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Thomas Schroeder

8 8 Share 0 Like 1 0 More According to you, what Starch and Dextrin Based Adhesives is the basic use of a SpecialChem - May 12, 2004 sealant?

Edward M. Petrie, Member of SpecialChem Technical Expert Team. To form a protection barrier Introduction To seal two substrates Raw Materials together Manufacture of Starch Adhesive To fill gaps or holes Manufacture of Dextrin Adhesives Additives and Modifiers Common Applications and Formulations

Introduction

An extremely large and well-known class of adhesives is based on starch and These play a very large part in industrial Article - Trends in Plastic Surface dextrin. Modifications to Improve Adhesion production, especially the packaging industry. Starch and dextrin are principally used for bonding paper products. Most corrugated boxboard for making cartons is bonded with starch based adhesives, and other porous substrates can be easily joined with these Article - Carbon Nanotubes Improve Conductivity of Adhesives versatile adhesives. Article - Formulating w ith Starch and dextrin adhesives are readily available, low in cost, and easy to apply from water dispersion. They are considered to be Polyurethane Prepolymers the least expensive class of paper packaging adhesive. Formulated starch and dextrin adhesives can be applied hot or cold. These adhesives are generally provided to the end-user as powder and mixed with water prior to use to form a relatively thick paste. Starch and dextrin cure by the loss of moisture. Since these adhesives cure to a thermosetting structure, they have excellent heat resistance. Another significant advantage is their very slow curing rate allowing ample assembly time. Disadvantages include poor Article moisture resistance and mold growth. Keeping Low -isocyanate Emission Polyurethane Adhesives Flow ing Although starch and dextrin have been used as adhesives for many decades, there are several important reasons why these natural Exploiting VAE Technology offers adhesives will not be entirely replaced by synthetic products. The following advantages ensure that they continue to fill particular Water-based Adhesive Advantage 1 niches in the market place. Starch and Dextrin Based Adhesives Availability is good and cost is relatively low SEBS for High Performance Hot Quality is stable Melts Good adhesion to cellulose and many porous substrates POSS – Nanoreinforcement of Insoluble in oils and fats Structural Adhesives Non-toxic and biodegradable Heat resistant

This article will review the raw materials that go into starch and dextrin adhesives, the methods of adhesive manufacture, and the additives that are commonly used in formulations for improvement of specific properties. Finally, the common applications and formulations of starch and based adhesives will be reviewed. dextrin Opinion Space - Do you think that starch and dextrin based-adhesives w ill play a larger part in the future of Adesives & Sealants industry? Raw materials Share your opinion now on this hot Both starch and dextrin (dextrin is starch which is processed further) are considered carbohydrate polymers, and they come from topic w ith other basically the same sources. Starch is a natural polymer, a polysaccharide, derived from the seeds, roots, and leaves of plants. Only SpecialChem4Adhesives members! a few plants yield starch in sufficient quantity to be economical. These are corn, wheat, potato, rice, tapioca, and sago. The quality of the starch must be high to produce high quality adhesives.

Starch is made up of two molecules: amylose and amylopectin. Amylose consists of long helical chains, and amylopectin has a branched structure. Starch does not have a uniform composition. Its molecular structure and amylose / amylopectin ratio vary according to plant source. Thus, the processing characteristics and end properties will vary similarly. The most important methods of differentiation between starches are the molecular weight of the amylose fraction and the ratio of amylose to amylopectin.

While amylopectin can be supplied in cold water soluble form, amylose is insoluble. Amylose can be dissolved with strong alkali, by cooking with formaldehyde, or by cooking in water at 150-160 °C under pressure. Upon cooling or neutralization such amylose dispersions will form firm gels at concentrations higher than 2% and precipitate at concentrations lower than 2%. Amylose fractions are never truly soluble in water and in time will form crystalline aggregates by hydrogen bonding - a process known as retrogradation or setback. Retrogradation is the cause of viscosity instability in certain starch based adhesives. The amylopectin is more soluble and less prone to retrogradation. The percent of amylose in various starches and their resulting properties are summarized in Table 1.

