WO2018184037A1

WO2018184037A1 - Pencil lead formulation

Info

Publication number: WO2018184037A1
Application number: PCT/ZA2018/050017
Authority: WO
Inventors: Gary Botha
Original assignee: Ecograf (Pty) Ltd.
Priority date: 2017-03-31
Filing date: 2018-04-03
Publication date: 2018-10-04
Also published as: GB201705218D0

Abstract

The invention relates to a pencil lead for writing, drawing and/or painting, wherein the pencil lead comprises a synthetic rubber compound, graphite and modified starch. The invention further relates to a method for producing the pencil lead and a writing instrument comprising the pencil lead.

Description

PENCIL LEAD FORMULATION

FIELD OF THE INVENTION

This invention relates to graphite pencil leads for writing, drawing, and/or painting. This invention further relates to writing instruments formed from the pencil lead and a method of manufacture of such pencil leads.

BACKGROUND OF THE INVENTION

The principle of pencil leads for writing, drawing, and/or painting is known. Clay and natural graphite-based pencil leads for writing, drawing, and/or painting are leads that are mostly solidly embedded in wood or other materials that are capable of being sharpened, and also leads that are retained in sliding manner inside a rigid enclosure. Examples of such are wood-encased pencils and pencil leads for mechanical pencils, also known as propelling pencils or clutch pencils, for example.

The manufacturing process of the conventional wood encased pencil and the associated leads has not changed much in over 200 years. Considering climate change and global warming a need exists to eliminate the wood usage and energy consumption typically used in conventional pencil manufacture. Conventiona! pencils suffer from the following disadvantages:

• Trees have to be grown for a period of on average 16 years.

• Trees have to be cut and transported to a saw mill.

• Wood has to be cured in kilns for 14 days to expel excess

moisture (reduce the moisture content from about 40 % down to about 3% (wt%)). This is to prevent the wood from cracking during and after the pencil making process.

• Wood has to be cut into slats.

• 60% of the tree is utilized - 40% sawdust generated.

• On average 20 000 pencils are made from one tree.

• 800 000 trees are cut annually and used to make 17 billion pencils.

• On average one tree can provide enough oxygen for two people per annum.

• Natural graphite is mined and has to be beneficiated (the carbon content is raised from 75% to 98%) before it can be used to increase its purity to writing quality.

• Graphite has to be mixed with clay and water into a paste consistency, extruded into thin strands, dried for 48 hours (ambient temperature 120 deg C) and then fired at 1000 deg C for 12 hours to produce the inner core pencil leads. These inner core leads have to be dipped in wax to improve writing smoothness.

• The lead cores must be glued into wood slats (to prevent slippage) to create a sandwich. The sandwiches are then cut into strips and shaped into hex, round or triangular shapes.

• The pencils then have to go through a lengthy paint process before being marketed.

In attempt to address these problems, so-called woodless pencils are known where the pencil consists largely or entirely of the lead, which is formed to the writing instrument shape. These can be solid sticks of natural graphite and clay composite (as found in a 'graphite pencil'), about the diameter of a common pencil and have no casing other than a wrapper or label. They are used primarily for art purposes as the lack of casing allows for covering larger spaces more easily, creating different effects, and providing greater economy as the entirety of the pencil is used.

Disadvantages of the current woodless pencil lead formulations are:

• Pure natural graphite pencils have to be coated with varnish or covered to prevent such material being transferred to the hands.

• Pure graphite pencils are usually sharpened with conventional sharpeners but generate/ leave a powdery residue due to lack of graphite elasticity.

• Pure graphite pencils are extremely fragile since they are not protected by a wooden body or similar material which means they are extremely breakable

• The leads retained inside a rigid enclosure, solidly embedded in wood or other materials and the lead in all so called woodless pencils, are all manufactured by extrusion process.

Pure graphite pencil leads are traditionally manufactured by mixing natural graphite, clay and water to create a basic paste. The paste is extruded through an aperture at about 10 ton cm^"2 to produce soft filaments which are dried in electric ovens for a period of about 48 hours at about 120 deg C so as to increase the green strength and reduce the moisture content of the filaments. If moisture levels in the lead filaments are too high they tend to burst under high temperature conditions.

After the stage described above, the pure natural graphite and clay mixture is processed (fired) at very high working temperatures (about 1000 deg C) for approximately 12 hours. To complete the process the pure graphite is then dipped in oil so that the product provides better softness during writing.

The different hardness and tonalities in pencil lead change due to different percentages of graphite and clay in the mixture. After the firing process the leads are very brittle (show very little or no elastomeric properties) and the leads often leave a residue on the hands when handled.

For these reasons, pure graphite pencils should not be used by children, which limits their consumption to teenagers and adults.

