edge of the two opposite edges a tongue and at the second edge of the two opposite edges a corresponding groove.

Insulation panels having coupling parts - and especially insulation panels having tongue and groove coupling parts - can be installed in such a way that the insulation properties at the joint between insulation panel are not affected to a large extent.

A preferred building element is characterized in that the building element is an insulation panel comprising the rigid polyurethane foam and having a thickness; wherein the rigid polyurethane foam comprises closed cells, wherein the closed cells are anisotropic in dimension and have a largest cell distance, wherein the largest cell distance is substantially oriented in the thickness direction of the insulation panel. Such embodiments provide insulation panels that have even increased compression strength in the thickness direction of the insulation panel.

The fifth aspect of the invention is a thermal insulation characterized in that the thermal insulation comprises two building elements according to the fourth aspect of the invention, wherein the two building elements are insulation panels, wherein the two insulation panels are coupled at their corresponding opposite edges by means a tongue and groove connection. At least at one side of the thermal insulation a sealing tape is applied on the surface of the insulation panels at the corresponding opposite edges coupled by means of the tongue and groove connection. The sealing tape covers the joint between the two insulation panels at the corresponding opposite edges coupled by means of the tongue and groove connection. Such thermal insulation provides better durability of the insulation properties at the joint between insulation panels.

The sixth aspect of the invention is a method to produce a rigid polyurethane foam using a rigid polyurethane foam formulation as in any embodiment of the first aspect of the invention, characterized in that the method comprises the steps:

- mixing the one or more polyols, the one or more blowing agents and the one or more optional additives of the rigid polyurethane formulation;

- adding the polyisocyanate of the rigid polyurethane formulation, thereby forming a polyisocyanate and polyol blend. This way, a rigid polymer foam with good characteristics can be made.

The seventh aspect of the invention is a method to produce a rigid polyurethane foam comprising the following steps:

- providing a polyisocyanate;

- making a polyol blend comprising one or more polyols, one or more blowing agents and optionally one or more additives;

- contacting the polyisocyanate with the polyol blend, wherein the one or more polyols comprise at least 5% by weight of lignin.

Preferably the one or more polyols comprise at least 8% by weight of lignin, more preferably at least 10% by weight of lignin, even more preferably at least 12% by weight of lignin and most preferably at least 15% by weight of lignin. When the polyisocyanate is contacted with the polyol blend, the polyisocyanate and the polyols start to react and the foaming starts, such that a rigid polyurethane foam is formed. With the aid of the method of the sixth and seventh aspect of the invention a rigid polyurethane foam according to the invention, as described above, is obtained. Preferably a rigid polyurethane foam formulation as described above is used to perform this method. The advantages, preferred embodiments and specific embodiments described for the rigid polyurethane foam formulation and the rigid polyurethane foams, also apply to the building elements according to the invention.

Preferably to make the polyol blend, a lignin source, such as a lignin powder is provided, wherein preferably the lignin source comprises at least 90% by weight of lignin. The purity of the lignin powder is then at least 90%. The powder can be dispersed in a fossilbased polyol which is provided in liquid form. For example polyester polyols for rigid foam production, can be provided in liquid form at room temperature. The one or more polyols of the method can for example consist of lignin and polyester polyols, wherein the polyester polyols are supplied in liquid form and the lignin powder can then be dispersed in the polyester polyols and this to form the polyol blend. For example the lignin powder can first be dispersed in the polyester polyols and then optionally the polyols can be mixed with the one or more blowing agents and the one or more additives to form the polyol blend. The addition of lignin powder and the other components to the polyester polyol can also be done simultaneously or in a specific order. For example the lignin powder can first be added in the fossil-based polyols and for example be dispersed at room temperature. The mixing time can for example be between 5 and 10 minutes. Then the other components of the polyol blend can be added to the polyols. The mixing can be done at room temperature and/or for example the mixing time can be between 2 and 5 minutes.

The lignin powder preferably has 95% purity and even more preferably 98% purity. The impurities of the lignin powder, being the components that are not lignin, can be water and ashes. For example a lignin powder of 95% purity can have for example about 3 weight percent water.