Corn Wheat Rice Tapioca Potato Sago Source Seed Seed Seed Root Root Pith Granule Diameter, Microns 5-26 3-35 3-8 5-35 15-100 10-70 Gelation Temp, °C 62-72 58-64 68-78 49-70 59-68 60-67 Amylose, % 28 25 19 20 25 26 Amylose, % 28 25 19 20 25 26

Table 1: Properties of Commercial Starches

Most starches contain 20-30% by weight of amylose although certain specialty types can have as little as 0% or as high as 80%. Because of the amylose fraction, starch suspended in cold water is essentially unable to act as an adhesive because the starch is so tightly bound in crystalline regions. These granules must be opened through processing to obtain adhesive bonding.

Manufacture of Starch Adhesive

Heating in water is the simplest method of breaking up the starch granules. On heating in water, starch granules first swell and then burst open with a resulting thickening of the suspension. The temperature at which this thickening of the suspension occurs is called the gelation temperature. For most starches in pure water, gelation occurs between 57 and 72°C (see Table 1). In this form the starch is not a true solution but a colloidal suspension.

The heating process can take one of two forms. In the first, salts (usually the chlorides of metals such as calcium, magnesium, and zinc) are added to a suspension of the starch in water, and the adhesive is produced by control of the temperature and the time of stirring. In the second and primary method, caustic soda is added to the starch suspension and later on in the process the product is neutralized with acids.

The starch paste now has a high viscosity and acts as an adhesive. If the concentration of the starch is above 7% then the cooked paste is very viscous and difficult to pump. Above 15% starch solids content, the cooked paste forms an immobile rubbery mass on cooling. Suspensions with a higher amylose / amylopectin ratio will have a higher viscosity.

Pregelation starch is produced by physical modification of the starch to impart the ability to form a paste easily in cold water. The process consists of heating the starch slurry to a temperature above the gelation temperature and then quickly drying the starch before retardation can occur. The main process equipment used is a heated-drum drier or hot roll.

Unmodified starches, such as produced by the steps outlined above, are in powder form (pregelation) or high viscosity, low solids content pastes which do not show a stable viscosity with time. Therefore, several treatments have been developed to provide liquid adhesives that are not subject to retrogradation and have a viscosity and rheology more suitable for many applications. These treatment methods include: alkali treatment, acid treatment, and oxidation.

The gelation temperature can be lowered by the addition of an alkali, such as sodium hydroxide, to the starch suspension. If sufficient alkali is added, the starch can be induced to gel at room temperature.

Acid modification of starch is achieved by heating the starch at 49-54°C with small amounts of aqueous mineral acid, followed by neutralization with a base. This tends to give a much thinner solution at the same solids content when compared to unmodified starch. Starches prepared in this manner are often known as thin-boiling or high-fluidity starches. Acid modification is often used when applications require higher solids content but a controlled, lower viscosity.

Oxidization of starch is usually obtained by treatment with aqueous alkaline hypochlorite. A starch suspension at pH 8-10 is treated with hypochlorite (5-10% Cl based on starch) for a long enough time to produce the desired viscosity. Acid is liberated during the reaction, so base must be added to maintain the pH for optimum reactivity. Dried oxidized starch is generally whiter than unmodified starch. Because of the low color, they are often used for the sizing and coating of printing papers. These oxidized or chlorinated starches behave similarly to acid modified starches. However, the oxidized starch has greater tack and adhesive character and, thus, is often used in adhesive applications.

The Stein-Hall Process

The first major commercial process for producing starch adhesives was the Stein-Hall Process. This was introduced in the 1930s for the purpose of producing an adhesive for corrugated boxes. [2] The Stein-Hall system combines the properties of an 8-9% cooked starch paste and a 25% starch slurry in a two-component, two-container process as indicated in Table 2. Sodium hydroxide is used to lower the gelation temperature and borax to increase the tack. Crosslinkers and special starches may be used to impart water- resistance.