A need exists to manufacture a pencil lead and pencil lead based writing instrument which addresses the abovementioned problems.

In particular, a need exists to produce a pencil lead for writing, drawing, and/or painting that is not associated with the disadvantages described above and which may be produced using low extrusion pressure, compression molding or injection molding, low heat, short production time, has a low carbon footprint and is not limited to adult use in the field.

SUMMARY OF THE INVENTION

The present invention provides pencil lead for writing, drawing and/or painting, wherein the lead comprises a synthetic rubber compound, graphite and modified starch.

The graphite is preferably synthetic graphite and is more preferably recycled synthetic graphite.

There are two types of synthetic graphite. The largest type of synthetic graphite is termed "graphite electrodes" and is made from petroleum coke primarily by public companies such as GrafTech International, SGL, Showa Denko, SEC and Toyo Tanso. Graphite electrodes are used in the electric-arc furnaces for melting steel and/or iron and producing ferroalloys.

The second type of synthetic graphite is synthetic graphite blocks known as "isotropic graphite." used primarily in the solar industry. This is a synthetic product made basically the same way as electrodes are made using petroleum coke. The only difference is that a different structure coke is used in making electrodes. An isotropic-type coke is used in making graphite blocks and an anisotropic coke is used in making electrodes.

When these electrodes or blocks are machined, a granular graphite or graphite dust results. This is known as "recycled synthetic graphite" and can be used in the present invention.

In a preferred embodiment, the recycled synthetic graphite is derived from recovered electrodes generated from the metal smelting industry.

The pencil lead may additionally comprise any one or more of zinc oxide, mineral oil, a processing promoter for synthetic rubber, microcrystalline wax, sulphur, a vulcanization accelerator and stearic acid.

The present invention preferably provides pencil lead for writing, drawing and/or painting, wherein the lead comprises a synthetic rubber compound, graphite, modified starch, zinc oxide, mineral oil, a processing promoter for synthetic rubber, microcrystalline wax, sulphur, a vulcanization accelerator and stearic acid.

The present invention allows for pencil leads to be produced by injection molding within 5 minutes, which pencil lead is then able to be used immediately for writing, drawing and/or painting with no other processing required.

The present invention relates to the addition of modified starch to rubber compounds. The addition of the modified starch reduces the cost of the compound by at least 40%, promotes elasticity in the end product and leaves a skin on the outer surface of the pencil. The synthetic rubber compound can be selected from styrene butadiene rubber and silicon rubber or a mixture thereof.

The modified starch can be selected from potato starch, maize starch, cassava starch, sorghum starch.

The mineral oil (also known as "white oil") can be selected from highly-refined mineral petroleum oils consisting of complex straight and branched-chain saturated hydrocarbons. Mineral oil is odorless and colorless with the highest purity and lowest toxicity possible.

The processing promoter for synthetic rubber is preferably a plasticizer (peptizer) for natural and isoprene rubber, as well as for their blends with other synthetic rubbers, for example Aktiplast (registered trade mark (RTM)). It preferably comprises a blend of zinc salts of higher molecular, mostly unsaturated fatty acids. Fatty acids are considered to be carboxylic acids with a long (4 to 28C) aliphatic chain, which is either saturated or unsaturated. Preferably the fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28.

The microcrystalline wax is a specific type of wax produced by de-oiling petroleum. It is used in cosmetics and beauty products as a viscosity agent, binder and emollient, and is often considered an alternative to paraffin wax. Unlike paraffin wax, microcrystalline wax has fine crystals, and is generally darker, more viscous, denser, tackier and more elastic than paraffin waxes. It has elastic and adhesive characteristics, which are related to its non-straight chain components. The structure of microcrystalline wax is small and thin, making these waxes more flexible than paraffin wax. The fine crystal structure also enables microcrystalline wax to bind solvents or oil, preventing the sweating-out of compositions. The sulphur is preferably elemental sulphur.

The rubber accelerator is preferably 2-(4-Morpholinothio) benzothiazole (MBS). Other accelerators are thiazoles, mercaptobenzothiazole (MBT), sulphonamides, N-Oxydiethylene-2-benzothiazole sulfonamide (NOBS), dithiocarbamates and guanidines.

In a preferred embodiment, a pencil lead formulation according to the present invention is as follows:

Styrene Butadiene Rubber or Silicone Rubber 5% - 20% w/w

Graphite 50% - 95% w/w

Modified Starch 5% - 20% w/w

Zinc Oxide 1 % - 1 .5% w/w

White Oil 2% - 5% w/w

Aktiplast (RTM) 1 % - 2% w/w

Micro Wax 1 % - 2% w/w

Sulphur 2% - 3% w/w

MBS (rubber accelerator) 0.5% - 1 .5% w/w

Stearic acid 0.4% - 1 .5% w/w

In this specification, all amounts are expressed as percentage weight for weight (% w/w).

