CA2350112C - Process to improve thermal properties of natural fibre composites - Google Patents
Process to improve thermal properties of natural fibre composites Download PDFInfo
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- CA2350112C CA2350112C CA 2350112 CA2350112A CA2350112C CA 2350112 C CA2350112 C CA 2350112C CA 2350112 CA2350112 CA 2350112 CA 2350112 A CA2350112 A CA 2350112A CA 2350112 C CA2350112 C CA 2350112C
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L97/00—Compositions of lignin-containing materials
- C08L97/02—Lignocellulosic material, e.g. wood, straw or bagasse
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L1/00—Compositions of cellulose, modified cellulose or cellulose derivatives
- C08L1/02—Cellulose; Modified cellulose
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
- C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
- C08L23/04—Homopolymers or copolymers of ethene
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
- C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
- C08L23/10—Homopolymers or copolymers of propene
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L2205/00—Polymer mixtures characterised by other features
- C08L2205/14—Polymer mixtures characterised by other features containing polymeric additives characterised by shape
- C08L2205/16—Fibres; Fibrils
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
- C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
- C08L23/10—Homopolymers or copolymers of propene
- C08L23/14—Copolymers of propene
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
- C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
- C08L23/16—Elastomeric ethene-propene or ethene-propene-diene copolymers, e.g. EPR and EPDM rubbers
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L97/00—Compositions of lignin-containing materials
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- Injection Moulding Of Plastics Or The Like (AREA)
Abstract
Lignocellulosic and cellulosic fibres are melt processed with polyolefin plastic and an imide monomer first in a high shear thermo-kinetic mixer and then in a plastic extruder or injection molder to produce reinforced plastic composite products.
Description
BACKGROUND OF THE INVENTION
This invention relates to processing natural fibre together with polyolefin plastic for manufacture of reinforced composites and particularly, to process for improving adhesion between plastic and fibre and reinforcing composite by improving its strength, creep and heat deformation properties.
Incorporation of discontinuous fibres into a polymer matrix to make reinforced composites is well known. For example, Goettler U.S. Pat. No. 4,376,144 describes vinyl chloride polymer composite of this type in which an isocyanate bonding agent was used to disperse fibre and to improve adhesion thereto. However, the environmental regulation restricts the use of isocyanate in many of such industrial applications.
1t is also known that the dispersion of short oellulosic fibres into a polymer matrix can be improved by pretreating the fibre with a lubricant and a plastic polymer. U.S.
Pat. No.
3,943,079 to Hawed describes such a fibre treatment. Boustany and Coran U.S.
Patent No. 3,697,364 described a predispersion process with rubber latex or other substances which reduce fiber-to-fiber interaction and improved composite properties when they are incorporated in a plastic matrix. In general, a "predispersed" fibre is not easy to feed t because of its low bulk density. Moreover, pretreating the fibre and then mixing the treated fibre into polymer matrix demands both time and equipment. The present invention simplifies the fibre dispersion and composite manufacturing process by first incorporating an imide additive and then activating the additive at a suitable process temperature range which promotes in-situ fibre dispersion and fibre-to-polyolefin adhesion.
SUMMARY OF THE INVENTION
It has now been discovered that reinforced natural fiber-polyolefin composite is simply prepared by melt processing polyolefinic plastic with particulate as well as short ellulosic fibre in a processing temperature range 200 to 240 °C in the presence of an imide dditive.
It has also been found that when the imide is activated with a co-additive during melt processing of lignocellulosic fibre particularly, wood fibre with polyolefin improved fiber-to-plastic adhesion at a processing temperature range of 180-230°C low enough to prevent undesired fibre degradation.
According to the present invention, reinforced composites are made of cellulose and lignocellulosic fibres dispersed in a polyethylene, polypropylene or ethylene-propylene copolymer matrix which includes a reinforcement additive which is chosen from a group of substituted maleimide with or without an activator, selected from thiazole, thiuram and dithiocarbamate derivatives. Composite containing 40 to 80 % of polyolefin by weight and 20 to 60% of natural fibre by weight, based on the total weight of composite is a subject of our study. The imide reinforcement additive used in this discovery has the general chemical structure:
This invention relates to processing natural fibre together with polyolefin plastic for manufacture of reinforced composites and particularly, to process for improving adhesion between plastic and fibre and reinforcing composite by improving its strength, creep and heat deformation properties.
Incorporation of discontinuous fibres into a polymer matrix to make reinforced composites is well known. For example, Goettler U.S. Pat. No. 4,376,144 describes vinyl chloride polymer composite of this type in which an isocyanate bonding agent was used to disperse fibre and to improve adhesion thereto. However, the environmental regulation restricts the use of isocyanate in many of such industrial applications.
1t is also known that the dispersion of short oellulosic fibres into a polymer matrix can be improved by pretreating the fibre with a lubricant and a plastic polymer. U.S.
Pat. No.
3,943,079 to Hawed describes such a fibre treatment. Boustany and Coran U.S.
Patent No. 3,697,364 described a predispersion process with rubber latex or other substances which reduce fiber-to-fiber interaction and improved composite properties when they are incorporated in a plastic matrix. In general, a "predispersed" fibre is not easy to feed t because of its low bulk density. Moreover, pretreating the fibre and then mixing the treated fibre into polymer matrix demands both time and equipment. The present invention simplifies the fibre dispersion and composite manufacturing process by first incorporating an imide additive and then activating the additive at a suitable process temperature range which promotes in-situ fibre dispersion and fibre-to-polyolefin adhesion.
SUMMARY OF THE INVENTION
It has now been discovered that reinforced natural fiber-polyolefin composite is simply prepared by melt processing polyolefinic plastic with particulate as well as short ellulosic fibre in a processing temperature range 200 to 240 °C in the presence of an imide dditive.
It has also been found that when the imide is activated with a co-additive during melt processing of lignocellulosic fibre particularly, wood fibre with polyolefin improved fiber-to-plastic adhesion at a processing temperature range of 180-230°C low enough to prevent undesired fibre degradation.
According to the present invention, reinforced composites are made of cellulose and lignocellulosic fibres dispersed in a polyethylene, polypropylene or ethylene-propylene copolymer matrix which includes a reinforcement additive which is chosen from a group of substituted maleimide with or without an activator, selected from thiazole, thiuram and dithiocarbamate derivatives. Composite containing 40 to 80 % of polyolefin by weight and 20 to 60% of natural fibre by weight, based on the total weight of composite is a subject of our study. The imide reinforcement additive used in this discovery has the general chemical structure:
z - R3 RI
Formula 1 where R,, can be a selection from hydrogen, alkyl, aryl, cyclohexyl and N-substituted imide , imide ester; R2 and R3 can be H, alkyl or aryl group.
Subjecting lignocellulosic or cellulose flour or fibre to the shear forces, resulting from a first stage intensive mixing any proportion of polyolefin plastic ranging between 15 to 85% by dry basis weight of composite and a fiber proportion ranging from 20 to 50 parts with the afore mentioned reinforcement additive ranging from 0.05 - 5%, effects dislogging of fibre bundles, their dispersion in the molten plastic matrix and improve fibre-to-plastic adhesion. By reinforcement additive is meant an auxiliary material which is first decomposed at the processing temperature and is then facilitated dispersion of fibre and adhesion between fibre and plastic. The additive is believed to produce the aforesaid effect only above the decomposition temperature of the said additive during melt mixing and processing by reducing the surface energy difference between polymer plastic and fibre or by improving acid-base interaction. Regardless of the correct explanation, it is observed that decomposition of the imide additive in the melt mixture of polyolefin polymer and natural fibre under thermo-kinetic action resulted in mechanical strength enhancement of composite. It is in the aforesaid sense of decomposition of additive and abetting the thermo-kinetic forces in dispersing and adhering the fibre to molten polymer.
The invention also includes reinforced composite, which is a melt-processed mixture of natural fibre, polyolefm, imide additive and a minor amount of an activator, dibenzthiazyl disulfide or other thiazole derivative. By activator it is meant a secondary additive which is believed to reduce melt processing temperature of the aforesaid mixture by decomposing imide additive at a temperature below the processing temperature of the said composite. The ratio of the activator proportion to the aforementioned imide additive of the invention falls within a range of 0.01 to 0.3.
Processing of the above mentioned polyolefins and lignocellulosic fibres with the imide additive and activator lead to pre-mixed composites which upon further processing results into composite product having a heat distortion temperature above 90°
centigrade at 1.8 MPa load and a relative creep of less than 200% at 30% load of the respective flexural strength of composites at 40° centigrade.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Natural cellulosic fibres are cellulosic and lignocellulosic fibres containing lignin, cellulose and hemicellulose. Examples of natural cellulosic fibres include wood-based fibres, seed-based fibres such as cotton, and bast fibres such as flax and kenaf. Among wood-based fibres hardwood and softwood fibres and woodflours are preferred.
Wood flour in particle size range 20 mesh to 200 mesh can be used and a preferred range is between 40 and 100 mesh. The wood fibres have aspect ratio in the range 10 to 200 and more preferred range is between 50 and 100. Lignocellulosic fibers of the present invention are the wood fibres and recycled newsprint with an aspect ratio of 10 to 250.
