[0001] The present invention relates to fuel pellets and briquettes comprising compressed, combustible biological material, particularly sawdust, their production and use.

BACKGROUND

[0002] Wood pellets and briquettes are a source of wood fuel, generally made from compacted sawdust or wood residue which is left over when lumber is produced or furniture is manufactured. Such fuel sources can be produced with high density and low water content (below 10%) which allows them to be burned with a very high combustion efficiency. The regular geometry and small size of wood pellets permit their automatic feeding by mechanical devices or by pneumatic conveying. The high density of wood pellets permits compact storage and rational transport over long distance. They can conveniently be blown from a tanker to a storage bunker or silo on a customer's premises. In addition to compressed sawdust, fuel pellets can be made from other combustible biological material, such as compressed straw.

[0003] A broad range of pellet stoves, central heating furnaces and other heating appliances designed to use fuel pellets have been developed and marketed since 1999. With the surge in the price and the increasing regulation of fossil fuels, the demand for pellet heating has increased in Europe and North America, and a sizable industry is emerging. In fact, it is estimated that by 2020 the market for fuel pellets will increase to 100 million tons per year.

[0004] However, existing fuel pellets suffer from a number of disadvantages that limit their use or at least inhibit their performance. For example, untreated fuel pellets frequently have limited integrity which can lead to breakage and/or dusting of the pellets particularly during transportation and storage. Not only do these problems lead to loss of useful fuel but also they can increase the danger of dust inhalation or dust explosion.

[0005] Thus, to improve the integrity of fuel pellets so that they can be easier to ship and store without crumbling or weathering, attempts have been made to add binder materials to the pellets during manufacture. U.S. Patent No. 4,236,897, for example, discloses the addition of a synthetic polymeric thermoplastic material to wood pellets as a binder which is uniformly distributed throughout the fuel pellet. Since, however, wood pellets are to be burned as fuel, the use of binders therein has in some places been regulated and restricted to prevent potentially harmful emissions from the binder materials when combustion of the pellets occurs.

[0006] In addition, International Publication No. WO2002/050220 discloses a method for decreasing the dusting tendency and the water sensitivity of fuel pellets by surface treating the pellets with vegetable oil, such as rape oil, preferably by atomizing or spraying, so that the oil is absorbed by the fuel pellets and the pellets are hardened.

[0007] Another problem particularly with wood pellets is that the wood in the pellets can undergo oxidation when in prolonged contact with air leading to the emission of harmful carbon monoxide and other materials such as aldehydes. These harmful emissions can and have lead to fatal accidents when wood pellets are shipped and stored without adequate ventilation. The nature of this problem has been described, for example, in Svedberg et al; "Hazardous Off-Gassing of Carbon Monoxide and Oxygen Depletion During Ocean Transportation of Wood Pellets"; Annals of Occupational Hygiene, Vol. 52(4), (2008) at pp. 259-266.

[0008] According to the present invention, it has now been found that the integrity of fuel pellets can be improved, while at the same time the problem of gaseous emission from the pellets is significantly reduced, by applying a barrier coating of a polymeric material to the external surface of the pellets. By ensuring that the thickness of the polymer coating is kept to a minimum and by proper selection of the polymer material, the generation of hazardous emissions from combustion of the polymer coating can be minimized or avoided. At the same time the water resistance of the pellets is also improved.

[0009] US Published Patent Application No. 2010/0282632 discloses a plurality of pellets comprising: at least one of a polymer, synthetic, biomass, or mineral based material, the pellets having a specific gravity of at least about 0.01 to about 0.3 g/cm ³ , the pellets being dimensionally stable, substantially dust-free, substantially non-hygroscopic, and resistant to settling and compression. The pellets may be substantially encapsulated with a coating selected from the group consisting of a coupling agent, a urethane, an epoxy, an acrylic, a silicone, an oleoresinous vehicle, a latex, a water reducible resin, and blends thereof. However, the pellets are intended for use as an insulating material and hence are generally formed of an expanded, foamed, or multi-cellular material so as to provide the required low specific gravity.

SUMMARY

[0010] In one aspect, the present invention resides in a fuel source comprising compressed combustible biological material having an external barrier coating of a polymeric material.

[0011] Conveniently, the fuel source has a specific gravity of at least 0.8 g/cm ³ .

[0012] Conveniently, the fuel source has a surface area to volume ratio greater than 0.05 mm ^"1 .