To 400 lbs w ater add w ith mixing, 100 lbs corn starch follow ed by 17 lbs caustic soda in 3 gal w ater. Upper Tank (Carrier Starch) Heat w ith live steam to 71°C, agitate 15 min and then dilute w ith 500 lbs of cold w ater

Lower Tank (Suspended To 1520 lbs w ater add 17 lbs borax, allow to dissolve 2-3 mins and add 500-600 lbs starch and suspend Starch) uniformly. Gradually empty content of upper tank into the low er tank w ith efficient mixing and agitate until viscosity is consistent. Finally add preservative and other additives as required.

Table 2: Stein-Hall Tw o-Container Process for Starch Based Corrugating Adhesive 3

The high viscosity cooked starch paste serves as a carrier to prevent the starch granules in the slurry portion from settling out. When the adhesive is applied and heated to above its gelatinization temperature, the granules gelatinize, and a very viscous and tacky paste is produced. In corrugated box manufacture, the suspended starch should gelatinize immediately on contact with the heated fluting rolls and develop instant tack at 60-70°C. Starch solids may range from 20-30%.

Single Container or "No-Carrier Process"

This entire operation can also be carried out in a one container system where there is no carrier portion of the adhesive. In this process, a solution of caustic soda is added to a slurry of starch to swell the starch granules uniformly until they reach the required adhesive viscosity. An acid is then added to stop this swelling process short of complete gelatinization, and borax is finally added.

When preparing such single container adhesives, it is essential to control carefully the concentrations and temperature of the starch slurry and the caustic solution, and to stop the swelling at the correct stage to ensure that the required viscosity is achieved. The main features of the single container starch system is that: Less heat is required for bond formation (as the starch granules are already partially swollen) There is less free water Viscosity allows increase in machine speeds and reduction in warped board production Superior storage and shear stability.

Manufacture of Dextrin Adhesives

Dextrin adhesives are manufactured by dry-roasting starch in the presence of an acid catalyst. Through treatments with heat and acid, the starch molecules are hydrolyzed into small fragments, and then repolymerized into highly branched, readily soluble polymer molecules of moderate size. Corn starch is commonly used because of its low cost and ready availability, although other starches are also employed.

Unlike starch, dextrins are soluble in water. The severity of the heat and acid treatment determines the degree of solubility, which is the basis for classifying dextrins. Finished dextrins are very fine powder varying in color from pure white to brown.

There are three main types of dextrin adhesives: white dextrins, canary or yellow dextrins, and British gums. The differences are determined by the roasting time and the temperature and amount of catalyst used. Table 3 describes the properties of these dextrin adhesives.

Property White Dextrin Yellow Dextrin British Gum Roasting Temp., °C 120-130 135-160 150-180 Roasting Time, hrs 3-7 8-14 10-24 Catalyst Concentration High Moderate Small Solubility 1-95% in w ater* Very soluble in w ater 1-95% in w ater* Viscosity Stability Prone to retrogradation Good stability Good stability, higher viscosity than Yellow Color White - Buff Yellow - Tan Yellow - Brow n

Table 3: Description of Common Dextrin Adhesives * Solubility depends on exact reaction conditions

The British gums are dextrins with the highest molecular weight and they are the strongest adhesives among the dextrins, but maximum usable solids content is only approximately 25%. Yellow dextrins are the lowest in molecular weight. Materials of this type are available that give a viscosity of only 1700 cps at 60% concentration. The white dextrins have the lowest color of the three. Their molecular weight is intermediate between those of the other two.

The range of solubilities of dextrin adhesives are advantageous in that there can be formulated a range of solutions of very different viscosities leading to a variety of gel strengths. These formulations can then be further modified as described in the next section. Generally the same additives and processes used for starch adhesives are used for formulating dextrin adhesives.