Bast fibre either flax or kenaf is preferred. Preferred aspect ratio is 10 to 250, with a more preferred range of 20 to 100. In some instances, it is desirable to use mixtures of fibres having two widely different aspect ratios. A preferred aspect ratio of fibre ranges from 10 to 50. An increased mixing time is good for fibre-to-plastic adhesion and reduced mixing time prevents undesired fibre breakage and increase process economy.
The plastic polymer of the invention is selected from the group of thermoplastic polyolefins. The term "thermoplastic" refers to plastics materials which, soften on heating and harden on cooling and the materials retain this property when reprocessed The plastic matrix contained in the composite is described as being "polyolefin plastic"
and includes polyethylene, polypropylene and copolymers of ethylene and propylene.
Copolymer comprises of a major portion of propylene and a minor portion of ethylene.
The flours described as "particulate" to distinguish from cellulosic or lignocellulosic short fibres having high aspect ratio (length divided by average diameter) and from inorganic fillers of any shape and size as taught in U.S. Pat. No.
Formula 1 where R,, can be a selection from hydrogen, alkyl, aryl, cyclohexyl and N-substituted imide , imide ester; R2 and R3 can be H, alkyl or aryl group.
Subjecting lignocellulosic or cellulose flour or fibre to the shear forces, resulting from a first stage intensive mixing any proportion of polyolefin plastic ranging between 15 to 85% by dry basis weight of composite and a fiber proportion ranging from 20 to 50 parts with the afore mentioned reinforcement additive ranging from 0.05 - 5%, effects dislogging of fibre bundles, their dispersion in the molten plastic matrix and improve fibre-to-plastic adhesion. By reinforcement additive is meant an auxiliary material which is first decomposed at the processing temperature and is then facilitated dispersion of fibre and adhesion between fibre and plastic. The additive is believed to produce the aforesaid effect only above the decomposition temperature of the said additive during melt mixing and processing by reducing the surface energy difference between polymer plastic and fibre or by improving acid-base interaction. Regardless of the correct explanation, it is observed that decomposition of the imide additive in the melt mixture of polyolefin polymer and natural fibre under thermo-kinetic action resulted in mechanical strength enhancement of composite. It is in the aforesaid sense of decomposition of additive and abetting the thermo-kinetic forces in dispersing and adhering the fibre to molten polymer.
The invention also includes reinforced composite, which is a melt-processed mixture of natural fibre, polyolefm, imide additive and a minor amount of an activator, dibenzthiazyl disulfide or other thiazole derivative. By activator it is meant a secondary additive which is believed to reduce melt processing temperature of the aforesaid mixture by decomposing imide additive at a temperature below the processing temperature of the said composite. The ratio of the activator proportion to the aforementioned imide additive of the invention falls within a range of 0.01 to 0.3.
Processing of the above mentioned polyolefins and lignocellulosic fibres with the imide additive and activator lead to pre-mixed composites which upon further processing results into composite product having a heat distortion temperature above 90°
centigrade at 1.8 MPa load and a relative creep of less than 200% at 30% load of the respective flexural strength of composites at 40° centigrade.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Natural cellulosic fibres are cellulosic and lignocellulosic fibres containing lignin, cellulose and hemicellulose. Examples of natural cellulosic fibres include wood-based fibres, seed-based fibres such as cotton, and bast fibres such as flax and kenaf. Among wood-based fibres hardwood and softwood fibres and woodflours are preferred.
Wood flour in particle size range 20 mesh to 200 mesh can be used and a preferred range is between 40 and 100 mesh. The wood fibres have aspect ratio in the range 10 to 200 and more preferred range is between 50 and 100. Lignocellulosic fibers of the present invention are the wood fibres and recycled newsprint with an aspect ratio of 10 to 250.
Bast fibre either flax or kenaf is preferred. Preferred aspect ratio is 10 to 250, with a more preferred range of 20 to 100. In some instances, it is desirable to use mixtures of fibres having two widely different aspect ratios. A preferred aspect ratio of fibre ranges from 10 to 50. An increased mixing time is good for fibre-to-plastic adhesion and reduced mixing time prevents undesired fibre breakage and increase process economy.
The plastic polymer of the invention is selected from the group of thermoplastic polyolefins. The term "thermoplastic" refers to plastics materials which, soften on heating and harden on cooling and the materials retain this property when reprocessed The plastic matrix contained in the composite is described as being "polyolefin plastic"
and includes polyethylene, polypropylene and copolymers of ethylene and propylene.
Copolymer comprises of a major portion of propylene and a minor portion of ethylene.
The flours described as "particulate" to distinguish from cellulosic or lignocellulosic short fibres having high aspect ratio (length divided by average diameter) and from inorganic fillers of any shape and size as taught in U.S. Pat. No.
5,494,948.(Mica-reinforced polypropylene resin composition; Ikezawa, Yuji , Nagoya, Japan (Sumitomo Chemical Company, Limited, Osaka, Japan; February 27, 1996).The "short fibres"
are described as chopped organic fibres having aspect ratio less than 500 to distinguish them from inorganic fibres of any length and diameter. A "plastic matrix" is a polymer which forms the dominant phase surrounding the flour and/or fibre. A "composite" is a combination of plastic and cellulose or lignocellulosic fibre or flour or both in which flour and fibre are dispersed in plastic matrix.
The additive of the invention is an unsaturated imide of heterocyclic structure. The preferred form of the additive is meta substitutions of benzene ring with maleimide, represented by the structural formula:
z- R3 R~
where R ~ , R2 = H and R,= ~o~=~
Formula 2 The additive is used in the composites of the invention in sufficient amount to achieve good dispersion of fibres and/or flours in polyolefin as well as reinforcement effect on the resulting composites. The "reinforcement" which refers to improvement of properties, is used to distinguish from the properties of finished composites without any imide additive.
The amount of imide additive can be as low as 0.05 parts by weight by 100 parts by weight of the composite, up to 3 parts by weight or more, on the same basis.
The amount of imide additive required can also expected to vary with the amount of cellulose and lignocellulosic flour or fibre, the type and nature of fibre or flour as well as the type of polyolefin plastic used.
are described as chopped organic fibres having aspect ratio less than 500 to distinguish them from inorganic fibres of any length and diameter. A "plastic matrix" is a polymer which forms the dominant phase surrounding the flour and/or fibre. A "composite" is a combination of plastic and cellulose or lignocellulosic fibre or flour or both in which flour and fibre are dispersed in plastic matrix.
The additive of the invention is an unsaturated imide of heterocyclic structure. The preferred form of the additive is meta substitutions of benzene ring with maleimide, represented by the structural formula:
z- R3 R~
where R ~ , R2 = H and R,= ~o~=~
Formula 2 The additive is used in the composites of the invention in sufficient amount to achieve good dispersion of fibres and/or flours in polyolefin as well as reinforcement effect on the resulting composites. The "reinforcement" which refers to improvement of properties, is used to distinguish from the properties of finished composites without any imide additive.
The amount of imide additive can be as low as 0.05 parts by weight by 100 parts by weight of the composite, up to 3 parts by weight or more, on the same basis.
The amount of imide additive required can also expected to vary with the amount of cellulose and lignocellulosic flour or fibre, the type and nature of fibre or flour as well as the type of polyolefin plastic used.
The mechanism of the reinforcement is not known, however, it is thought that the two active functional groups of maleimide moiety in the additive react with polyolefin and hydroxyl group of cellulose and lignocellulose fibres or flours, forming a chemical bond herewith.
The reinforcement additive can be premixed with fibre or flour, or it can be preblended with a small amount of powder form of polyolefin, in order to facilitate dispersion of the additive in the composite. This premixing process is optional depending on the type of mixing equipment used in order to melt the polyolefin plastic and to disperse the fibres in molten polymer. The reinforcement additive can be incorporated into the composites of the invention by mixing the reinforcement additive therewith, at the same time the flour or fibres are combined with the polyolefin plastics. A high speed thermo-kinetic mixer such as K-mixer can be used to melt-mix polyolefin, fibre and reinforcement additive without predispersing the said additive.
The reinforcement additive can also contain other imide compounds, such as 4,4'-diantipyrylmethane, 2- methyl, N-phenylmaleimide, N-cyclohexylmaleimide, N-ethyl-maleimide. Mixtures of one or more of the other imide compounds can be present along with the N, N - 1,3 phenylenedimaleimide as in Structure formula 2, but their effect is inferior thereto in the present invention.
An unexpected beneficial result is noted in that the cellulosic flour or fibre and polyolefin plastic compositions, which contain the reinforcement additive of the present invention show a significant improvement of the final properties.
The effectiveness of the reinforcement additive of the present invention is surprising, since it would not be expected that an imide additive which is designed to use as a crosslinking agent for elastomers, could be so effective even without any activator, in improving dispersity of natural fibres or woodflour and thereby reinforcing a plastic matrix containing polyethylene, polypropylene or their copolymer.