[0013] Conveniently, the fuel source comprises pellets having a maximum cross- sectional dimension of about 4 to about 8 mm and a length of about 8 to about 30mm.

[0014] In one embodiment, the combustible biological material comprises a ligneous material, particularly sawdust.

[0015] Conveniently, the external barrier coating reduces the CO emission from said fuel source by at least 50%, such as at least 70%, as compared with an identical fuel source without said external barrier coating.

[0016] Conveniently, the barrier coating comprises less than 10 wt%, such as from about 0.01 to about 10 wt%, of the total weight of the fuel source.

[0017] Conveniently, the interior of the fuel source is substantially free of said polymeric material.

[0018] Conveniently, the barrier coating comprises a cured emulsion polymer.

[0019] Conveniently, the emulsion polymer is selected from vinyl ester-based, acrylic- based, and styrene/acry lie-based emulsion polymers.

[0020] In one embodiment, the polymer or parts of the polymer have a glass transition temperature, T _g , of at least 50 °C.

[0021] In a further aspect, the invention resides in a method of producing a fuel source, which method comprises:

(a) compressing and forcing sawdust through a die to produce wood pellets;

(b) spraying an aqueous dispersion of an emulsion polymer onto the external surface of said wood pellets; and (c) drying the coated wood pellets to form a cured external polymeric barrier coating on the pellets.

DESCRIPTION

[0022] Described herein is a fuel source comprising a combustible biological material compressed into the form a pellets or briquettes and provided with an external barrier coating of a polymeric material to improve the mechanical integrity and water resistance of the fuel source and to reduce gaseous emissions from the fuel source during storage and transportation.

[0023] Although any combustible biological material, such as straw, can be used in the present fuel source, in practice the fuel source will normally be composed of a ligneous material, such as compacted sawdust or other wood residue which is left over when lumber is produced or furniture is manufactured. Wood pellets and briquettes are generally produced by compressing the wood material which has first passed through a hammer mill to provide a uniform dough-like mass. This mass is then fed to a press where it is squeezed through a die having a hole or holes of the size required. The high pressure of the press causes the temperature of the wood to increase greatly, and the lignin plastifies slightly forming a natural "glue" that holds the pellets/briquettes together as they cool.

[0024] The resultant fuel source typically has a specific gravity of at least 0.8 g/cm ³ , such as from about 0.8 to about 1.4 g/cm ³ and a low water content, that is below 10 wt , such as from about 1 to about 10 wt , and hence comply with DIN 51731.

[0025] In one preferred embodiment, the fuel source has a surface area to volume ratio greater than 0.05 mm ^"1 , such as greater than 0.5 mm ^"1 . One example of such a fuel source is wood pellets having a maximum cross-sectional dimension of about 4 to about 8 mm and a length of about 8 to about 30 mm.

[0026] After production, the fuel source described herein is provided with an external barrier coating of a polymeric material which substantially covers the external surface of the fuel source but leaves the interior of the fuel source substantially free of the polymeric material. Generally, the external barrier coating comprises no more than 10 wt of the total weight of the fuel source, such as from about 0.05 to about 10 wt of the total weight of the fuel source. [0027] The polymeric material used to produce the external barrier coating is generally an aqueous emulsion polymer and in particular a vinyl ester-based, an acrylic-based, a styrene/acrylic-based and/or a styrene butadiene rubber-based emulsion polymer.

[0028] One preferred type of emulsion copolymer comprises a vinyl ester-based polymer selected from vinyl acetate (co-)polymers, vinyl acetate-ethylene copolymers, vinyl acetate- vinyl versatate; vinyl acetate-alkyl maleates, vinyl acetate- vinyl benzoate, vinyl acetate-acrylic copolymers, and combinations of these polymer types. Vinyl acetate- ethylene (VAE) emulsion copolymers are well-known. Such VAE copolymers useful herein can comprise from about 60 wt to about 95 wt of vinyl acetate and from about 5 wt to about 50 wt of ethylene, based on total monomers therein. More preferably, VAE copolymers will comprise from about 70 wt to about 92 wt of vinyl acetate and from about 8 wt to about 30 wt of ethylene, based on total monomers therein.