In a very preferred embodiment, the one or more polyols comprise a fossil-based polyol, wherein the fossil-based polyol is provided in liquid form, and wherein, to make the polyol blend, the lignin source is dispersed in the fossil-based polyol. By dispersing the lignin in the fossil-based polyol, the distribution of the lignin in the polyol blend is good, such that the obtained foam is more or less homogeneous. Before dispersing the lignin source in the fossil-based polyol, the lignin source can be submitted to a drying step to remove some water of the lignin source. With the aid of the drying step the water content of the lignin source can be reduced, such that the purity of the lignin source can be increased. Preferably, if the lignin source is a lignin powder, the water content of the lignin powder after the drying step is at most 3% by weight, preferably at most 1% by weight and more preferably at most 1% by weight. Preferably the purity of the lignin after the drying step is at least 96%, more preferably a least 98% and even more preferably at least 99%.

Preferably the polyol blend comprises one or more viscosity reducing agents, wherein the dynamic viscosity of the polyol blend measured at 20°C is preferably below 10000 mPa*s, more preferably at most 7500 mPa*s. With the aid of the viscosity reducing agents, the lignin is well distributed in the polyol blend and this while a significant amount of lignin can be dispersed in the polyol blend. The one or more polyols can for example comprise at least 10% by weight of lignin, even 15% and this while the polyol blend is capable of reacting with the polyisocyanate.

Surprisingly it was also found that by using viscosity reducing agents, normal amounts of catalysts can be used.

A mixing device can be used to make the polyol blend. Mixing can be performed at room temperature or at another temperature. Preferably the mixing speed is at most 500 revolutions per minute (RPM), for example 300 RPM or less. When the mixing speed is not too high, the bubble formation is low, such that after making the polyol blend, the polyol blend can be rapidly contacted with the polyisocyanate. Also preferably the mixing time is less than 2 hours, preferably less than 1 hour, for example 30 minutes or less, such that the production speed of the rigid foam is sufficiently high. The polyol blend and the polyisocyanate are preferably contacted at temperatures between 20 °C and 35°C.

In a specific embodiment, the method relates to the production of an insulation panel comprising a first facer and preferably a second facer. If two facers are present, the rigid polyurethane is embedded between the facers. The rigid polyurethane foam is produced as described above. Preferably, after contacting the polyol blend with the polyisocyanate to form the polyurethane foam, the contacted polyol blend and polyisocyanurate, when the polyol blend and the polyisocyanurate are still reactive, thus before and/or during the foaming, is sprayed/poured upon the first facer, after which preferably the second facer is applied upon the foam. During production this first facer is preferably located at the bottom and the second facer is located on top, such that from the top to the bottom is present, the second facer, the rigid polyurethane foam and the first facer. During the production, when the contacted polyol blend and polyisocyanurate is sprayed/poured upon the first facer, the first facer can have a temperature above room temperature, for example a temperature of at least 30°C and at most 120°C, preferably at least 50°C and at most 90°, for example 70°C. Also the second facer can have a temperature above room temperature. This increases the reactivity of the contacted polyol blend and polyisocyanurate. With the aid of the facer or facers, high amounts of lignin can be used to produce the rigid polyurethane foam. The temperature of the first facer can for example be obtained by using a heated spray table upon which the first facer is displaced when the contacted polyol blend and polyisocyanurate is sprayed/poured upon the first facer. Other ways to heat the facers are also possible.

A preferred method is characterized in that the first facer and the second facer comprise or consist of an aluminum sheet, more preferably wherein the aluminum sheet has a thickness of more than 5 micrometer, more preferably wherein the aluminum sheet has a thickness of more than 40 micrometer, and more preferably of less than 75 micrometer, even more preferably of less than 60 micrometer.

The aluminum sheet provides for gas tightness of the insulation panel, beneficial for the durability of the insulation properties of the insulation panel. Preferably, the facers used in the method according to the invention comprise or consist of a multi-layered laminate; wherein the multi-layered laminate comprises a polymer film. Preferably the polymer film is provided on the outer surface of the multi-layered laminate. Preferably the polymer film is a polyethylene film or a polyvinylidene film. Preferably the multi-layered laminate comprises one or more aluminum layers. The aluminum layers are provided for gas tightness of the insulation panel. Preferably the aluminum layers have a thickness more than 5 micrometer and preferably of less than 20 micrometer. The multi-layered laminate comprises one or more than one paper layers, preferably Kraft paper layers. The presence of the paper layers provides rigidity to the facer.

Insulation panels comprising facers comprising or consisting a such multi-layer laminates have optimized properties.