The effect of obtaining a reinforcing effect on the composite of the present invention is of course, to provide a natural fibre- or wood flour-filled polyolefin composite, which has maximum strength, stiffness, heat deflection temperature (HDT) and resistance to creep.
Creep and mechanical properties of the modified wood fibre reinforced thermoplastic composites were reported by Sain, Law and Balatinecz (J. Appl. Polym. Sci.,77, 2000, 260) and Sain and Kokta (J.Appl.Polym.Sci.,54, 1994, 1545). With optimum reinforcement effect which thought to increase not only the adhesion between fibre or woodflour and polyolefin matrix but chemically attach some part of the additive of the present invention to the composite to impart resistance to deformations under stress and heat.
The fibre or flour, polyolefin and additive can be melt-mixed by following the teaching of Goettler U.S. Pat. No. 4,376,144 and of Hamed..U.S. Pat. No. 3,943,079 to produce composite premix of the present invention. A high intensity thermo-kinetic mixer such as K-mixer is advantageously used, and the materials, polyethylene or polypropylene or propylene-ethylene copolymer, cellulosic and lignocellulosic flour or fibre, reinforcement additive and other ingredients, can all be charged initially. The order of addition of material is not critical. However, other charging sequence can be used if desired.
The temperature of melt-mixing of the present invention should be sufficiently high to decompose the reinforcement additive and to melt polyolefin plastic. The dump temperature of the premixed composite should be above 190° centigrade.
The preferred dump temperature range is 200 to 230° centigrade. Higher temperature is preferred for better reinforcement properties of resulting composite. However, other dump temperature can be selected depending on the mixing equipment. Higher temperature can be used, but excessive heat can be harmful over a period of time. The residence time of the compound in the mixer should not exceed more than few minutes for a high intensity mixer and longer time can be used for extruder or other low speed mixer. Effective mixing time for a high speed mixer is between 30 seconds to 3 minutes. Usually, it will be economical to mix as rapidly as possible. Precaution should be taken to avoid excessively high temperature and/or long residence time that might burn fibre or woodflour.
A mixing temperature of the present invention lower than 200 centigrade may be used only if an activator is added along with the reinforcement additive. For a discussion of activator used for elastomer crosslinking, see Handbook of Elastomers, Ch. 8 p.249 et seq (A.Y: Coran, author), Ed. A.K. Bhowmik and H.L. Stephens, Mercel Dekker Inc., New York, 1988. Activators of this invention usually can be an organic compound containing an active sulfur (S.) or active hydrogen (H.). The examples of effective activators are various thiazole, thiuram , dithiocarbamate compounds.
Preferred compounds are thiazole and its derivative. Typical thiazole and its derivative are mercaptobenzthiazole and dibenzothiazyl disulfide. A compatible blend of two or more activators can be used. Example of a preferred blend is dibenzthiazyl disulfide and tetramehtylene thiuram disulfide. Other activators, which also can be used, are mercaptobenzthiazol, tetraethyl thiuram disulfide, zinc diethyl dithiocarbamate and some other sulfur donor such as tetraethyl thiuram tetrasulfide, tetramethyl thiuram tetrasulfide.
The reinforcement additive can be premixed with fibre or flour, or it can be preblended with a small amount of powder form of polyolefin, in order to facilitate dispersion of the additive in the composite. This premixing process is optional depending on the type of mixing equipment used in order to melt the polyolefin plastic and to disperse the fibres in molten polymer. The reinforcement additive can be incorporated into the composites of the invention by mixing the reinforcement additive therewith, at the same time the flour or fibres are combined with the polyolefin plastics. A high speed thermo-kinetic mixer such as K-mixer can be used to melt-mix polyolefin, fibre and reinforcement additive without predispersing the said additive.
The reinforcement additive can also contain other imide compounds, such as 4,4'-diantipyrylmethane, 2- methyl, N-phenylmaleimide, N-cyclohexylmaleimide, N-ethyl-maleimide. Mixtures of one or more of the other imide compounds can be present along with the N, N - 1,3 phenylenedimaleimide as in Structure formula 2, but their effect is inferior thereto in the present invention.
An unexpected beneficial result is noted in that the cellulosic flour or fibre and polyolefin plastic compositions, which contain the reinforcement additive of the present invention show a significant improvement of the final properties.
The effectiveness of the reinforcement additive of the present invention is surprising, since it would not be expected that an imide additive which is designed to use as a crosslinking agent for elastomers, could be so effective even without any activator, in improving dispersity of natural fibres or woodflour and thereby reinforcing a plastic matrix containing polyethylene, polypropylene or their copolymer.
The effect of obtaining a reinforcing effect on the composite of the present invention is of course, to provide a natural fibre- or wood flour-filled polyolefin composite, which has maximum strength, stiffness, heat deflection temperature (HDT) and resistance to creep.
Creep and mechanical properties of the modified wood fibre reinforced thermoplastic composites were reported by Sain, Law and Balatinecz (J. Appl. Polym. Sci.,77, 2000, 260) and Sain and Kokta (J.Appl.Polym.Sci.,54, 1994, 1545). With optimum reinforcement effect which thought to increase not only the adhesion between fibre or woodflour and polyolefin matrix but chemically attach some part of the additive of the present invention to the composite to impart resistance to deformations under stress and heat.
The fibre or flour, polyolefin and additive can be melt-mixed by following the teaching of Goettler U.S. Pat. No. 4,376,144 and of Hamed..U.S. Pat. No. 3,943,079 to produce composite premix of the present invention. A high intensity thermo-kinetic mixer such as K-mixer is advantageously used, and the materials, polyethylene or polypropylene or propylene-ethylene copolymer, cellulosic and lignocellulosic flour or fibre, reinforcement additive and other ingredients, can all be charged initially. The order of addition of material is not critical. However, other charging sequence can be used if desired.
The temperature of melt-mixing of the present invention should be sufficiently high to decompose the reinforcement additive and to melt polyolefin plastic. The dump temperature of the premixed composite should be above 190° centigrade.
The preferred dump temperature range is 200 to 230° centigrade. Higher temperature is preferred for better reinforcement properties of resulting composite. However, other dump temperature can be selected depending on the mixing equipment. Higher temperature can be used, but excessive heat can be harmful over a period of time. The residence time of the compound in the mixer should not exceed more than few minutes for a high intensity mixer and longer time can be used for extruder or other low speed mixer. Effective mixing time for a high speed mixer is between 30 seconds to 3 minutes. Usually, it will be economical to mix as rapidly as possible. Precaution should be taken to avoid excessively high temperature and/or long residence time that might burn fibre or woodflour.
A mixing temperature of the present invention lower than 200 centigrade may be used only if an activator is added along with the reinforcement additive. For a discussion of activator used for elastomer crosslinking, see Handbook of Elastomers, Ch. 8 p.249 et seq (A.Y: Coran, author), Ed. A.K. Bhowmik and H.L. Stephens, Mercel Dekker Inc., New York, 1988. Activators of this invention usually can be an organic compound containing an active sulfur (S.) or active hydrogen (H.). The examples of effective activators are various thiazole, thiuram , dithiocarbamate compounds.
Preferred compounds are thiazole and its derivative. Typical thiazole and its derivative are mercaptobenzthiazole and dibenzothiazyl disulfide. A compatible blend of two or more activators can be used. Example of a preferred blend is dibenzthiazyl disulfide and tetramehtylene thiuram disulfide. Other activators, which also can be used, are mercaptobenzthiazol, tetraethyl thiuram disulfide, zinc diethyl dithiocarbamate and some other sulfur donor such as tetraethyl thiuram tetrasulfide, tetramethyl thiuram tetrasulfide.
In use activators has the effect not only to lower processing temperature well below the decomposition temperature of imide reinforcement additives, but also of improving efficiency of decomposed reinforcement additive.
The effective temperature range for melt-mixing plastics, fibre, wood flour, reinforcement additive along with activator is between 175 to 230° centigrade.
Preferred temperature range for composites prepared with an activator is between 185 to 225°
centigrade. The effectiveness of the activator of the invention is surprising, since it allows melt-mixing and subsequent processing to be carried out at a significantly low temperature thereby preventing any undesired decomposition or burning of lignocellulosic materials.
Again, the time of mixing will usually be minimized, and will of course, depend on a number of factors, type of mixer, degree of shear obtained, type of activator, the proportion of the ingredients, the batch size and batch temperature. The premixed composite of this invention is then subjected to optional pelletization or granulation process to make the composite of this invention easily processible in extruder or other molding equipment.
The proportions of the ingredients will be usually be dictated by the properties described in the final product. The amount of polyolefin used will be at least sufficient to process the compounded premix in a conventional processing equipment such as extruder, injection molder etc. without the requirement of additional plastic addition.
Usually at least 15 parts of polyolefin plastic by weight per 100 parts by weight of composite.
Generally no more than 85 parts of polyolefin plastic by weight per 100 parts of composite by weight will be used, although higher plastic levels can be used if desired.
