Reinforced Polymer Composites With Wood Fibers Grafted With Silanes

(1) Field of the Invention

This invention relates to polymer composites. More particularly, this invention relates to a technique for grafting organic materials as such, or mixed with an inorganic material, with a silylating agent. Bonding the silylated material with a polyolefin resin produces either composites having improved physicomechanical properties, or composites having only strongly bonded components.

(2) Description of Prior Art

The modification of the physical and mechanical properties of thermoplastic, thermosetting or rubbers by the addition of a filler is well known in the art. Greater degrees of property improvement result when the filler is uniformly and highly dispersed throughout the polymer matrix and/or when its chemical interface provides a possible site weakness in the composite.

Various means for improving the compatibility of the filler with the polymer matrix by providing an interaction between the filler and the polymer are known in the art. For example, when a cellulosic material, such as wood fibers, or cotton fibers, are used as a filler in the polymers containing methylol groups such as phenolic urea or melamine resins, a reaction occurs between the methylol groups and the hydroxyl groups on the cellulosic filler. Inorganic fillers such as calcium carbonate, clay or asbestos have been encapsulated and dispersed in a polyolefin in order to obtain improved properties. Some materials such as maleic anhydride or others were bonded by the effect of the initiating agents such as peroxides. In such a system, the polymer used to coat the filler should be chemically bonded to the filler in order to obtain optimum properties such as high strength.

The E.P. Pat. No. 0008143-Solvay & Cie, issued in Feb. 20, 1980, and the Japanese Pat. No. J5 8011-537- Showa Den Kokk, issued on Jan. 22, 1983, both teach the application of organic and/or inorganic fillers in presence of a peroxide and an acid anhydride, resulting in composite materials having relatively little improvements in their mechanical strengths.

The Japanese Pat. No. J5 719 2-466 Showa issused in May 5, 1981, teaches the addition of an unsaturated silane coupling agent onto the polyolefin matrix material by the effect of a catalyst peroxide and then by adding a wood flour to act as a filler. The composite materials produced according to this reference have improved impact strength, but, in comparing their relative improvement values with those realised by the present invention they were found much lower, because the silane coupling agent was grafted onto the polymer resin and was not grafted onto the cellulosic filler material. Grafting and bonding of a silylating agent onto cellulosic fibers whether premixed or not with an inorganic filler leads to producing composite materials having dramatic higher physicomechanical properties beside their high durability and resistivity to heat, cold and solvents. It should be noted that the silane ratios added according to the present invention can be lowered by prehydrolysing in acid medium which leads to cost economy.

According to the present invention, the silane grafting and bonding process creates strong attachments between the mixture of components and the silylating agent. The grafting process could either be partial or complete. The process could be completed by the effect of a hot molten polymer resin, which provides sites for the formation of free radicals with desired degrees of loading by the initiation effect to attach by chemical bonding to the other side of the silylating agent molecules, which are pregrafted and prebonded with the mixture of materials. Thus, for example, the flow properties of polyethylene are improved by the incorporation of the grafted filler mixture in the presence of maleic anhydride and a free radical catalyst such as benzoyl peroxide. The composition ratios range from 1 to 99% by weight of the grafted and bonded silylated materials.

An object of this invention is to provide an improved technique for compatibilizing particulate silylated materials with polymeric matrix material or rubber.

Another object of this invention is to provide a process for grafting a silylating agent onto an organic material which may be previously mixed with an inorganic material.

The organic material is selected from the group of starch and cellulosic materials. Any effective method can be applied for grafting and bonding a silylating agent onto the said organic materials or organic and inorganic mixture of materials, so as to make the filler or bonding materials compatible for thermoplastic or thermosetting polymers or rubber with controlled weight ratios according to desired applications.

One embodiment of this invention proves that the resultant mechanical strengths of composites produced from bonding thermoplastic polymer with silylated mixture of organic and inorganic materials are much better compared to those of composites produced by bonding the same polymer resin with each individual silylated mixture components.

A further embodiment of this invention is not only to render a silylated mixture of an organic and an inorganic materials compatible as a filler for thermoplastic polymers to improve their physical and mechanical properties, but also to bond, glue or coat said silylated mixture of materials, or the silane grafted cellulosic material only, with a polymer resin which is thermoplastic or thermosetting, or rubber, which leads to several useful applications such as fabrication of boards or panels, plastic coating, glueing, or to be adapted for other useful applications.