The eighth aspect of the invention is a rigid polyurethane foam formulation comprising a polyisocyanate, one or more polyols, one or more blowing agents and optionally one or more additives, wherein the one or more polyols comprise at least 1% by weight of unmodified lignin and comprise at least 1% by weight of modified lignin. The eighth aspect also relates to a rigid polyurethane foam formed from the rigid polyurethane foam formulation and a method to form such a foam.

In this aspect, the foam formulation does not only comprise unmodified lignin, but also modified lignin. The modified lignin can be chemically modified and/or depolymerized lignin, for example one or more of the following modified lignin: epoxidized lignin, hydrolyzed lignin, transesterified lignin, oxypropylated lignin, catalytically depolymerized lignin and/or chemically depolymerized lignin and/or depolymerized lignin by pyrolysis, etc. The modified lignin can comprise one type of modified lignin, but can also be a blend of two or more types of modified lignin. Drying of lignin powder, such as kraft lignin powder, is not considered as a modification according to the invention. When lignin powder is dried to remove excess water, the dried lignin powder still comprises unmodified lignin. Lignin powder derived from industrial processes, such as kraft lignin powder, is viewed as a powder comprising unmodified lignin. The modified lignin can be derived from the modification of lignin provided in the form of lignin powder.

Preferably the one or more polyols comprise at least 15% by weight of the combination of unmodified lignin and modified lignin, more preferably at least 20% by weight of the combination of unmodified lignin and modified lignin, most preferably at least 25% by weight of the combination of unmodified lignin and modified lignin. The higher the amount of the combination of unmodified lignin and modified lignin, the more bio-based polyols are present. For example the one or more polyols could also comprise fossilbased polyols such as polyether polyols and/or polyester polyols. The polyester polyols can be aromatic polyester polyols. For example the one or more polyols consist of lignin, modified lignin and polyester polyols.

Preferably, for the one or more polyols, the amount of unmodified lignin by weight, is higher than the amount of modified lignin by weight. The modification of lignin requires energy and/or chemicals, such that a high amount of unmodified lignin is desirable.

The method according to the eighth aspect of the invention is a method to produce a rigid polyurethane foam comprising the following steps:

- providing a polyisocyanate;

- making a polyol blend comprising one or more polyols, one or more blowing agents and optionally one or more additives;

- contacting the polyisocyanates with the polyol blend, wherein the one or more polyols comprise at least 1% by weight of unmodified lignin and at least 1% by weight of modified lignin.

It is noted that the advantages, preferred embodiments and specific embodiments of the disclosure described before the disclosure of the eighth aspect of the invention, also apply to the eighth aspect of the invention, as long as they are not contrary to each other. A ninth aspect of the invention is a rigid polyurethane foam formulation comprising a polyisocyanate, one or more polyols, one or more blowing agents and optionally one or more additives, wherein the one or more polyols comprise at least 5% by weight of modified lignin. The ninth aspect also relates to a rigid polyurethane foam formed from the rigid polyurethane foam formulation and a method to form such a foam. Here the foam formulation can for example not comprise a significant amount of unmodified lignin. Then the lignin of the foam formulation substantially comprises modified lignin.

The modified lignin can be chemically modified lignin, for example one or more of the following types of modified lignin: epoxidized lignin, hydrolyzed lignin, transesterified lignin, oxypropylated lignin, etc. The modified lignin can be depolymerized lignin such as catalytically depolymerized lignin and/or chemically depolymerized lignin. The modified lignin can comprise one type of modified lignin, but can also be a blend of two or more types of modified lignin. Drying of lignin powder is not considered as a modification according to the invention. When lignin powder is dried to remove excess water, the lignin powder still comprises unmodified lignin. Lignin powder derived from industrial processes, such as kraft lignin powder, is viewed as a powder comprising unmodified lignin. The modified lignin can be derived from the modification of lignin, that is provided in the form of lignin powder. When lignin is modified, it is possible that a small amount of lignin will not get modified, such that we can state that the lignin of the foam formulation substantially comprises modified lignin.