In this specification, all amounts expressed have a tolerance of 2.5% higher or lower in relation to percentages expressed.

Surprisingly, it has been revealed that the deliberate use of modified starch as a constituent of the pencil lead according to the invention results in reducing the cost of the pencil lead and increasing the green strength. A major advantage of the pencil lead according to the present invention over other pure graphite and wood covered pencils is:

• The pencil leads according to the present invention can be produced by an injection molding process which allows a production time of under 5 minutes.

• When produced by extrusion, the pencil leads require a significant reduction of extrusion pressure.

• Short mixing time of all ingredients.

• Low temperature curing at approximately 185 deg C in a feeder barrel for the mold.

• Short cure time - 10 minutes using extrusion or compression molding techniques with approximately 4 minutes residency time in the mold.

• Does not leave a residue on the hands when handled due to a "skin " that is formed on the outer surface of the pencil due to the combination of rubber and starch.

A significant advantage of the present invention is that it is possible to produce the pencil leads by injection molding or when extruded, requiring a significantly lower extrusion pressure. In particular, the modified starch additive in the pencil lead composition serves to lower the pressing and extrusion pressure at the head of the extruder during extrusion. A reduction in pressure of at least 30%, and possibly by at least 50% is achieved compared with conventionally extruded pencil lead compositions.

In addition, the modified starch lends a more intense colour property to the impression left by the pencil lead and improves its sliding behaviour when writing. In pencil leads, the line drawn is full with high optical density, while colored pencil leads create rich, lustrous shades.

The microcrystalline wax used is preferably malleable at 20 deg C, has a firm to brittle hardness, a coarse to fine crystalline structure, is translucent to opaque, but not glassy, and melts without decomposition above 40° C. The wax is readily flowable slightly above its melting point and its consistency and solubility are strongly dependent on temperature.

It is also advantageous if the microcrystalline wax is constituted from at least one wax from the group including stearates, montan waxes, amide waxes, and paraffins.

Mixtures of two or more waxes may also be used. In this context, it is particularly preferable if the microcrystalline wax consists of calcium stearate or includes calcium stearate.

The pencil leads according to the invention are unfired pencil leads that are usable immediately after extruding or injection molding without further heat treatment or impregnation.

Besides pencil and colored pencil leads, the composition may also be used to produce special leads such as leads for draftsman's pencils.

Graphite or graphite in combination with carbon black or colourising pigments are preferred as colourising pigments for pencil leads. For coloured pencil leads, combinations of graphite and white or achromatic fillers such as hexagonal boron nitride, layered silicates and similar with colourising pigments such as azo pigments, pthalocyanines, dioxazines, quinacridones, iron oxides, carbon black, ultramarine, and iron-cyan complexes have proven most advantageous.

A preferred composition for a coloured pencil lead consists of:

Styrene Butadiene Rubber or Silicone Rubber 5% - 20% w/w Pigment and graphite 50% - 95% w/w

Modified Starch 5% - 20% w/w Zinc Oxide 1 % - 1 .5% w/w

White Oil 2% - 5% w/w

Plasticizer 1 % - 2% w/w

Micro Wax 1 % - 2% w/w

Sulphur 2% - 3% w/w

MBS (rubber accelerator) 0.5% - 1 .5% w/w

Stearic acid 0.4% - 1 .5% w/w

Injection molding has proven to be effective as the method for producing a colored pencil lead according to the invention, although extrusion can also be used.

An approximately 40% reduction of extrusion pressure in the extruder head during production of a pencil lead is achieved according to the invention in accordance with Example 1 compared with a pencil lead that is extruded according to the prior art.

Advantages of the present invention include:

• The lead does not require a conventional sharpener to create a point and can be sharpened on any rough surface.

• The pencils require significantly less energy and time in the manufacturing process.

• Can be injection molded (5 minutes to make).

The pencil lead according to the invention may be produced as follows:

a. Mixing all components of the pencil lead to produce a pencil lead dough;

b. Injection molding (or extruding) the pencil lead dough at a temperature in the range from 130 to 200 deg C. to form pencil lead strands;

c. Cooling and solidifying the pencil lead strand; and d. Cutting and/or shaping the endless pencil lead strand to final length, particularly the required pencil length.

The pencil lead cross section may have any shape - round, angular or a combination thereof.

During extrusion of the pencil lead according to the present invention, it is also possible to combine different pencil lead recipes in one extrusion head to create a multicomponent pencil lead by multiple coextrusion.

As such, the problems associated with the prior art are addressed by using a food grade polymer, starch and recycled waste synthetic graphite.