50- 80 parts by weight of the composite by weight of the polyolefin selected from polyethylene, polypropylene and a mixture of polypropylene and ethylene propylene copolymer has been used in the composite of this invention.
To make the composite 20-50 parts by weight of the cellulose and cellulosic fibres selected from wood flour, bleached kraft, thermomechanical pulp, recycled newsprint and flax having particle size of 20 - 100 mesh have been used effectively. Given the wide variety of formulations which can be used effectively within the scope of the invention, the optimum ratios of polyolefin to fibre or flour can be readily determined by statistical experimentation.
As stated before, the effective level of reinforcement additive selected from maleimidopropionate, hydroxysuccinimide ester derivative, dimaleimides and hydroxyl succinimide derivative in the composite of the invention is from 0.05 to 5 parts per 100 parts of composite by weight. All part of the other ingredients must be added in the melt-mixing stage. The premixed composite of this invention obtained from melt-mixing process is then pelletized or granulated as desired for further processing or it can be used as is for making compression molded products.
When activator is added to reduce the melt-mixing and subsequent processing temperatures, the amount of activator is normally small. For example, activator /imide additive ratio is approximately between 0.01 and 0.3. The preferred level of the activator selected from thiazole, thiuram and dithiocarbomate compounds such as dibenzthiazyldisulphide, 2-mercaptobenzthiazole, tetramethylthiuramdisulphide and cyclohexylbenzthiazyl sulphonamide, in the composites of the invention is from 10 to 20% by weight of the reinforcement additive. The activator is generally added together with other ingredients during the melt-mixing stage. A preferred way to introduce activator is premixing fibre or flour with imide additive and activator before adding to the mixer.
Processing of premixed composite to give the formation of a product is further performed in an extruder, compression molder or injection molder. The processing temperature for making products from the premixed composite of this invention is very critical because further reinforcement of the properties of finished product depends on the processing temperature. Premixed composites prepared without activator according to this invention can be processed in any temperature range between 180 to 240°
centigrade. Effective processing temperature for extrusion, injection molding or compression molding according to this invention is between 200 to 240° centigrade for composites without any added activator. More preferred temperature for activator-free composites is between 220 to 235° centigrade. Temperature above 230° centigrade can be used provided precaution has been taken to prevent thermal degradation of cellulosic fibre during the process.
Preferred processing temperature for premixed composites containing an activator of the present invention is between 180 to 240°C. More preferred temperature range is 200 to 230°C.
According to this invention, the premixed composites contain the aforementioned ingredients when processed in extruder, injection or compression molding equipment produce composite finished products having superior stiffness, strength, heat deformation resistance and creep resistance properties compared to composites products without imide additive. The preferred temperature for injection molding is between 220 to 240°
centigrade and for compression molding is between 210 to 230°C.
Injection, compression and extrusion processes are used to produce test samples to evaluate performance of the composites. Also good surface finish of the composite products has been achieved by employing a higher processing temperature range between 220 to 230°
centigrade.
A better understanding of the invention can be obtained by reference of the following specific examples, in which all parts are by weight unless otherwise indicated.
EXAMPLE I
In order to compare the effect of various imide additives of this invention in a composite formulation, a series of compounds were prepared containing the imide additives, as well as controls. Imide compounds having at least one unsaturation were used as property modifiers. The imide compounds which were used in the composites are: N-cyclohexyl maleimide (Imide A); N-succinimidyl 3-maleimidopropionate (Imide B); 3-maleimidobenzoic acid N-hydroxysuccinimide ester (Imide C); N, N' 1,3-phenylenedimaleimide (imide D). These imide compounds differ in their chemical structure and melting points. Table 1 gives the composition. Samples H and I
contained no fibers. Samples A to D contained polyethylene and approximately 40% wood flour and about 0.75% of Imide A, Imide B , Imide C and Imide D respectively. Samples E
and F
contained polypropylene and approximately 40% wood flour and about 0.75% Imide A
and Imide D respectively. Sample H contained a mixture of about 45%
polypropylene, 15% ethylene-propylene copolymer, 40% wood flour and about 0.75% of Imide D.
Table 1. Composite formulations Composition A B C D E F G H I J K
EP Copolymer 15 Wood flour 40 40 40 40 40 40 40 40 40 (60/80 mesh) Imide A 0.75 0.75 Imide B 0.75 Imide C 0.75 Imide D 0.75 0.75 0.75 Imide A: N-cyclohexyl maleimide; Imide B:N-succinimidyl 3-maleimidopropionate;
Imide C: 3-maleimidobenzoic acid N-hydroxysuccinimide ester; Imide D: N, N' 1,3 phenylenedimaleimide.
Mixing was done in a laboratory K mixer fitted with high speed mixing screw.
The polyolefin plastics and fibers were compounded in this high speed thermokinetic mixer and the dump temperature was about 200-205° centigrade. The residence time of samples in compounding machine varied between 1.5 to 2.5 min.
After the mixes were discharged they were allowed to cool and then granulated with a laboratory scale BrabenderTM granulator. The granulated products were then extruded and injection molded to prepare test specimens according to ASTM standard methods.
The extrusion temperature was between 190 to 230° centigrade depending on the extrusion zones, head and die. For injection molding a temperature of about 230°
centigrade was used.
Samples were then tested using Instron machine for tensile and flexural properties.
ASTM standards D 638M-89 and D 790M-90 were used for tensile and flexural tests.
Tensile and flexural data are presented in Table 2. The addition of various imides causes Table 2. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa A 2.3 30.8 2.5 58.3 3.8 B 2.4 31.2 2.6 62.1 3 .1 C 2.4 32.5 2.6 63.9 2.9 D 2.6 33.8 2.9 68.7 3.4 E 2.7 34.8 3.2 72.5 2.6 F 2.9 38.6 3.2 75.1 2.8 G 2.6 36.7 2.8 68.8 3.6 H 0.7 20.8 1.0 33.6 7.9 I 1.1 30.9 1.4 48.9 9.8 J 2.3 22.8 2.3 51.3 3.8 K 2.3 28.7 2.9 64.7 2.5 sharp increase in the mechanical properties of the composites. Typically it results in an increase of modulus and strength of composites, but a decrease in the elongation.
Depending on the type of imide additives the improvement of modulus and strength were IS
different. For example, imide A resulted in the minimum improvement and imide D
induced maximum improvement in strength and modulus. The observation is true for polyethylene as well as for polypropylene matrices. Elongation was somewhat improved by addition of about 15 parts ethylene-propylene copolymer. The standard deviation for mechanical properties varied within ~ 5% indicating good reproducibility of data. The addition of ethylene-propylene copolymer marginally reduces the strength properties over polypropylene-based composites containing an imide additive. However, the properties were significantly higher than unmodified PP-wood flour composites. Composites made with polypropylene showed more improved tensile and flexural properties over composites prepared from polyethylene both with and without Imide modifiers.
Therefore, the drop in mechanical strength of virgin polymers, namely, polyethylene and polypropylene due to addition of 40 parts of wood flour were more than compensated by the addition of only 0.75 parts of any of the four imide additives and the best imide additive for the composite property reinforcement was Imide D followed by Imide C, Imide B and Imide A.
EXAMPLE II
An illustrative of composite compositions comprising of both polyolefin plastic and imide additive is separately treated with three different kinds of fiber in a high speed mixer and then separately extruded and injection molded. To evaluate the effect of imide additives on composites made by using various fiber sources, a comparison was run in which fibers were selected from agro-sources, from recycled fiber source as well as from processed pulp such as, bleached kraft. The composites were made by the same process Table 3. Composite formulations Composition L M N O P Q R S
EP Copolymer Bleached kraft 20 20 30 30 Flax 40 40 Imide C 0.75 Imide D 0.75 0.75 0.75 as described in Example I. The composites were prepared as shown in Table 3, following, in which all parts are by weight. .The dump temperature for bleached kraft and ONP (old newsprint) did not exceed 205°C and for flax it below 200°C.
Samples L, N, P and R were controls, containing no imide additive. Sample M, O, Q and S were prepared with Imide D additive. Samples L and M contained ONP as fiber source, samples N, O, P and Q contained bleached kraft as fiber source and samples R
and S
contained flax as fiber source. Samples L, M, P and Q contained 30 parts fiber, samples N
and O contained 20 parts fiber and samples R and S contained 40 parts fiber.
The samples after compounding in high speed mixer were injection molded at 220°C to 230°C using a laboratory injection molder. Test results are set forth in Table 4, following.
Table 4. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa L 1.9 24.5 2.3 52.8 5.3 M 1.85 30.7 2.4 61.6 5.5 N 1.7 34.5 2.0 56.6 5.4 O 1.6 39.1 2.1 64.3 5.4 P 2.15 32.4 2.7 60.9 3.1 Q 2.3 43.3 2.9 70.3 3.8 R 2.3 22.2 3.8 50.0 2.0 S 2.5 27.9 3.9 59.4 1.9 The tensile test results in Table 4 indicate that composites containing imide D additive (samples M, O, Q and S) give very good mechanical strength and the tensile modulus and tensile strengths are much higher than their controls. The flexural test data also indicate that the addition of imide additive significantly improved the flexural strength of composites filled with three different fibers. This indicates that imide D is a very good property enhancer for polyethylene and polypropylene composites filled with ONP, bleached kraft or flax fiber. Highest mechanical property was achieved for composite made with bleached kraft followed by ONP and flax. Increase in bleached kraft concentration in composite from 20 parts to 30 parts resulted in further property improvement in presence of Imide D.