Preferably the one or more polyols comprise at least 5% by weight of modified lignin, more preferably at least 10% by weight of modified lignin, even more preferably at least 15% by weight of modified lignin, most preferably at least 20% by weight of modified lignin. The higher the amount of modified lignin, the more bio-based polyols are present. For example the one or more polyols could also comprise fossil-based polyols such as polyether polyols and/or polyester polyols. The polyester polyols can be aromatic polyester polyols. For example the one or more polyols consist of modified lignin and polyester polyols. The method according to the ninth aspect of the invention is a method to produce a rigid polyurethane foam comprising the following steps:

- providing a polyisocyanate;

- making a polyol blend comprising one or more polyols, one or more blowing agents and optionally one or more additives;

- contacting the polyisocyanates with the polyol blend, wherein the one or more polyols comprise at least 1% by weight of modified lignin; and preferably at least 5% by weight of modified lignin.

It is noted that the advantages, preferred embodiments and specific embodiments of the disclosure described before the eighth aspect of the invention and the disclosure of the eighth aspect of the invention, also apply to the ninth aspect of the invention, as long as they are not contrary to each other.

With the intention of better showing the characteristics of the invention, hereafter, as an example without any limitative character, several preferred embodiments are described, with reference to the accompanying drawings, wherein: figure 1 represents a wall comprising a thermal insulation according to the invention; and figure 2 shows a top view of the wall of figure 1.

Figure 1 represents a wall 10 comprising an inner wall 12, an outer wall 14 and thermal insultation 16 according to the invention. Figure 2 shows a top view of the wall of figure 1. The inner wall 12 and the outer wall 14 can be brick walls. The thermal insulation 16 comprises insulation panels 18 according to the invention.

The insulation panels 18 are rectangular and have a thickness T. The insulation panel comprises at least at two opposite edges coupling parts for coupling the insulation panel to another such insulation panel at their corresponding opposite edges comprising the coupling parts. In the example, the coupling parts comprise at the first edge of the two opposite edges a tongue 20 and at the second edge of the two opposite edges a corresponding groove 22. At the side of the outer wall 14, a sealing tape 24 is applied on the surface of the insulation panels at the corresponding opposite edges coupled by the tongue 20 and groove 22 joint. The sealing tape covers the joint between the two insulation panels.

The insulation panels 18 comprise two facers 26 in between which the rigid polyurethane foam 28 according to the invention is embedded. The rigid polyurethane foam 28 comprises lignin-based structures.

In the example, the facers 26 are each a multi-layered laminate. The multi-layered laminate comprises at the surface of the insulation panel a polyethylene film. The multilayered laminate further comprises an aluminum layer for providing gas tightness and one or more than one Kraft paper layer providing the rigidity of the facers.

The insulation panel 18 has thickness T. The rigid polyurethane foam 28 comprises closed cells 30. The closed cells 30 are anisotropic in dimension and have a largest cell distance Tc substantially oriented in the thickness direction of the insulation panel 18.

Examples 1 to 3

Three examples of rigid foam formulations are shown. Examples 2 and 3 are rigid foam formulations according to the invention, example 1 is a rigid foam formulation which does not comprise lignin. These foam formulations comprise the following amounts (parts by weight).

The foam formulations are used to form rigid polyurethane foams. These rigid polyurethane foams are preferably part of an insulation panel comprising two facers between which the foam is present. The facers for example comprise (multiple) layers of aluminum foil and/or kraft paper. Further the insulation panels preferably comprises tongue and grooves to interconnect such insulation panels. The used chemical substances are the same for all the examples, meaning that the same aromatic polyester polyol, the same one or more catalyst, etc. are used.

These foam formulations are used in a method to produce the rigid polyurethane foam. First a polyol blend is made by combining the following ingredients: the aromatic polyester polyol, the kraft lignin powder (95% purity) for examples 2 and 3, the one or more catalysts, the one or more silicone surfactants, the triethyl phosphate, the propylene carbonate, the water and the pentane. The aromatic polyester polyol is provided in liquid form and the lignin powder is dispersed in the polyol blend. To provide the polyol blend, a mixing device is used. Then the polyol blend is contacted with the MDI. The polyols, thus also the lignin for examples 2 and 3, will react with the MDI and foaming will occur, as such to form the rigid polyurethane foam. It was found that properties of the foam obtained with example 3 were much better than the properties of the foam obtained with example 2 and even much better than the properties of the foam obtained with example 1. The respective initial lambda values (W/m*K) were the following (examples 3, 2 and 1): 0.02195, 0.02212 and 0.02292, the respective compression strength values (kPa) were the following: 109, 103 and 85. The cream time, the string time and the flame resistance were comparable. This means that a higher amount of lignin provides a rigid polyurethane foam with a higher compressive strength and better heat insulating properties. The cream time is the time when the liquid polyurethane starts to expand, due to the gas reaction. Then, crosslinking of the foam occurs, the speed of which speed can be measured by the so called string time.