Dextrins are often preferred to starch, as they need less or no cooking and much less water to form the paste. This means that drying time is reduced and there is little likelihood of mold infection in warmer climates. As adhesives, dextrins are used in many of the same applications as starch adhesives. Dextrins are also used as a core binding material in the manufacture of foundry cores. Here the main function is to give good green strength and to prevent deformation of the core before being fired in the oven.

Additives and Modifiers

Properties of starch and dextrin adhesives that can be modified through formulation variables include: viscosity, solids content, stability, tack, slip, substrate penetration, drying rate, flexibility, water and fungus resistance, and cost. The formulations generally depend on (1) the type of starch and modification as indicated above and (2) the additives employed.

Frequently dextrin adhesives and sometimes starch based adhesives are formulated with borax (sodium tetraborate) to obtain high tack at moderate concentrations and viscosities with good aging characteristics. They also have good adhesion and machining properties. The borax is generally added in amounts of up to 10% based on dry starch. Sodium hydroxide is also added to convert the borax to more active sodium metaborate. Applications include case sealing, carton sealing, bag seams, tube winding, and laminating.

Plasticizers are widely used to control brittleness of the glue line and to regulate the speed of drying. Common plasticizers include glycerin, glycols, sorbitol, glucose, and sugar. These types of plasticizers act as a hygroscopic agent to decrease the drying rate of the film. Plasticizers based on saps, polyglycols, and sulfonated oil derivates lubricate the layers within the dried adhesive and, thus, impart flexibility. Urea, sodium nitrate, salicylic acid, and formaldehyde plasticize by forming a solid solution with the dried adhesive.

Chemicals such as calcium chloride, urea, sodium nitrate, thiourea, guanidine salts, etc. are used as liquefiers to reduce viscosity or as humectants to control open time and speed of drying. They are usually added at about 5-20% based on dry starch. Colloid stabilizers such as soaps and sodium chloride are sometimes used to retard the tendency to retrogradation.

Native starches are capable of yielding high degrees of water resistance, which may be necessary for labeling, bill posting, and other applications. Improved cold water resistance can be obtained with polyvinyl alcohol or polyvinyl acetate blends. These adhesives will also dissolve in hot water, which is often a benefit. Optimal moisture resistance is obtained through the addition of thermosetting resins, such as urea formaldehyde or resorcinol formaldehyde.

Mineral fillers such as kaolin clay, calcium carbonate, titanium dioxide, etc. are often used in starch and dextrin adhesives at concentrations of 5-50%. These are used to reduce cost and control penetration into porous substrates. Thixotropic agents such as bentonite are also often used in certain applications to control viscosity.

Other additives that are found in starch and dextrin based adhesive include preservatives, bleaches, and defoamers. Commonly used preservatives used to prevent microbial activity include formaldehyde (35% solids) at 0.2-1.0%, copper sulfate at about 0.2%, zinc sulfate, benzoates, fluorides, and phenols. Possible toxic effects must be carefully considered before choosing a preservative. Common bleaching agents include sodium bisulfite, hydrogen and sodium peroxide, and sodium perborate. Organic solvents are sometimes added to improve the adhesion to waxed surfaces. Common Applications and Formulations

The majority of starch and dextrin adhesive are used in the paper and textile industries. The principal end uses of starch and dextrin adhesive are shown in Table 4. Some of the most common applications for these adhesives are described below.