[0029] Another preferred type of emulsion polymer comprises acrylic emulsion copolymers made of acrylic ester co-monomers. The alkyl acrylates that can be used to prepare the acrylic ester copolymer emulsions include alkyl acrylates and alkyl methacrylates containing 1 to 12, preferably 1 to 10 carbon atoms in the alkyl group. The polymer backbone in the acrylic ester copolymer can be either hydrophilic or hydrophobic and it can comprise polymerized soft monomers and/or hard monomers. The soft and hard monomers are monomers which, when polymerized, produce soft or hard polymers, or polymers in between. Soft polymers have a T _g of less than 30 °C, whereas hard polymers have a T _g of at least 30 °C. Preferred soft acrylic ester monomers are selected from alkyl acrylates containing 2 to 12 carbon atoms in the alkyl group and include ethyl acrylate, propyl acrylate, n-butyl acrylate, lauryl acrylate and 2-ethylhexyl acrylate. The hard acrylic ester monomers are selected from alkyl methacrylates containing up to 3 carbon atoms in the alkyl group, isobornyl acrylate, isobornyl methacrylate and from non-acrylic monomers such as styrene and substituted styrenes, acrylonitrile, vinylchloride, and generally any compatible monomer the homopolymer of which has a T _g above 30° C.

[0030] The emulsion copolymer used herein can also comprise from about 0.1 wt to about 10 wt , based on total monomers in the copolymer, of one or more ethylenically unsaturated crosslinkable or self-cross-linking co-monomers having, for example, at least one amide, epoxy, or alkoxysilane group. Examples of such suitable crosslinkable or self cross-linking co-monomers include N-methylol (meth)acrylamide and esters thereof, acryloxy-propyltri(alkoxy)silanes, methacryloxypropyltri(alkoxy)silanes, vinyl trialkoxysilanes, vinylmethyldialkoxysilanes, alkylvinyldialkoxysilanes, diacetoneacrylamide, allyl acetoacetate, vinyl acetoacetate, acetoacetoxyethyl acrylate or methacrylate, allyl glycidyl ether, methacryloyl glycidyl ether, butadiene monoepoxides,

1.2- epoxy-5-hexene, l,2-epoxy-7-octene, l,2-epoxy-9-decene, 8-hydroxy-6,7-epoxy-l- octene, 8-acetoxy-6,7-epoxy-l-octene, N-(2,3-epoxy)propylacrylamide, N-(2,3- epoxy)propylmethacrylamide, 4-acrylamidophenyl glycidyl ether, 3-acrylamidophenyl glycidyl ether, 4-methacrylamidophenyl glycidyl ether, 3-methacrylamidophenyl glycidyl ether, N-glycidyloxymethylacrylamide, N-glycidyloxypropylmethacrylamide, N- glycidyloxyethylacrylamide, N-glycidyloxyethylmethacrylamide, N- glycidyloxypropylacrylamide, N-glycidyloxypropylmethacrylamide, N- glycidyloxybutylacrylamide, N-glycidyloxybutylmethacrylamide, 4-acrylamidomethyl-2,5- dimethylphenyl glycidyl ether, 4-methacrylamidomethyl-2,5-dimethylphenyl glycidyl ether, acrylamidopropyldimethyl-(2,3-epoxy)propylammonium chloride, methacrylamidopropyldimethoxy- (2, 3 -epoxy)propy lammonium chloride , glycidyl methacrylate, Cl-C9-hydroxyalkyl methacrylates and acrylates, such as n-hydroxy ethyl, n- hydroxypropyl or n-hydroxybutyl acrylate and methacrylate and combinations of these crosslinkable or self-crosslinking co-monomers.

[0031] The emulsion copolymer can also contain, in addition to or instead of the crosslinkable or self cross-linking co-monomers, minor amounts of multifunctional in-situ cross-linking co-monomers. Thus the copolymers used herein can optionally comprise from about 0.1 wt to about 10 wt , based on total monomers in the copolymer, of one of more of these multifunctional cross-linking co-monomers. Examples of suitable multifunctional cross-linking co-monomers include alkylene glycol diacrylates and dimethacrylates, such as ethylene glycol diacrylate, 1 ,2-propylene glycol diacrylate, 1,3 -propylene glycol diacrylate,

1.3- butylene glycol diacrylate, 1,4-butylene glycol diacrylates or methacrylates and ethylene glycol diacrylates or methacrylates, 1 ,2-propylene glycol dimethacrylate, 1,3- propylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylates, hexanediol diacrylate, pentaerythritol diacrylate. Others comprise monomers like divinyl benzene, vinyl methacrylate, vinyl acrylate, vinyl crotonate, allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl phthalate, cyclopentadienyl acrylate, divinyl adipate, diallyl adipate, allyl methacrylate, methylenebisacrylamide and triallyl cyanurate. and combinations of these monomers.