The cellulosic, inorganic or polymer resin materials could be in different states and forms. The states could be solids, dissolved or melted solids, liquids or solutions. The forms could be powders, particles, crystals, fibers, sheets, papers, cartons, threads, cloth, gravels, chips, boards, panels, pre-shaped forms, etc.

This invention can be optimized by adjusting the methods and/or conditions of preparations. Thus, it shows its most useful application with polyethylene and polypropylene reinforced with wood pulps mixed or not with calcium carbonte, asbestos or clay and grafted with silylating agent, in the present or absence of maleic anhydride. It will be more specifically described hereafter in details, regarding composites of linear low density polyethylene-wood pulp/asbestos, linear low density polyethyelene-wood pulp/asbestos, polypropylene/aspen wood pulp/calcium carbonate. However, this invention is not limited to such materials, nor to their weight ratios. It also shows advantages when used with any kind or any weight ratio of silylating agents in grafting with any kind or any weight ratio of organic fillers selected from the group of starch or cellulosic material such as cotton fibers, other wood pulps, stem or vegetable fibers, wood flours, starch, waste papers, cartons, or cellulosic cloth or any natural or synthetic cellulosic material which may be mixed with any kind or any weight ratio of inorganic fillers selected from a compound containing hydroxyl group, clay, calcium carbonate, asbestos, sand, wollastonite, quartz, glass, mica, diatomaceous earth, silica talc, kaolinite, aramide, hydrate or partially hydrate metal oxides, metal powders, metal compounds, carbon, graphite, synthetic, or a natural occurring material and if naturally occurring they could be used in either purified or unpurified form. Also other organic materials such as lignin or lignin compounds can be mixed with said cellulosic materials and also more than one kind of inorganic material can be mixed or bonded with the cellulosic materials.

In addition, for the preparation of composites by compounding the silylated mixture of materials with rubbers, thermosettings or thermoplastic polymers, the polymers may be those described in said U.S. Pat. No. 4,317,765.

The ethylenically unsaturated carboxylic acid or acid anhydride as an auxiliary agent used in the practice of this invention is preferably a dicarboxylic acid such as maleic acid, maleic anhydride, furmaric acid, citraconic acid, or itaconic acid. Maleic anhydride is the preferred auxiliary agent. Monocarboxylic acids such as acrylic acid and methacrylic acid may also be used.

Instead of maleic anhydride polymaleic anhydride, succinic anhydride or any acid/acid anhydride and the like may be used.

The means for generating free radicals is preferably accomplished by using a free radical catalyst such as a peroxide (for example, t-butyl perbenzoate, dicumyl peroxide, methylethylketone peroxide, benzoyl peroxide, di-t-butyl peroxide, and 2,5-dimethyl-2,5-di(t-butyl peroxy) hexane. A more detailed compilation of free radical initiators which may be used is set forth at pages 11-3, to 11-51 of "Polymer Handbook", Interscience Publishers (1966). Also the grafting process can be carried out by the effect of gamma radiation, U.V. radiation or any other effective process, for grafting the silylating agent.

The silylating agents according to this invention are gamma-aminopropyltriethoxysilane, a gamma-methacryloxypropyl trimethoxysilane and propyl triethoxy silane. However, this invention is not limited to such silanes, nor to the disclosed weight ratios. They may advantageously be replaced by other kinds or other weight ratios of silylating agents, for example, vinyltriethoxysilane, vinyltri(2-methoxy-ethoxy)silane, beta-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane, gamma-glycidoxypropyltrimethoxysilane, gamma-mercaptopropyltrimethoxysilane, gamma-aminopropyltriethoxysilane, n-beta-(aminoethyl), gamma-aminopropyltrimethoxysilane, n-beta-(aminoethoxyl)gamma-aminopropyltrimethoxysilane and gamma-chloropropyltrimethoxysilane or any other silylating agent having the general formula ##STR1## or an oligomer thereof, wherein R.sub.1, R.sub.2 and R.sub.3 are the same or different and are selected from the group comprising alkoxy with 1 to 6 carbon atoms, alkyl, alkenyl, cycloalkyl with 1 to 6 carbon atoms, aryl aralkyl and organo-functional radicals.