According to the invention, a writing instrument which has the following characteristics can now be commercially manufactured.

• The pencil has acceptable marking and writing characteristics to its conventional counterpart.

» The pencil has a very low carbon footprint, in that from start to finish the pencil takes under one hour to produce and uses far less energy consumption in the manufacturing process.

• Utilising synthetic graphite in the formulation (preferably a synthetic recycled graphite) greatly reduces costs and is energy saving.

» The pencil and stubs of the pencil are fully recyclable. By milling the pieces into a powder, the powder can then be re-introduced into the formulation. The conventional wooden pencil would not be cost effective to recycle.

• Once all the ingredients are combined and after 15 minutes of mixing, a dough like compound is produced.

• This dough can either be compression molded, extruded or injection molded. • Once the pencils come out of the molding process, they can be used immediately.

• Pencil lead compounds and formulations according to the prior art can only be extruded and must be cured.

The invention will now be described with reference to the following non-limiting examples.

Example 1

Dark marking pencil (HB)

Styrene Butadiene Rubber 5% w/w

Graphite 82.1 % w/w

Modified Starch 5% w/w

Zinc Oxide 1 % w/w

White Oil 2% w/w

Aktiplast (RTM) 1 % w/w

Micro Wax 1 % w/w

Sulphur 2% w/w

MBS (rubber accelerator) 0.5% w/w

Stearic acid 0.4% w/w

Example 2

Medium dark marking pencil (2H)

Silicone Rubber 10% w/w

Graphite 77.1 % w/w

Modified Starch 5% w/w

Zinc Oxide 1 % w/w

White Oil 2% w/w

Plasticizer 1 % w/w Micro Wax 1 % w/w

Sulphur 2% w/w

MBS (rubber accelerator) 0.5% w/w

Stearic acid 0.4% w/w

Example 3

Light marking pencil (4H)

Styrene Butadiene Rubber 15% w/w

Graphite 72.1 % w/w

Modified Starch 5% w/w

Zinc Oxide 1 % w/w

White Oil 2% w/w

Aktiplast (RTM) 1 % w/w

Micro Wax 1 % w/w

Sulphur 2% w/w

MBS (rubber accelerator) 0.5% w/w

Stearic acid 0.4% w/w

Claims

1 . A pencil lead for writing, drawing and/or painting, wherein the lead comprises a synthetic rubber compound, graphite and modified starch.

2. The pencil lead according to claim 1 additionally comprising any one or more of zinc oxide, mineral oil, a processing promoter for synthetic rubber, microcrystalline wax, sulphur, a vulcanization accelerator and stearic acid.

3. A pencil lead according to claim 1 or 2 comprising:

i- Styrene Butadiene Rubber or Silicone Rubber 5% - 20% w/w ii. Graphite 60% - 95% w/w iii. Modified Starch 5% - 20% w/w iv. Zinc Oxide 1 % - 1 .5% w/w v. White Oil 2% - 5% w/w vi. Plasticizer 1 % - 2% w/w vii. Micro Wax 1 % - 2% w/w viii. Sulphur 2% - 3% w/w ix. MBS (rubber accelerator) 0.5% - 1 .5% w/w

X. Stearic acid 0.4% - 1 .5% w/w

A colored pencil lead according to any one of claims 1 to 3 comprising: i. Styrene Butadiene Rubber or Silicone Rubber 5% - 20% w/w ii. Pigment and graphite 50% - 95% w/w iii. Modified Starch 5% - 20% w/w iv. Zinc Oxide 1 % - 1 .5% w/w v. White Oil 2% - 5% w/w vi. Plasticizer 1 % - 2% w/w vii. Micro Wax 1 % - 2% w/w viii. Sulphur 2% - 3% w/w ix. MBS (rubber accelerator) 0.5% - 1 .5% w/w x. Stearic acid 0.

4% - 1 .5% w/w

5. A pencil lead according to any one of claims 1 to 4 wherein the wax is formed from at least one wax of the group comprising fatty acids, stearates, montan-waxes, amide-waxes and paraffins.

6. Method of producing a pencil lead according to any one of claims 1 to 5, wherein the lead is formed by injection molding.

7. A method of producing a pencil lead according to claim 1 including the following steps:

a. Mixing all components of the pencil lead to produce a pencil lead dough;

b. Injection molding the pencil lead dough in mold at a temperature in the range from 130 to 200 deg C. to form a pencil lead strands; c. Cooling and solidifying the pencil lead strand; and

d. Cutting and/or shaping the pencil lead strand to final length.

8. A writing instrument consisting essentially of a pencil lead according to any one of claims 1 to 5.

9. A writing instrument comprising a pencil lead according to any one of claims 1 to 5 or comprising a pencil lead produced according to the method of claim 6 or 7.