[0032] In one practical embodiment, the emulsion polymer dispersion used herein comprises an aqueous dispersion having a minimum filming temperature (MFT) of from about 0 to 50 °C, comprising at least one carbonyl-containing, soft latex polymer A having an MFT of below about 20 °C; at least one hard latex polymer B having an MFT of above about 25 °C; and at least one poly-functional carboxylic hydrazide C Examples of these poly-functional carboxylic hydrazide are adipic dihydrazide, oxalic dihydrazide, isophtalic dihydrazide and polyacrylic polyhydrazide. Such a dispersion is disclosed in, for example, U.S. Patent No. 5,596,035, the entire contents of which are incorporated herein by reference.

[0033] Both during polymerization and thereafter, the copolymer dispersion used herein is stabilized in the form of an aqueous copolymer emulsion or latex. The copolymer dispersion therefore will be prepared in the presence of and will contain a stabilization system which generally comprises emulsifiers, in particular nonionic emulsifiers and/or anionic emulsifiers. Mixtures of nonionic and anionic emulsifiers can also be employed.

[0034] The amount of emulsifier employed will generally be at least 0.5 wt , based on the total quantity of main co-monomers in the copolymer dispersion. Generally emulsifiers can be used in amounts up to about 8 wt , based on the total quantity of main co- monomers in the copolymer dispersion. The weight ratio of emulsifiers nonionic to anionic may fluctuate within wide ranges, between 1:50 and 50:1 for example.

[0035] Emulsifiers employed with preference herein are nonionic emulsifiers having alkylene oxide groups and/or anionic emulsifiers having sulfate, sulfonate, phosphate and/or phosphonate groups. Such emulsifiers, if desired, can be used together with molecularly or dispersely water-soluble polymers (protective colloids), preferably together with polyvinyl alcohol. Preferably also the emulsifiers used contain no alkylphenolethoxylates (APEO).

[0036] Examples of suitable nonionic emulsifiers include acyl, alkyl, oleyl, and alkylaryl ethoxylates. These products are commercially available, for example, under the name Genapol ^® , Lutensol ^® or Emulan ^® . They include, for example, ethoxylated mono-, di-, and tri-alkylphenols (EO degree: 3 to 50, alkyl substituent radical: C ₄ to C12) and also ethoxylated fatty alcohols (EO degree: 3 to 80; alkyl radical: Cs to C36), especially Ci2-Ci ₄ fatty alcohol (3-40) ethoxylates, C13-C15 oxo-process alcohol (3-40) ethoxylates, C16-C18 fatty alcohol (11-80) ethoxylates, C ₁₀ oxo-process alcohol (3-40) ethoxylates, C ₁₃ oxo- process alcohol (3-40) ethoxylates, polyoxyethylenesorbitan monooleate with 20 ethylene oxide groups, copolymers of ethylene oxide and propylene oxide having a minimum ethylene oxide content of 10% by weight, and the polyethylene oxide (4-40) ethers of oleyl alcohol. Particularly suitable are the polyethylene oxide (4-40) ethers of fatty alcohols, more particularly of oleyl alcohol, stearyl alcohol or Cn alkyl alcohols.

[0037] The amount of nonionic emulsifiers used in preparing the copolymer dispersions herein is typically about 1% to about 8% by weight, preferably about 1% to about 5% by weight, more preferably about 1% to about 4% by weight, based on the total main monomer quantity. Mixtures of nonionic emulsifiers can also be employed.