Silanes grafted and bonded onto a hydrophilic cellulosic fibers whether mixed or not with an inorganic particles according to this invention contribute to the wrapping and bonding these materials with the hydrophobic polymer resin. The selected percentages of silylating agent added were 0.1-8 (filler weight percent) of gamma-aminopropyltiethoxysilane, gamma methocryloxypropyltrimethoxysilane or propyltriethoxysilane. The presence of maleic anhydride may lead to the formation of a little part of maleiated polymer resin by the effect of the free radical initiator present and the heat melt of the polymer. The selected weight percentage of maleic anhydride is 0-5% of the mixture of materials used. The selected percentage of the free radical initiator was 0.1-5% by weight with respect to the filler, which acts to initiate the hydroxylic cellulosic functional groups, part of the heat melt polymer matrix, the maleic anhydride present and may eliminate the ethoxyl or methoxyl groups of silane molecules to be grafted onto the initiated cellulosic molecules of the filler. This takes place during the hot melt mixing on the roll mill or extruder. Furthermore, the oxygen atoms of the maleiated polymer attach to some of the hydrogen atoms of cellulosic hydroxylic functional groups through the formation of hydrogen bonding. The superior mechanical properties according to this invention are achieved by grafting 2% by weight of the filler of gamma-aminopropyltriethoxysilane onto dry chemicalthermomechanical aspen wood pulp only or onto aspen wood pulp mixed with 10-30 weight percent of calcium carbonate with respect to wood pulp. The wood pulp is ground at mesh size 60 and the calcium carbonate at mesh size 150. These superior mechanical properties are due to further combination with oxygen atoms of the maleiated polymer and with the silane amino functional group by eliminating water molecule. A silylating agent when being hydrolyzed by an aqueous alkaline or acidic solution may decrease the added silane percentage and give composites having the same mechanical strengths by formation of a silane monolayer. All the above-mentioned weight ratios can be varied to optimize the results. Also the addition of wax slightly improve the results.

Some aspects of this invention, during its preparation, are the easiness in its chemical technique, time and energy saving, no waste of chemicals and lower in costs. In a further embodiment, the organic filler used in the present invention for preparing composites have low cost economy, light weight, less machine wear, no health hazard and renewable nature. The resultant composite materials in this invention have high durability and less surface fracture and superior stability under extreme temperature conditions compared to other composites filled with fiber glass or mica or the like.

A method for preparing composite materials according to the present invention is as follows:

(1) Forming a silylated solution by dissolving in an organic solvent a silylating agent (hydrolyzed or not) possibly in the presence of benzoylperoxide. Maleic anhydride may be added or not. The polymeric matrix material could be dissolved in the same silylated solution or not. Grinding the organic material at available mesh size the latter to be mixed or not with an inorganic material.

(2) The organic materials may be selected from the group consisting of starch and cellulosic materials. The inorganic material is selected from a compound containing hydroxyl group, clay, calcium carbonate, asbestos, sand, wallstonite, glass, quartz, mica, diatomaceous earth, silica, talc, kaolinite, aramide, hydrate or partially hydrate metal oxides, metal powders, metal compounds, carbon, graphite, or a natural occurring material and if naturally occurring it could be used in either purified or unpurified form.

(3) Wetting by spraying said materials of step (2) with the silylated solution of step (1), then evaporating the solvent from the wetted mixture material by passing hot air current or by subjecting to conditions effective to produce a grafted cellulosic materials or grafting and bonding silylated mixture materials.

(4) Mixing polymeric or copolymeric matrix materials which are thermoplastic or thermosetting, or rubber with lower weight ratios of the prepared silylated material(s) of step (3) to give reinforced and bonded composite material(s) or mixing the same silylated material(s) in high weight ratios with the same matrix materials to give only bonded composite materials.

This invention will now be further described by non-limiting examples.

20 g of dry and ground at mesh 60 of semichemical pulp of bagasse, chemical thermomechanical wood pulp of aspen or birch were placed in a beaker of 500 ml. capacity. A mixture of 0.16g benzoyl peroxide, 0.2g maleic anhydride and 0.2, 0.4 or 0.8g of gamma-aminopropyltriethoxysilane, gamma-methacryloxypropyltrimethoxysilane or propyltriethoxysilane were well dissolved together in 15ml acetone as a solvent and were added by spraying to the pulp while stirring. The stirring continued for another 30 minutes. The beaker contents were then subjected to a warm air current for 1-2 hours. 30% of linear low density polyethylene were added to be melt first on a hot roll-mill (at 140.degree. C.). The remaining 70% of the polyethylene previously were mixed with silane-pulp to form a homogenized mixture by using a blender, and were added later on to the roll-mill. All composite quantity was collected and remixed on the roll-mill every half minute and for 15 minutes, then the melted composite was collected to cool down to room temperature and ground to mesh 20 to be molded for testing in accordance with ASTM standard D 1822-79. The results are given below in tables 1, 2 and 11.