Application Starch Dextrin Fiberboard cases including corrugated boxes and solid board X X Paper bags X X Cartons X X Wallpaper, hanging pastes, sizes and binders X Cigarette manufacture X X Bottle and jar labeling X X Other labeling and gum tape X X Plasterboard production X X Tube w inding and lamination X X Envelope manufacture X X Book binding X

Table 4: Principal End-Uses of Starch and Dextrin Adhesives

Most corrugated boxboard for making cartons is bonded with starch based adhesives. A fraction of the starch needed to formulate the adhesive is gelatinized with aqueous caustic. This is blended with a concentrated suspension of unmodified starch granules. (See Table 1) A typical starch adhesive formulation also includes bentonite as a thixotroping agent, borax to speed gelatinization, and a small quantity of preservatives as a mold inhibitor. The paste is applied cold to the corrugated flutes and the liners. Upon subsequent exposure to heat, the starch granules swell and burst, forming a strong bond.

Starch based adhesives are also commonly used for bag manufacture. Normally three adhesives are used in this application: seam, cross-pasting, and bottoming. Initially in bag production the paper is formed into a long tube with a side-seam adhesive. The viscosity and solids content of these adhesives are relatively low. Generally they are made from high soluble white dextrin or acid modified starch. The bottom adhesive is made from a more viscous paste, and it generally has greater tack. Bottom pastes are normally composed of white dextrins or starch. Typical formulations for seam and bottoming adhesives are shown in Table 5 and 6. Cross paste adhesives are used for the manufacture of multiwall bags. Generally mineral fillers are added to this adhesive so that it does not bleed through the paper.

Formulation A 4

Water 49.7 pbw White dextrin (94% solids) 39.7 White dextrin (13-15% solids) 5.0 Borax 4.0 Preservative 0.01 Antifoaming agent 0.03 Cook to 85°C for 20-30 mins; cool to room temperature 1.2 and add 50% Aqueous NaOH

Formulation B 3 (Can be used for seams or bottoms) Water 67.9 pbw Dextrin 28.0 Heat to 72°C and add Borax 25.6 Soap 3.2 Heat to 88°C and hold; adjust solids to 27-28% 0.3 and add Preservative

Formulation B 3 (Insoluble adhesive w hich should be used w ithout delay) Water 17.0 pbw White dextrin 70.0 Soap 0.2 Urea-formaldehyde 7.0 Cook w ith live steam to 95°C, dilute w ith cold w ater, 1.4 and add Ammonium chloride

Table 5 : Typical Side-Seam Adhesives for Paper Bags

Formulation A 4 Water 76.7 pbw Starch 19.2 Borax 3.1 Preservative 0.2 Soap 0.8 Cook to 93°C for 20-30 mins; adjust solids to 30%

Formulation B 3 (Can be used for seams or bottoms) Water 67.4 pbw Starch 6.4 Dextrin 25.6 Soap 0.2 Soap 0.2 Heat to 88°C, hold 20 min, cool to 57°C, 0.3 adjust to 25% solids, and add Preservative

Table 6 : Typical Bottom Adhesives for Paper Bags

Laminating adhesives must exhibit high tack, low permeation of the substrate, and non-curling behavior. The adhesive must also meet the requirements of the laminating equipment. White dextrin is often used in this application. A typical formulation is shown in Table 7. Sugar, sodium nitrate, or urea is often added at 30-50% of the dextrin to increase solids and to control open time. Foil laminations require resin based adhesive for adhesion and water resistance. Alkaline starch types yield excellent adhesion to aluminum foil and good heat resistance, but poor water resistance.

Water 54.6 pbw White dextrin 20.2 Clay 13.5 Urea 6.7 Borax 5.0 Cook to gel

Table 7 : Typical Formulation for a Laminating Adhesive 4

Today most paperboard cartons are sealed with liquid glues or hot melt adhesive. These are generally preferred because of their high speed of operation and ability to bond strongly to many different substrates. However, dextrin adhesives are used in certain applications where low cost must be achieved. Carton sealing adhesive require a low and stable viscosity, and they must set quickly. A formulation for a white dextrin adhesive for carton sealing is provided in Table 8.