EXAMPLE III
An illustrative of another embodiment, there are charged to a high speed mixer variable quantities of polypropylene and wood flour and a constant amount of 0.75 part Imide D
additive. In one composition 25 parts of polypropylene was replaced by an equal amount of ethylene-propylene copolymer. Five compositions as given in Table 5 were melt mixed a high speed in a thermo-kinetic mixer and dumped at 200°C. The compounds were then injection molded in the temperature range 220° centigrade to 230° centigrade.
Table 5. Composite formulations Composition T U V W X
EP Copolymer 25 Wood flour 20 30 40 50 50 (20/40mesh) Imide D 0.75 0.750.75 0.750.75 The tensile and flexural properties of composites with variable amount of wood flour are illustrated in Table 6. Increasing wood flour content from 20 to 50 wt% showed further improvement of mechanical properties over pure polyolefin as in composition H
and I of Table 2 as well as improvement of mechanical strength over unmodified controls as set forth in Examples I and II. Strength improvement was maximum for composition Table 6 Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa T 1.73 30.8 1.81 52.8 5.4 U 2.1 33.7 2.32 61.3 4.1 V 2.4 34.6 2.85 63.8 3.2 W 3.2 36.8 4.1 68.3 2.7 X 2.7 33.5 2.8 60.8 3.6 containing 40 part of wood flour and with 50 part of wood flour the strength properties decreased marginally over composite containing. 4'0 part wood flour. However, all composites in samples T to X showed improved tensile and flexural properties over pure polyethylene and polypropylene.
EXAMPLE IV
In still another embodiment using thermo mechanical pulp (TMP), polypropylene and an imide additive, Imide D, there are charged to an high speed thermo-kinetic mixer 70 parts of polypropylene, 30 parts of TMP and four different concentrations of Imide D
additive ranging from 0.1 part to 5 parts as given in Table 7. The dump temperature was 205°
centigrade and then the composite samples Y, Z, AA and AB were injection molded in Table 7. Composite formulations Composition Y Z AA AB AC
Imide D 0.1 1 2 5 0 the temperature range 220° centigrade to 230°centigrade. Test results of the composites are given in Table 8, following.
Table 8. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa Y 2.5 34.9 3.3 68.2 2.8 Z 2.5 44.7 3.45 76.3 3.1 AA 2.6 41.9 3.3 75.2 2.8 AB 3 .2 3 7.8 3 .7 72.5 2.0 AC 2.6 34.4 3.2 61.3 2.6 The tensile strength in Table 8 indicates that concentration of Imide D
additive has a strong improvement effect on mechanical properties. One part of Imide D
improved tensile and flexural strengths by about 30% over control sample AC. and further increase of Imide D additive to 2 parts and 5 parts resulted in a marginal decrease in strengths of composites. However, the tensile and flexural properties of composites Y, Z, AA and AB
are significantly higher than controls without imide additives as well as pure polyolefins.
Use of only 0.1 part of imide also resulted in improved flexural strength compared to the control. Test results indicate that use of Imide D additive in any concentration range between 0.1 to 5 parts improves tensile and flexural properties of polyolefin-natural fiber composites.
EXAMPLE V
As illustrative to yet another embodiment, there are charged to a high speed thermo-kinetic mixer 70 parts polypropylene, 30 parts TMP and 0.75 part Imide D. The dump temperature was 205° centigrade. These compounded samples as given in Table 9 were then injection molded at three different temperatures 190°, 210°
and 230° centigrade respectively for samples AD, AE and AF respectively. The composite samples after injection molding were tested for tensile and flexural properties.
Table 9.
Composition AD AE AF
Injection molding temperature,190 210 230 C
Imide D 0.75 0.75 0.75 Table 10 gives the tensile and flexural strength and modulus values for these composites.
Table 10. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa AD 2.1 27.8 2.4 61.5 2.9 AE 2.5 36.7 3.1 72.5 3.1 AF 2.6 41.9 3.3 75.2 2.8 Use of high injection temperature improved tensile and flexural strengths of composite. A
temperature of 230° centigrade produced highest improvement in strength and modulus.
Injection temperature above 230° centigrade was not successful because it initiated burning of TMP. High processing temperature provided somewhat darker samples with excellent surface finish. Injection temperature lower than 210°
centigrade developed strength and modulus properties inferior to virgin polyolefins and no reinforcement effect was observed. It is because Imide D does not decompose below 195°
centigrade. It is then essential to process polyolefin-natural fiber compositions with imide additive above the decomposition temperature of the imide.
EXAMPLE VI
The following illustrate embodiments of the invention in which composites are prepared using activators which decrease the melt-mixing and processing temperatures of composite. Several activators are used either alone or in combination. The activators used are dibenzthiazyl disulfide (MBTS); 2-mercaptobenzthiazole (MBT), Tetramethyl thiuram disulfide (TMT); and cyclohexyl benzthiazyl sulphenamide (CBS). Three different imide additives are used and they are N, N' 1,3 phenylenedimaleimide (Imide D); methyl, 1,3 phenylenedimaleimide (Imide E); 4-(Maleimidomethyl)-1-cyclohexanecarboxylic acid N-hydroxysuccinimide (Imide F) Table 11. Composite formulations Composition AG AH AI AJ AK
MBTS 0.3 0.1 0.3 TMT 0.1 0.1 CBS 0.2 MBT 0.1 0.1 Imide D 1.0 1.0 1.0 Imide E 0.75 Imide F 0.75 MBTS: dibenzthiazyl disulfide; MBT:2-mercaptobenzthiazole, TMT: Tetramethyl thiuram disulfide; CBS: cyclohexyl benzthiazyl sulphenamide. E:4 methyl, 1,3 phenylenedimaleimide; F:4-(Maleimidomethyl)-1-cyclohexanecarboxylic acid N-hydroxysuccinimide To a high shear thermo-kinetic mixer, the ingredients shown in compositions AG
to AK
are charged separately and mixed at high speed until the dump temperature reached 195°
centigrade. The compositions were then separately injection molded in the temperature range 200 to 210° centigrade to produce test samples. The test results are shown in Table 12 , following.
Table 12. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa AG 2.4 43.8 3.6 72.2 3.9 AH 2.1 38.7 2.97 68.5 3.1 AI 2.1 37.9 2.9 66.9 3.2 AJ 1.97 32.7 2.5 63.8 4.8 AK 2.3 42.7 3.5 71.8 4.1 The tensile test results in Table 12 indicate that addition of activators helped to reduce process temperature without significant sacrifice in strength and modulus. The processing temperature used in this case is 210° centigrade but the properties are significantly better than composition AE for PP composite of Table 10, Example V and sample M for PE
composite of Table 4, Example II which can be used as controls. It is further found that MBTS is most effective and a combination of MBTS and TMT is least effective in reducing the processing temperature. The suitable concentration range for activator is found to be in the range of 10 to 40% by weight of Imide additive. It is also evident from results in Table 12 that Imide E and Imide F additives are less effective than Imide D
additive in reinforcing tensile and flexural properties of the said composites because the tensile and flexural strength and modulus for samples AH and AI are inferior to sample AG.
EXAMPLE VII
The following illustrates another embodiment in which the samples H, I, L, M, P, Q, R, S, Y, Z, AA, AB, AD and AG are melt mixed in a high speed thermo-kinetic mixer at 200°
centigrade dump temperature followed by injection molding at 230°
centigrade except for sample AD which was processed at 190° centigrade, to prepare test samples for measuring heat distortion temperature and creep properties. The heat deflection temperature was measured according to ASTM standard D 648 at 1.8 MPa load. The creep test was conducted at 40° centigrade for 1 day according to ASTM
standard D
2990. The stress level for creep measurement was 30% of the room temperature ultimate flexural strength of respective molded materials. The test results are given in Table 13 and Table 14 as follows:
Table 13. Thermal properties Sample H L M AG
HDT, C 51 92 105 107 Initial strain 28 6.3 4.8 4.2 (x 10'3 mm/mm) Relative creep 380 250 189 181 (%) Table 14. Thermal properties Sample I P Q R S Y Z AA AB AD
HDT,C 54 98 108 96 105 101 111 113 116 110 Initial 20 3.7 2.8 - - - - 2.4 - 2.7 strain (x 10-3 mm/mm) Relative 425 195 140 - - - - 121 - 135 creep (%) It is evident from Table 13 results that heat distortion temperature and creep resistance of composites are significantly higher than virgin PE (sample H). Again the addition of Imide D (sample M and AG) further increased heat distortion temperature and creep resistance of unmodified composite (sample L). Again, increase of concentration of Imide D has a positive effect on heat distortion temperature and creep resistance (sample M and sample AG).