Examples 4 to 6

Three more examples of rigid foam formulations are shown. Examples 5 and 6 are rigid foam formulations according to the invention, example 4 is a rigid foam formulation which does not comprise lignin. These foam formulations comprise the following amounts (parts by weight). The foam formulations are used to form rigid polyurethane foams. These rigid polyurethane foams are preferably part of an insulation panel comprising two facers between which the foam is present. The used chemical substances indicated in the first column, are the same for all the examples, meaning that the same aromatic polyester polyol, the same one or more catalyst, etc. are used.

These foam formulations are used in a method to produce the rigid polyurethane foam. First a polyol blend is made by combining the following ingredients: the aromatic polyester polyol, the specific kraft lignin powder for examples 5 and 6, the one or more catalysts, the one or more silicone surfactants, the triethyl phosphate, the propylene carbonate, the water and the pentane. The aromatic polyester polyol is provided in liquid form and the lignin powder is dispersed in the polyol blend. For this a mixing device is used. Then the polyol blend is contacted with the MDI. The polyols, thus also the lignin for examples 5 and 6, will react with the MDI and foaming will occur, as such to form the rigid polyurethane foam.

It was found that properties of the foam obtained with example 6 were better than the properties of the foam obtained with example 5 and even better than the properties of the foam obtained with example 4. The respective initial lambda values (W/m*K) were the following (examples 4, 5 and 6): 0.02271, 0.02226 and 0.02220, the respective compression values (kPa) were the following: 82, 88 and 92. The cream time and the string time were comparable. This means that a higher amount of lignin provides higher compressive strength and better heat insulating properties.

Examples 1 - 6 are lab scale examples.

Examples 7 to 8

Another lab trial compared Example 8 which is a rigid foam formulations according to the invention, while Example 7 which is a rigid foam formulation which does not comprise lignin. The lignin of Example 8 is a modified lignin having average molecular weight according to mass equal to 1000 g/mol.

The lambda value (W/m*K) for rigid foam made with the formulation of Example 7 was 0.02259. The lambda value (W/m*K) for rigid foam made with the inventive formulation of Example 8 was 0.02122. Example 8 provides rigid foams with improved thermal insulation properties.

Flame resistance properties can be measured with the test method EN 13501-1 and EN 15715. The rigid foam made with the formulation of Example 7 showed a flame height of 122.5 mm; whereas the rigid foam made with the inventive formulation of Example 8 showed a flame height of 105 mm. Thus, rigid foam made with the inventive formulation of Example 8 has better flame retardancy than the rigid foam made with the foam formulation of comparative Example 7.

Examples 9 to 10

Example 9 and 10 relate to trials in which rigid polyurethane foam was made on an industrial production line comparing two different rigid polyurethane foam formulations. Example 9 is a rigid polyurethane foam formulation which does not comprise lignin. Example 10 is a rigid polyurethane foam formulation according to the invention which comprises lignin.

Rigid foam made with the foam formulation of Example 10 had 24 % higher compression strength than rigid foam made with the foam formulation of Example 9.

Rigid foam made with the foam formulation of Example 10 had 2 % lower lambda value than rigid foam made with the foam formulation of Example 9, indicating the improved thermal insulation value of rigid polyurethane foams according to the invention.

The flame height has been measured of rigid polyurethane foams made with the formulation of Examples 9 and 10. The rigid polyurethane foam made with the foam formulation of Example 9 showed flame height 20.3 mm, whereas the rigid polyurethane foam made with the foam formulation of Example 10 only showed flame height 16.7 mm, indicating the improved flame retardancy of rigid polyurethane foams made with foam formulations according to the invention.

The difference of the flame heights measured for Examples 7 - 8 on the one hand and measured for Examples 9 - 10 on the other hand is due to the different sample sizes that are used in the flame tests. The present invention is in no way limited to the herein above-described embodiments; on the contrary, such foam formulation, foams and methods can be realized according to various variants, without leaving the scope of the present invention, as defined by the appended claims.