Trends & Innovations Materials & Solutions Training & Education Channels Community Pulse Open Innovation New s Editorials R&D Highlights Patent Watch Articles Market Reports Industry Events Glossary Newsletters 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.