[0038] Examples of suitable anionic emulsifiers include sodium, potassium, and ammonium salts of linear aliphatic carboxylic acids of chain length C12-C2 ₀ , sodium hydroxyoctadecanesulfonate, sodium, potassium, and ammonium salts of hydroxy fatty acids of chain length C12-C2 ₀ and their sulfonation and/or sulfation and/or acetylation products, alkyl sulfates, including those in the form of triethanolamine salts, alkyl(Cio-C2o) sulfonates, alkyl(Cio-C2o) arylsulfonates, dimethyl-dialkyl (Cs-Cis) ammonium chloride, and their sulfonation products, lignosulfonic acid and its calcium, magnesium, sodium, and ammonium salts, resin acids, hydrogenated and dehydrogenated resin acids, and their alkali metal salts, dodecylated sodium diphenyl ether disulfonate, sodium lauryl sulfate, sulfated alkyl or aryl ethoxylate with EO degree between 1 and 10, for example ethoxylated sodium lauryl ether sulfate (EO degree 3) or a salt of a bisester, preferably of a bis-C4-Cis alkyl ester, of a sulfonated dicarboxylic acid having 4 to 8 carbon atoms, or a mixture of these salts, preferably sulfonated salts of esters of succinic acid, more preferably salts, such as alkali metal salts, of bis-C4-Cis alkyl esters of sulfonated succinic acid, or phosphates of polyethoxylated alkanols.

[0039] The amount of anionic emulsifiers used can typically range from about 0.1% to about 3.0% by weight, preferably from about 0.1% to about 2.0% by weight, more preferably from about 0.5% to about 1.5% by weight, based on the total main monomer quantity. Mixtures of anionic emulsifiers can also be employed.

[0040] The polymer dispersion may also include an anti-oxidant or oxygen absorbing compound, such as ascorbic acid, to improve the resistance of the final barrier coating to CO _x emission. If present, suitable levels of anti-oxidant in the polymer dispersion vary from about 0.01 wt to about 5 wt .

[0041] In addition, the polymer dispersion may include an optical brightener and/or an inorganic salt with a characteristic emission spectrum to provide a marker in the barrier coating to allow, for example, different pellet batches to be identified with different manufacturers. Examples of suitable marker materials include boron salts, salts of Group 1A or IIA metals, such as salts of lithium, sodium, potassium, calcium, strontium and barium and/or transition metal salts such as copper salts. Examples for optical brighteners comprise derivates of stilbene, coumarine, diphenyl pyrazolines, naphtalimids and pyrenyltriazines.

[0042] Generally the polymer dispersion employed herein has a solids content of from about 5 wt to about 60 wt and a Brookfield viscosity ranging from about 5 mPas to about 5,000 mPas at 23°C.

[0043] The polymer dispersion can be applied to the external surfaces of the pellets in any convenient manner, such as by spraying, after which the coated pellets are dried at a temperature of from about 30 °C to about 180 °C for a period of from about 0.25 to 5 minutes so as to produce the desired barrier coating on the surface of the pellets. The resultant coating is found to reduce the CO emission from ligneous pellets by at least 50 , in many cases at least 70% as compared with identical uncoated pellets tested under the same conditions and to improve the mechanical durability (abrasion resistance) of the pellets. In addition, provided the polymer or parts of the polymer have a Tg of at least 50 °C it is found that the pellets have low tendency to block or stick together.

[0044] The invention will now be more particularly described with reference to the following non-limiting Examples.

[0045] In the Examples, the mechanical durability of the pellets was measured by means of a commercially available New Holmen Portable Lignotester (available at TekPro Ltd, UK, Model NHP-100). In the test, a sample of pellets is loaded into a test chamber where the pellets are cascaded in a 70 mbar air stream (generated in a pressurized chamber) causing the pellets to collide with each other and the perforated hard surfaces within the test chamber. After a 60 second test cycle the pellets are ejected for manual weighing. The mechanical durability is the difference between pellet weight before and after the test recorded as a percentage. [0046] The determination of CO emission from the pellets was conducted by a Drager MSI EM200 0 ₂ /CO flue gas analyzer system having a CO range up to 8,000 ppm (available at Drager Safety AG & Co. KGaA, Germany). The program "CO measurement" under "flue gas measurement" was chosen for each measurement. A stainless steel gas probe 30 cm in length equipped with an integrated Viton hose of 2.5 m in length was connected to gas analyzer for each measurement. After each measurement the analyzer was disconnected from the gas probe.

[0047] Each CO emission measurement was conducted in a desiccator having a volume of 11 liters and equipped with a wire mesh tray to hold a batch of pellets to be tested above the bottom of the desiccator. A hole was drilled into the top of the desiccator lid to hold the gas probe, which was mounted in a silicon stopper sealing the hole in the desiccator lid so that the probe extended through the wire mesh to the bottom of the desiccator. For each measurement, 2 kg pellets were placed on the wire tray and the desiccator was sealed and kept in a climate-controlled room (23 °C and 50 % humidity) for a storage period of 1 week.