0.8 g of silylating agents mentioned in example No 1, 2, was dissolved in 15 ml of acetone and was added as droplets for 5 minutes to 20 g of aspen wood pulp (mesh size 60) while stirring for 10 minutes. After this addition, stirring continued for another 20 minutes. The mixture was left at 45.degree. C. in a circulated air oven for 1/2 hour, before being mixed with the polymer on the roll mill. (Table 3)

Same procedure as in example No. 1, but in the absence of silylating agent.

Table 4 indicates the mechanical properties of polyethylene composites filled with aspen pulp treated with maleic anhydride, as explained in example 7.

Some improvements appeared when aspen pulp treated with maleic anhydride, in presence of free radical initiator. This slight improvement of tensile strength, energy & modulus is due to the effect of maleic anhydride coupling agent.

It may be seen from these results that there is no significant increase in mechanical properties in the ungrafted aspen or birch pulp-polyethylene composites, Table 5, or ungrafted aspen pulp-polypropylene composites, Table 6.

The Suzorite mica (200 mesh coated), or glass fiber 0.8 mm (coated), was mixed with the linear lower density.

The mechanical results are shown on Table No. 7.

The results presented in Table 7 show that the composites of grafted aspen wood pulp with a silylating agent give superior mechanical properties compared with composites of mica or glass fiber. In addition, the price of coated mica or fiber glass is much higher than the costs of the grafted wood fibers subject of the present invention.

After boiling in water for 4 hours:

The mechanical results after boiling in tap water for 4 hours are indicated on Table 8.

It is very exciting to see that the silane grafted aspen pulp-polyethylene composites (as example 1) remain virtually unaffected by boiling. The samples remained stronger than polyethylene, mica, or fiber glass.

After being kept at 105.degree. C. for seven days:

Measuring at -40.degree. C.:

The measurements of composite samples were carried out in Instron Environmental Chamber System (Model 3111).

Finally, with respect to the results presented in Table 10 dealing with composites, their behaviour at -40.degree. C. shows that grafted aspen pulp with silane A-1100 composites do not lose their reinforcing properties even at -40.degree. C. and keep their reinforcing advantages vis-a-vis glass fibers or mica.

According to the procedure described in the previous example No. 1, wherein wood pulps are substituted by clay, asbestos or calcium carbonate. The results are given in Tables 11 and 14.

As described in example No. 1, wherein aspen wood pulp was well mixed in weight ratios from 10-30 with calcium carbonate, asbestos or clay. The mixing was carried before spraying the silylated solution.

Mixing of cellulosic and inorganic materials improve the mechanical strengths as shown in Tables 12, 13 & 14.

As described in examples 1 and 18, wherein the linear low density polyethylene is substituted with isotactic polypropylene.

The results are shown in Table 15.

Particles of aspen wood of 5% water content were wetted by spraying with the organic solution described in example No. 1. A warm air current was then passed in order to evaporate the solvent present in the wetted particles and to form grafted particles with a silylating agent. A powdered linear low density polyethylene of mesh 40 was well mixed with the silylated wood particles in 8% weight ratio. The silylated and mixed wood particles were then molded by hot pressing at 180.degree. C. for 5 minutes and at density 610 kgm/m.sup. 3 for obtaining board samples having a thickness of 11.1 mm. The internal bond strength is determined by testing specimens in tension perpendicular to the plane of the board specimens. The test was carried out according to CSA 0188 in reference to commercial boards. The results are shown in Table 16.

Differently to what was described in example 24, the wood particles are not sprayed with the silylated solution, but are mixed with a ground composite material at mesh 40 prepared as described in examples 1 or 18, which comprise 20 weight percent of the filler. The results are shown in Table 16.

Differently to what was described in example 24, the acetone solvent was substituted by methyl ethyl ketone, and the linear low density polyethylene was substituted by atactic polystyrene which was previously dissolved in the silylated solution. The results are shown in Table 16.