Results from Table 14 revealed that both creep resistance and heat distortion temperature of virgin PP can be significantly improved by adding natural fiber and Imide additive.
There is almost 100% increase in the heat distortion temperature of PP when a composite is prepared from this virgin PP in combination with 30 parts of bleached kraft and 0.75 part of Imide D. The same composite sample (sample Q) has improved creep resistance of virgin PP (sample I) and unmodified composite (sample P) significantly by yielding a relative creep of 140% against 195% for composite without Imide D additive.
Again increase of Imide D concentration from 0.75 parts to 2 parts in composite (sample AA) further improved heat distortion temperature and relative creep. On the other hand, a lower processing temperature during molding process such as for Sample AD, the heat distortion temperature and the relative creep resistance are inferior to the composites which is processed at higher temperature with same chemical ingredients (sample Q). in other word, higher process temper~u~e improved creep resistance and heat distortion temperature as well. Again, both wood fiber (bleached kraft) and nonwood fiber (Samples R and S) showed improvement in croep resistance and heat distortion temperature when Imide additive is added. Results in Table 12 further indicate that increasing imide additive concentration fi~om 0.1 part to 5 part improved heat distortion temperature of composites by about 15° centigrade (samples Q, Y, Z, AA, A13). All these composites have a very smooth surface finish with no unpleasant odour and they are also extruded to obtain strips of very high surface finish.
Although the invention has been illustrated by typical examples, it is not limited thereto.
Changes and modifications of the examples of the invention herein chosen for purposes of disclosure can be made which do not constitute depaitume from the spirit and scope of the invention.
The effective temperature range for melt-mixing plastics, fibre, wood flour, reinforcement additive along with activator is between 175 to 230° centigrade.
Preferred temperature range for composites prepared with an activator is between 185 to 225°
centigrade. The effectiveness of the activator of the invention is surprising, since it allows melt-mixing and subsequent processing to be carried out at a significantly low temperature thereby preventing any undesired decomposition or burning of lignocellulosic materials.
Again, the time of mixing will usually be minimized, and will of course, depend on a number of factors, type of mixer, degree of shear obtained, type of activator, the proportion of the ingredients, the batch size and batch temperature. The premixed composite of this invention is then subjected to optional pelletization or granulation process to make the composite of this invention easily processible in extruder or other molding equipment.
The proportions of the ingredients will be usually be dictated by the properties described in the final product. The amount of polyolefin used will be at least sufficient to process the compounded premix in a conventional processing equipment such as extruder, injection molder etc. without the requirement of additional plastic addition.
Usually at least 15 parts of polyolefin plastic by weight per 100 parts by weight of composite.
Generally no more than 85 parts of polyolefin plastic by weight per 100 parts of composite by weight will be used, although higher plastic levels can be used if desired.
50- 80 parts by weight of the composite by weight of the polyolefin selected from polyethylene, polypropylene and a mixture of polypropylene and ethylene propylene copolymer has been used in the composite of this invention.
To make the composite 20-50 parts by weight of the cellulose and cellulosic fibres selected from wood flour, bleached kraft, thermomechanical pulp, recycled newsprint and flax having particle size of 20 - 100 mesh have been used effectively. Given the wide variety of formulations which can be used effectively within the scope of the invention, the optimum ratios of polyolefin to fibre or flour can be readily determined by statistical experimentation.
As stated before, the effective level of reinforcement additive selected from maleimidopropionate, hydroxysuccinimide ester derivative, dimaleimides and hydroxyl succinimide derivative in the composite of the invention is from 0.05 to 5 parts per 100 parts of composite by weight. All part of the other ingredients must be added in the melt-mixing stage. The premixed composite of this invention obtained from melt-mixing process is then pelletized or granulated as desired for further processing or it can be used as is for making compression molded products.
When activator is added to reduce the melt-mixing and subsequent processing temperatures, the amount of activator is normally small. For example, activator /imide additive ratio is approximately between 0.01 and 0.3. The preferred level of the activator selected from thiazole, thiuram and dithiocarbomate compounds such as dibenzthiazyldisulphide, 2-mercaptobenzthiazole, tetramethylthiuramdisulphide and cyclohexylbenzthiazyl sulphonamide, in the composites of the invention is from 10 to 20% by weight of the reinforcement additive. The activator is generally added together with other ingredients during the melt-mixing stage. A preferred way to introduce activator is premixing fibre or flour with imide additive and activator before adding to the mixer.
Processing of premixed composite to give the formation of a product is further performed in an extruder, compression molder or injection molder. The processing temperature for making products from the premixed composite of this invention is very critical because further reinforcement of the properties of finished product depends on the processing temperature. Premixed composites prepared without activator according to this invention can be processed in any temperature range between 180 to 240°
centigrade. Effective processing temperature for extrusion, injection molding or compression molding according to this invention is between 200 to 240° centigrade for composites without any added activator. More preferred temperature for activator-free composites is between 220 to 235° centigrade. Temperature above 230° centigrade can be used provided precaution has been taken to prevent thermal degradation of cellulosic fibre during the process.
Preferred processing temperature for premixed composites containing an activator of the present invention is between 180 to 240°C. More preferred temperature range is 200 to 230°C.
According to this invention, the premixed composites contain the aforementioned ingredients when processed in extruder, injection or compression molding equipment produce composite finished products having superior stiffness, strength, heat deformation resistance and creep resistance properties compared to composites products without imide additive. The preferred temperature for injection molding is between 220 to 240°
centigrade and for compression molding is between 210 to 230°C.
Injection, compression and extrusion processes are used to produce test samples to evaluate performance of the composites. Also good surface finish of the composite products has been achieved by employing a higher processing temperature range between 220 to 230°
centigrade.
A better understanding of the invention can be obtained by reference of the following specific examples, in which all parts are by weight unless otherwise indicated.
EXAMPLE I
In order to compare the effect of various imide additives of this invention in a composite formulation, a series of compounds were prepared containing the imide additives, as well as controls. Imide compounds having at least one unsaturation were used as property modifiers. The imide compounds which were used in the composites are: N-cyclohexyl maleimide (Imide A); N-succinimidyl 3-maleimidopropionate (Imide B); 3-maleimidobenzoic acid N-hydroxysuccinimide ester (Imide C); N, N' 1,3-phenylenedimaleimide (imide D). These imide compounds differ in their chemical structure and melting points. Table 1 gives the composition. Samples H and I
contained no fibers. Samples A to D contained polyethylene and approximately 40% wood flour and about 0.75% of Imide A, Imide B , Imide C and Imide D respectively. Samples E
and F
contained polypropylene and approximately 40% wood flour and about 0.75% Imide A
and Imide D respectively. Sample H contained a mixture of about 45%
polypropylene, 15% ethylene-propylene copolymer, 40% wood flour and about 0.75% of Imide D.
Table 1. Composite formulations Composition A B C D E F G H I J K
EP Copolymer 15 Wood flour 40 40 40 40 40 40 40 40 40 (60/80 mesh) Imide A 0.75 0.75 Imide B 0.75 Imide C 0.75 Imide D 0.75 0.75 0.75 Imide A: N-cyclohexyl maleimide; Imide B:N-succinimidyl 3-maleimidopropionate;
Imide C: 3-maleimidobenzoic acid N-hydroxysuccinimide ester; Imide D: N, N' 1,3 phenylenedimaleimide.
Mixing was done in a laboratory K mixer fitted with high speed mixing screw.
The polyolefin plastics and fibers were compounded in this high speed thermokinetic mixer and the dump temperature was about 200-205° centigrade. The residence time of samples in compounding machine varied between 1.5 to 2.5 min.
After the mixes were discharged they were allowed to cool and then granulated with a laboratory scale BrabenderTM granulator. The granulated products were then extruded and injection molded to prepare test specimens according to ASTM standard methods.
The extrusion temperature was between 190 to 230° centigrade depending on the extrusion zones, head and die. For injection molding a temperature of about 230°
centigrade was used.
Samples were then tested using Instron machine for tensile and flexural properties.
ASTM standards D 638M-89 and D 790M-90 were used for tensile and flexural tests.
Tensile and flexural data are presented in Table 2. The addition of various imides causes Table 2. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa A 2.3 30.8 2.5 58.3 3.8 B 2.4 31.2 2.6 62.1 3 .1 C 2.4 32.5 2.6 63.9 2.9 D 2.6 33.8 2.9 68.7 3.4 E 2.7 34.8 3.2 72.5 2.6 F 2.9 38.6 3.2 75.1 2.8 G 2.6 36.7 2.8 68.8 3.6 H 0.7 20.8 1.0 33.6 7.9 I 1.1 30.9 1.4 48.9 9.8 J 2.3 22.8 2.3 51.3 3.8 K 2.3 28.7 2.9 64.7 2.5 sharp increase in the mechanical properties of the composites. Typically it results in an increase of modulus and strength of composites, but a decrease in the elongation.