[0048] After storage and prior to each measurement, the hose of the probe was connected to the analyzer which was started and calibrated for 20 seconds (during this period approx. 150 cm ³ gas was used). The measurement time was 60 seconds (460 cm ³ gas volume). During the measurement, a pressure relief valve at the side of the desiccator was opened and immediately closed after the measurement.

Example 1

[0049] An acrylic polymer dispersion was produced in a cylindrical glass reactor fitted with an anchor stirrer, slow-add funnels, reflux condenser and a digitally controlled cooling/heating jacket. An aqueous phase was prepared by dissolving 1.0 parts (active material) of an anionic emulsifier based on an ethoxylated alkyl ether sulphate (with 7 EO, Na salt) in 53.7 parts of demineralized water. A monomer emulsion was prepared from 53.5 parts demineralized water, 0.5 active parts of the above used emulsifier, 70 parts methyl methacrylate, 30 parts 2-ethylhexyl acrylate, 2 parts methacrylic acid and 1 part acrylic acid. An aqueous solution of 0.35 parts of ammonium peroxodisulfate in 3.15 parts demineralized water was prepared as initiator solution. For starting the reaction 2.5 % of the emulsion was added to the aqueous phase. The polymerization start was conducted by addition of 14.3 % of the ammonium peroxodisulfate solution at 80 °C. The reaction mixture was kept at 80 °C for 15 min. After this stage, the rest of the monomer emulsion and initiator solution were metered into the reaction mixture over a period of 3 hours while a constant temperature of 80 °C was maintained. After this period the temperature was kept at 80 °C for a further 60 minutes for completion of the reaction, whereafter the product was allowed to cool.

[0050] The final emulsion had a solids content of 47.5 , a pH value of 2.4, a viscosity (Brookfield RVT, 23 °C, spindle 2, 20 rpm) of 290 mPas. The mean value of the volume average particle size (Laser Aerosol spectrometry) was d _w = 113 nm. Prior to spray coating the pH of the emulsion was adjusted to 5 by addition of a 5 % (w/w) solution of sodium hydroxide and diluted down to 35 % solids content. The viscosity of the diluted emulsion was 18 mPas (spindle 1, 20 rpm).

[0051] Commercial pellets made of coniferous sawdust (6 mm diameter) were placed on a steel mesh and spray-coated with the diluted emulsion by means of a common household spraying device equipped with a holding tank for about 500 ml liquid (e.g. as used for flowers). 200 to 250 g pellets were spray-coated at once. After the spraying the pellets were placed on a glass fiber scrim and dried immediately in an oven of 110 °C for 90 seconds. The procedure was repeated until 2 kg pellets were coated. The amount of polymer was determined to be 1.7 % (w/w) related to the mass of pellets. Prior to transfer to the desiccator the pellets were placed on a metal sheet for 2 h at room temperature to cool down and for further drying. The results of CO emission testing of the coated pellets and the uncoated pellets, as a control, are summarized in Table 1.

Table 1

[0052] From Example 1 it can be seen that the coating reduces the CO emission significantly. The higher 0 ₂ concentration in the gas phase in Example 1 indicates that the coating is a functional barrier that hinders access of oxygen to the pellets and reduces oxidative decomposition of ingredients of the wood resulting in CO formation. Example 2

[0053] The procedure of Example 1 was repeated to produce a further batch of coated wood pellets but with the coating comprising 1.5 % (w/w) related to the mass of pellets. The coated pellets were then tested for mechanical durability according to the method described above and the results are summarized in Table 2. The data in Table 2 show that the polymer coating improved the mechanical stability of the pellets.

Table 2

Examples 3 to 8

[0054] Commercially available polymer dispersions of different types were diluted to 35 % solids content and used to spray coat further batches of the pellets used in Example 1. The coated products were tested for blocking behavior, gloss and mechanical durability and the results are summarized in Table 3.

[0055] Blocking behavior was judged on an arbitrary scale of 1 (strong blocking) to 5 (no blocking). Strong blocking leads to adherence of the pellets to each other during industrial application at elevated temperatures and is undesirable. The gloss of the coated pellets was judged visually in comparison to the uncoated pellets which display very glossy appearance. An arbitrary scale of 1 (no gloss) to 3 (high gloss) was used.

Table 3