Water 11.0 pbw White dextrin 8.0 Preservative 0.2 Borax 1.2 Antifoaming agent 0.01 Cook to 85°C for 20 mins, cool to 48°C, and add 0.12 50% Aqueous NaOH

Table 8 : Typical Formulation for Carton Sealing: 42% Solids, 2500-3500 cps 3

Adhesive for tapes, labels, envelopes, and similar applications required noncurling ability and they should be nonblocking under humid conditions. Certain tape and label adhesive also must be capable of rewetting or remoistening. This generally requires that the formulation contain acid modified starch or high soluble white and yellow dextrins to allow high solids contents. A formulation for a remoistening adhesive that is often used for envelopes is given in Table 9.

Yellow dextrin (95% soluble) 65.7 pbw Water 32.9 Tributyl phosphate 0.2 Heat to 88°C for 30 min, cool to 60°C, and add Corn syrup 1.1

Table 9 : Formulation of Remoistening Adhesive 4

To prevent curling, several processes have been employed. In one process, the tape or label substrate with dried adhesive is run over a bar to produce microcracks in the adhesive layer. Another process uses a solvent deposition of the adhesive where a cold water soluble dextrin is suspended in an organic solvent. When this suspension is applied to paper and dries, it leaves behind discrete particles of adhesive, which will swell and produce the required tack on contact with water.

Dextrins and converted starches are often used for bonding labels. In bottle labeling, high solids starch based jelly gums represent tacky, slow drying, humidity resistant adhesives. They have good adhesion to polyethylene and treated polypropylene. Dextrin formulations are used where high concentration and speed are required.

References:

1. Lazarus, D.M., "Adhesives Based on Starch", Chapter 10, Adhesives and Adhesion, Vol. 7, K.W. Allen, ed., Applied Science, London, 1983. 2. Bauer, I.V., U.S. Patents 2,051,025 (1936), 2,102,937 (1937), 2,212,557 (1940). 3. Jarow enko, W., "Starch Based Adhesives", Chapter 12, Handbook of Adhesives, van Nostrand Reinhold, New York, 1977. 4. Baumann, M.G.D. and Conner, A.H., Chapter 22, "Carbohydrate Polymers as Adhesives", Handbook of Adhesive Technology, 2nd ed., A. Pizzi and K.L. Mittal, Marcel Dekker, New York, 2003.

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SpecialChem4Adhesives Members Reactions

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Helpfull and Informative - Jan 23, 2011 posted by PAWAN KAYAL, R&D - Applied/ Formulation/ Product development at Vinayak Packaging This would certainly expand to understand specialization in starch based adhesive. alert moderator | add y our reaction

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Good knowledge based information - Oct 11, 2010 posted by Anthony D'Souza, R&D - Applied/ Formulation/ Product development Thanks for sharing this excellent information,was very helpfu. alert moderator | add y our reaction

Very good but brief - Aug 22, 2010 posted by Bhaskar Reddy, Corporate Management at stardex Article summarises all the basic formulations. It would have been better if it include new developments in Dextrin making and new starch based adhesive through extrusion technology. Looking forward to see more advanced articles & e training on the subject alert moderator | add y our reaction

Excelent Comprehensive Overview on Starch based adhesives - Jul 28, 2009 posted by ANOOP GUPTA, Technical Service/ Customer Assistance at bOMBAY SILICATE WORKS A to Z of Starch and Dextrine Based adhesives , narrated in very informative way. Gives insight in to adhesives, Keep it up alert moderator | add y our reaction

Excelent open information - Jul 15, 2009 posted by omar vera, R&D - Applied/ Formulation/ Product development at Interquim S.A. Open information permit use it in industrial process and increase of starch consumption. I am very interested in uses with urea- formaldehyde polymers in several applications. alert moderator | add y our reaction

Is a very interesting review. - May 19, 2009 posted by Jose Lenin Ortiz Mongui, R&D - Basic Research All this information is a consise starting point for develoment of especialties in starch or dextrines adhesives. Thanks very much alert moderator | add y our reaction

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