Depending on the type of imide additives the improvement of modulus and strength were IS
different. For example, imide A resulted in the minimum improvement and imide D
induced maximum improvement in strength and modulus. The observation is true for polyethylene as well as for polypropylene matrices. Elongation was somewhat improved by addition of about 15 parts ethylene-propylene copolymer. The standard deviation for mechanical properties varied within ~ 5% indicating good reproducibility of data. The addition of ethylene-propylene copolymer marginally reduces the strength properties over polypropylene-based composites containing an imide additive. However, the properties were significantly higher than unmodified PP-wood flour composites. Composites made with polypropylene showed more improved tensile and flexural properties over composites prepared from polyethylene both with and without Imide modifiers.
Therefore, the drop in mechanical strength of virgin polymers, namely, polyethylene and polypropylene due to addition of 40 parts of wood flour were more than compensated by the addition of only 0.75 parts of any of the four imide additives and the best imide additive for the composite property reinforcement was Imide D followed by Imide C, Imide B and Imide A.
EXAMPLE II
An illustrative of composite compositions comprising of both polyolefin plastic and imide additive is separately treated with three different kinds of fiber in a high speed mixer and then separately extruded and injection molded. To evaluate the effect of imide additives on composites made by using various fiber sources, a comparison was run in which fibers were selected from agro-sources, from recycled fiber source as well as from processed pulp such as, bleached kraft. The composites were made by the same process Table 3. Composite formulations Composition L M N O P Q R S
EP Copolymer Bleached kraft 20 20 30 30 Flax 40 40 Imide C 0.75 Imide D 0.75 0.75 0.75 as described in Example I. The composites were prepared as shown in Table 3, following, in which all parts are by weight. .The dump temperature for bleached kraft and ONP (old newsprint) did not exceed 205°C and for flax it below 200°C.
Samples L, N, P and R were controls, containing no imide additive. Sample M, O, Q and S were prepared with Imide D additive. Samples L and M contained ONP as fiber source, samples N, O, P and Q contained bleached kraft as fiber source and samples R
and S
contained flax as fiber source. Samples L, M, P and Q contained 30 parts fiber, samples N
and O contained 20 parts fiber and samples R and S contained 40 parts fiber.
The samples after compounding in high speed mixer were injection molded at 220°C to 230°C using a laboratory injection molder. Test results are set forth in Table 4, following.
Table 4. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa L 1.9 24.5 2.3 52.8 5.3 M 1.85 30.7 2.4 61.6 5.5 N 1.7 34.5 2.0 56.6 5.4 O 1.6 39.1 2.1 64.3 5.4 P 2.15 32.4 2.7 60.9 3.1 Q 2.3 43.3 2.9 70.3 3.8 R 2.3 22.2 3.8 50.0 2.0 S 2.5 27.9 3.9 59.4 1.9 The tensile test results in Table 4 indicate that composites containing imide D additive (samples M, O, Q and S) give very good mechanical strength and the tensile modulus and tensile strengths are much higher than their controls. The flexural test data also indicate that the addition of imide additive significantly improved the flexural strength of composites filled with three different fibers. This indicates that imide D is a very good property enhancer for polyethylene and polypropylene composites filled with ONP, bleached kraft or flax fiber. Highest mechanical property was achieved for composite made with bleached kraft followed by ONP and flax. Increase in bleached kraft concentration in composite from 20 parts to 30 parts resulted in further property improvement in presence of Imide D.
EXAMPLE III
An illustrative of another embodiment, there are charged to a high speed mixer variable quantities of polypropylene and wood flour and a constant amount of 0.75 part Imide D
additive. In one composition 25 parts of polypropylene was replaced by an equal amount of ethylene-propylene copolymer. Five compositions as given in Table 5 were melt mixed a high speed in a thermo-kinetic mixer and dumped at 200°C. The compounds were then injection molded in the temperature range 220° centigrade to 230° centigrade.
Table 5. Composite formulations Composition T U V W X
EP Copolymer 25 Wood flour 20 30 40 50 50 (20/40mesh) Imide D 0.75 0.750.75 0.750.75 The tensile and flexural properties of composites with variable amount of wood flour are illustrated in Table 6. Increasing wood flour content from 20 to 50 wt% showed further improvement of mechanical properties over pure polyolefin as in composition H
and I of Table 2 as well as improvement of mechanical strength over unmodified controls as set forth in Examples I and II. Strength improvement was maximum for composition Table 6 Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa T 1.73 30.8 1.81 52.8 5.4 U 2.1 33.7 2.32 61.3 4.1 V 2.4 34.6 2.85 63.8 3.2 W 3.2 36.8 4.1 68.3 2.7 X 2.7 33.5 2.8 60.8 3.6 containing 40 part of wood flour and with 50 part of wood flour the strength properties decreased marginally over composite containing. 4'0 part wood flour. However, all composites in samples T to X showed improved tensile and flexural properties over pure polyethylene and polypropylene.
EXAMPLE IV
In still another embodiment using thermo mechanical pulp (TMP), polypropylene and an imide additive, Imide D, there are charged to an high speed thermo-kinetic mixer 70 parts of polypropylene, 30 parts of TMP and four different concentrations of Imide D
additive ranging from 0.1 part to 5 parts as given in Table 7. The dump temperature was 205°
centigrade and then the composite samples Y, Z, AA and AB were injection molded in Table 7. Composite formulations Composition Y Z AA AB AC
Imide D 0.1 1 2 5 0 the temperature range 220° centigrade to 230°centigrade. Test results of the composites are given in Table 8, following.
Table 8. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa Y 2.5 34.9 3.3 68.2 2.8 Z 2.5 44.7 3.45 76.3 3.1 AA 2.6 41.9 3.3 75.2 2.8 AB 3 .2 3 7.8 3 .7 72.5 2.0 AC 2.6 34.4 3.2 61.3 2.6 The tensile strength in Table 8 indicates that concentration of Imide D
additive has a strong improvement effect on mechanical properties. One part of Imide D
improved tensile and flexural strengths by about 30% over control sample AC. and further increase of Imide D additive to 2 parts and 5 parts resulted in a marginal decrease in strengths of composites. However, the tensile and flexural properties of composites Y, Z, AA and AB
are significantly higher than controls without imide additives as well as pure polyolefins.
Use of only 0.1 part of imide also resulted in improved flexural strength compared to the control. Test results indicate that use of Imide D additive in any concentration range between 0.1 to 5 parts improves tensile and flexural properties of polyolefin-natural fiber composites.
EXAMPLE V
As illustrative to yet another embodiment, there are charged to a high speed thermo-kinetic mixer 70 parts polypropylene, 30 parts TMP and 0.75 part Imide D. The dump temperature was 205° centigrade. These compounded samples as given in Table 9 were then injection molded at three different temperatures 190°, 210°
and 230° centigrade respectively for samples AD, AE and AF respectively. The composite samples after injection molding were tested for tensile and flexural properties.
Table 9.
Composition AD AE AF
Injection molding temperature,190 210 230 C
Imide D 0.75 0.75 0.75 Table 10 gives the tensile and flexural strength and modulus values for these composites.
Table 10. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa AD 2.1 27.8 2.4 61.5 2.9 AE 2.5 36.7 3.1 72.5 3.1 AF 2.6 41.9 3.3 75.2 2.8 Use of high injection temperature improved tensile and flexural strengths of composite. A
temperature of 230° centigrade produced highest improvement in strength and modulus.
Injection temperature above 230° centigrade was not successful because it initiated burning of TMP. High processing temperature provided somewhat darker samples with excellent surface finish. Injection temperature lower than 210°
centigrade developed strength and modulus properties inferior to virgin polyolefins and no reinforcement effect was observed. It is because Imide D does not decompose below 195°
centigrade. It is then essential to process polyolefin-natural fiber compositions with imide additive above the decomposition temperature of the imide.
EXAMPLE VI
The following illustrate embodiments of the invention in which composites are prepared using activators which decrease the melt-mixing and processing temperatures of composite. Several activators are used either alone or in combination. The activators used are dibenzthiazyl disulfide (MBTS); 2-mercaptobenzthiazole (MBT), Tetramethyl thiuram disulfide (TMT); and cyclohexyl benzthiazyl sulphenamide (CBS). Three different imide additives are used and they are N, N' 1,3 phenylenedimaleimide (Imide D); methyl, 1,3 phenylenedimaleimide (Imide E); 4-(Maleimidomethyl)-1-cyclohexanecarboxylic acid N-hydroxysuccinimide (Imide F) Table 11. Composite formulations Composition AG AH AI AJ AK
MBTS 0.3 0.1 0.3 TMT 0.1 0.1 CBS 0.2 MBT 0.1 0.1 Imide D 1.0 1.0 1.0 Imide E 0.75 Imide F 0.75 MBTS: dibenzthiazyl disulfide; MBT:2-mercaptobenzthiazole, TMT: Tetramethyl thiuram disulfide; CBS: cyclohexyl benzthiazyl sulphenamide. E:4 methyl, 1,3 phenylenedimaleimide; F:4-(Maleimidomethyl)-1-cyclohexanecarboxylic acid N-hydroxysuccinimide To a high shear thermo-kinetic mixer, the ingredients shown in compositions AG
to AK
are charged separately and mixed at high speed until the dump temperature reached 195°
centigrade. The compositions were then separately injection molded in the temperature range 200 to 210° centigrade to produce test samples. The test results are shown in Table 12 , following.
Table 12. Mechanical properties Sample Tensile Tensile FlexuralFlexural Breaking mod., strength,Modulus Strength,elongation, GPa MPa GPa MPa AG 2.4 43.8 3.6 72.2 3.9 AH 2.1 38.7 2.97 68.5 3.1 AI 2.1 37.9 2.9 66.9 3.2 AJ 1.97 32.7 2.5 63.8 4.8 AK 2.3 42.7 3.5 71.8 4.1 The tensile test results in Table 12 indicate that addition of activators helped to reduce process temperature without significant sacrifice in strength and modulus. The processing temperature used in this case is 210° centigrade but the properties are significantly better than composition AE for PP composite of Table 10, Example V and sample M for PE
composite of Table 4, Example II which can be used as controls. It is further found that MBTS is most effective and a combination of MBTS and TMT is least effective in reducing the processing temperature. The suitable concentration range for activator is found to be in the range of 10 to 40% by weight of Imide additive. It is also evident from results in Table 12 that Imide E and Imide F additives are less effective than Imide D
additive in reinforcing tensile and flexural properties of the said composites because the tensile and flexural strength and modulus for samples AH and AI are inferior to sample AG.
EXAMPLE VII
The following illustrates another embodiment in which the samples H, I, L, M, P, Q, R, S, Y, Z, AA, AB, AD and AG are melt mixed in a high speed thermo-kinetic mixer at 200°
centigrade dump temperature followed by injection molding at 230°
centigrade except for sample AD which was processed at 190° centigrade, to prepare test samples for measuring heat distortion temperature and creep properties. The heat deflection temperature was measured according to ASTM standard D 648 at 1.8 MPa load. The creep test was conducted at 40° centigrade for 1 day according to ASTM
standard D
2990. The stress level for creep measurement was 30% of the room temperature ultimate flexural strength of respective molded materials. The test results are given in Table 13 and Table 14 as follows:
Table 13. Thermal properties Sample H L M AG
HDT, C 51 92 105 107 Initial strain 28 6.3 4.8 4.2 (x 10'3 mm/mm) Relative creep 380 250 189 181 (%) Table 14. Thermal properties Sample I P Q R S Y Z AA AB AD
HDT,C 54 98 108 96 105 101 111 113 116 110 Initial 20 3.7 2.8 - - - - 2.4 - 2.7 strain (x 10-3 mm/mm) Relative 425 195 140 - - - - 121 - 135 creep (%) It is evident from Table 13 results that heat distortion temperature and creep resistance of composites are significantly higher than virgin PE (sample H). Again the addition of Imide D (sample M and AG) further increased heat distortion temperature and creep resistance of unmodified composite (sample L). Again, increase of concentration of Imide D has a positive effect on heat distortion temperature and creep resistance (sample M and sample AG).
Results from Table 14 revealed that both creep resistance and heat distortion temperature of virgin PP can be significantly improved by adding natural fiber and Imide additive.
There is almost 100% increase in the heat distortion temperature of PP when a composite is prepared from this virgin PP in combination with 30 parts of bleached kraft and 0.75 part of Imide D. The same composite sample (sample Q) has improved creep resistance of virgin PP (sample I) and unmodified composite (sample P) significantly by yielding a relative creep of 140% against 195% for composite without Imide D additive.
Again increase of Imide D concentration from 0.75 parts to 2 parts in composite (sample AA) further improved heat distortion temperature and relative creep. On the other hand, a lower processing temperature during molding process such as for Sample AD, the heat distortion temperature and the relative creep resistance are inferior to the composites which is processed at higher temperature with same chemical ingredients (sample Q). in other word, higher process temper~u~e improved creep resistance and heat distortion temperature as well. Again, both wood fiber (bleached kraft) and nonwood fiber (Samples R and S) showed improvement in croep resistance and heat distortion temperature when Imide additive is added. Results in Table 12 further indicate that increasing imide additive concentration fi~om 0.1 part to 5 part improved heat distortion temperature of composites by about 15° centigrade (samples Q, Y, Z, AA, A13). All these composites have a very smooth surface finish with no unpleasant odour and they are also extruded to obtain strips of very high surface finish.
Although the invention has been illustrated by typical examples, it is not limited thereto.
Changes and modifications of the examples of the invention herein chosen for purposes of disclosure can be made which do not constitute depaitume from the spirit and scope of the invention.
Claims (13)
1. A natural fiber-reinforced polyolefin resin composite prepared by first melt-mixing and then injection molding or extruding a mixture comprised of:
(a) 50 to 80 parts by weight of a polyolefin selected from polyethylene, polypropylene and a mixture of polypropylene and ethylene-propylene copolymer, (b) 20 to 50 parts by weight of natural fiber selected from wood flour, thermo mechanical pulp, bleached kraft, flax and recycled newsprint, 28a (c) an imide additive in an amount 0.05 to 5 parts by weight per 100 parts by weight of the sum of components (a) and (b), (d) said composite may contain an activator or a combination of activators selected from thiazole, thiuram and dithiocarbamate compounds in which said activator to said imide additive ratio falls within the range of 0.01 to 0.3 said composite having a heat distortion temperature above 90° centigrade at 1.8 MPa load and a relative creep of less than 200% at 30% load of the respective flexural strength of composites at 40° centigrade.
(a) 50 to 80 parts by weight of a polyolefin selected from polyethylene, polypropylene and a mixture of polypropylene and ethylene-propylene copolymer, (b) 20 to 50 parts by weight of natural fiber selected from wood flour, thermo mechanical pulp, bleached kraft, flax and recycled newsprint, 28a (c) an imide additive in an amount 0.05 to 5 parts by weight per 100 parts by weight of the sum of components (a) and (b), (d) said composite may contain an activator or a combination of activators selected from thiazole, thiuram and dithiocarbamate compounds in which said activator to said imide additive ratio falls within the range of 0.01 to 0.3 said composite having a heat distortion temperature above 90° centigrade at 1.8 MPa load and a relative creep of less than 200% at 30% load of the respective flexural strength of composites at 40° centigrade.
2. The natural fiber-reinforced composite of claim 1, wherein the imide is N, N' 1, 3 -phenylenedimaleimide.
3. The natural fiber-reinforced composite of claim 1 wherein the wood flour having particle size of 20 to 100 mesh.
4. The natural fiber-reinforced composite of claim 1 wherein the wood fiber, recycled newsprint and flax fiber having aspect ratio of 10 to 250.
5. The natural fiber-reinforced composite of claim 1 wherein the composite does not contain any activator.
6. The natural fiber-reinforced composite of claim 1 wherein the composite is melt-mixed and processed not below 200° centigrade without any activator compound.
7. The natural fiber-reinforced composite of claim 1 wherein the composite does contain an activator or a combination of activators.
8. The natural fiber-reinforced composite of claim 1 wherein the activators are selected from dibenzthiazyl disulfide; 2-mercaptobenzthiazole, Tetramethyl thiuram disulfide;
and cyclohexyl benzthiazyl sulphenamide.
and cyclohexyl benzthiazyl sulphenamide.
9. The natural fiber-reinforced composite of claim 1 wherein the activator is dibenzthiazyl disulfide.
10. The natural fiber-reinforced composite of claim 8 wherein the composite is melt-mixed and processed at a temperature of from 180° centigrade to 230° centigrade.
11. The natural fiber-reinforced composite of claim 1 wherein the imide compound is represented by the chemical structure Formula I wherein R1, is selected from hydrogen, alkyl, aryl, cyclohexyl, N-substituted imide and imide ester; R2 and R3 are selected from H, alkyl and aryl group.
12. The natural fiber-reinforced composite of claim 1, wherein imide compound is selected from maleimidopropionate; hydroxysuccinimide ester derivative;
dimaleimide; and hydroxysuccinimide derivative.
dimaleimide; and hydroxysuccinimide derivative.
13. A premixed composite in the form of granulates obtained by granulating a natural fiber-reinforced polyolefin composition obtained by melt-mixing a mixture consists of (a) 50 to 80 parts by weight of a polyolefin selected from polyethylene, polypropylene and a mixture of polypropylene and ethylene-propylene copolymer, (b) 20 to 50 parts by weight of natural fiber selected from wood flour, thermo mechanical pulp, bleached kraft, flax and recycled newsprint, (c) an imide compound in an amount 0.05 to 5 parts by weight per 100 parts by weight of the sum of components (a) and (b), with or without an activator or a combination of activators selected from thiazole, thiuram and dithiocarbamate compounds in which said activator to said imide additive ratio falls within the range of 0.01 to 0.3 said composite having a heat distortion temperature above 90° centigrade at 1.8 MPa load and a relative creep of less than 200% at 30% load of the respective flexural strength of composites at 40°
centigrade.
centigrade.
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