Starch based adhesive compositions, and the natural and synthetic alternatives thereof, find many industrial uses, such as in the industry producing corrugated box board, paper bags, paper boxes, in carton sealing or case sealing, in book-binding or tube winding, in wallpaper adhesives, in the production of laminated paper board, gummed tape or paper, envelope adhesives, school glues, bottle labelling and in textile sizing.
Starch or amylum is a natural polymeric material which is found in almost every plant, a carbohydrate consisting of a large number of glucose units. It is produced by all green plants as an energy store, for storing their glucose as a polysaccharide polymer, which is a water insoluble and much more compact form than glucose. It consists of two types of molecules: the linear and helical amylase and the branched amylopectin. Starch molecules arrange themselves in the plant in semi-crystalline granules, and each plant species has a unique starch granular size. Pure starch is a powder which is insoluble in cold water, but which may convert to a water soluble form when heated.
Many plants may be dried and milled or ground to obtain a starch-containing flour. This is a dry, simple and inexpensive process, and which leads to a flour containing starch together with other components, such as usually gluten and fiber. Starch in higher purity on the other hand is typically produced by a wet and more complex process, whereby a natural product such as corn is usually first softened to break the bonds between the individual components of the kernel, also called the steeping process, followed by wet milling or grinding, washing and screening or sieving to separate the other component parts in relatively high purity. Corn processing for instance leads to the so-called steepwater, containing all the water solubles, and further washing and separation leads to the starch, separated from the germ, the fiber and the gluten as co-products. The germ, gluten and fibers are dewatered and dried by use of filters, presses and dryers. The germ is usually further processed to recover corn oil, while the remaining fiber is typically returned for animal feed.
For the production of industrial and food starches, starch slurry from the washing and separation steps is usually dewatered in centrifuges to produce a starch “cake”, which is then dispersed into a stream of heated air, and flash dried to the desired water content of about 10%. Great care must be taken to prevent the starch from overheating in the presence of moisture, because of the risk for gelation to start, and such a product may also exhibit an undesirably high sensitivity to caustic soda.
Today the principal sources of commercial starches are corn or maize, potato, tapioca and wheat, but also rice, sweet potato, sago, arrowroot and mung bean are used commercially.
The quality of a starch product depends on its starting material, and as with any natural raw material, the properties of a harvested plant may be affected by many factors, such as temperature, hours of sunlight, humidity, rainfall, year of planting, soil moisture, plant population and fertilization rate.
The chemical makeup of the starch polymer makes it a good adhesive, a property which has been known and used since ancient history. Raw, untreated or ungelatinized starch, also labeled “pearl” starch, does not dissolve in cold water, i.e. in water at or below ambient temperature. The starch only becomes soluble in water when heated. In heated water, the granules swell and burst, the semi-crystalline structure is lost and the smaller amylase molecules start leaching out of the granule, forming a network which holds water and increasing the mixture's viscosity.
This transformation of the starch granule is called gelatinization. The gelatinization temperature, or gel temperature, of a starch in water is the temperature at which the mixture begins to thicken and to develop its bonding properties. Although it is often expressed as a single temperature unit, in reality it is a temperature range, dependent on the molecular size and composition of the starch, and hence strongly raw material dependent. The gel point of a starch in the adhesive industry is very important, as it sets the temperature required for the applied adhesive to gelatinize and to perform its bonding function after evaporation of the excess water.
For modern industrial applications however, with corrugating machines running at speeds in excess of 300 m/min, starch itself is too slow in developing its adhesive properties, i.e. its “tack”, and also its viscosity in water is too low. The “gelatinization point” of a pearl starch is typically 77° C. or higher. It is known that the gelatinization temperature of a starch may be reduced, for instance by the addition of a base. Sufficient amount of a strong base, such as caustic soda or soda ash, may even fully gelatinize starch at ambient temperature or below, and convert it into a cold water soluble form. Industrial starch-based adhesives are therefore made rather strongly basic. For gel point reduction, a need which is driven by reduced energy requirements and faster processing speeds, the starch-based adhesive therefore typically has a pH of at least 12.2 or above, and in many instances the pH may be above 13.0. The conventional starch adhesive therefore typically contains a high amount of free base, usually NaOH.
Starch may be pre-cooked, or pregelatinized and subsequently dried, such that it may be used to thicken instantly in cold water. Starch may be chemically modified to allow the starch to function properly under conditions frequently encountered during processing or storage, such as high heat, high shear, low pH, freeze/thaw and cooling. Typical modified starches for technical applications are cationic starches, hydroxyethyl starch and carboxymethylated starches.
The prime non-food application for starches is their use in the paper and corrugated cardboard industry, together consuming millions of metric tons of starch annually. Nearly all of the corrugated board produced today is bonded with a starch-based adhesive, primarily using unmodified native starches. An aqueous and raw starch-containing adhesive composition is applied onto the paper or cardboard surface, the water migrates into the porosity of the paper or cardboard and entrains the starch granules, which by applying heat and pressure, and assisted by the presence of a base, swell and gelatinize and act as a fast and strong adhesive by forming the network required for providing the desired bonding. Excess water, i.e. the water not absorbed by the starch granules, is then typically removed by vaporisation.
An adhesive containing only pearl starch, water and some caustic soda is too low in viscosity for a good application of the adhesive. Various methods have been developed to increase the viscosity.
The conventional method is the Stein-Hall method wherein separately a “carrier” is produced. For this purpose, typically 15-20% of the (primary) starch is heated in the presence of a significant amount of (primary) water and caustic soda to form a viscous liquid or paste. This viscous carrier of gelatinized starch is then used as the basis for suspending the typically 80-85% secondary amount of pearl starch in water complemented with an additional, i.e. a secondary, amount of water, to form the target adhesive composition. The viscosity of this aqueous adhesive composition is primarily set by the amount and the viscosity of the carrier used in the final adhesive formulation. Usually a small amount of a boron compound, most frequently borax, a product chemically known as di-sodium tetraborate decahydrate, is finally added, primarily for buffering the caustic soda. The anion structure of the borax builds further interchain linkages between the already gelatinized starch polymer molecules, thereby further increasing the viscosity of the adhesive composition. The pH of a typical Stein-Hall adhesive is about 12.5, but may sometimes be as high as 13.1.
There exist variants to the Stein-Hall process. The borax may for instance be added to the carrier, before mixing in the secondary starch and additional water, optionally already prepared in parallel as a secondary mixture, such as for instance described in JP 45020598 B4 or in JP45019600 B4. In the so-called “Roquette” or “Primsteam” process, only heat and no base is used in the production of the carrier. Caustic soda is added only in the secondary step, in order to reduce the gel point of the pearl starch in the final adhesive. This process has the disadvantage that the viscosity of the final adhesive is more difficult to control. In the “Minocar” process, the carrier forming step is monitored by closely controlling its viscosity during production. When the desired viscosity is reached, further gelatinization is stopped by adding extra water, extra starch and borax. In the Jet Cook process, all the water, half of the starch, and the required caustic soda and borax to give the desired final gel temperature of the adhesive are mixed together, and live steam is injected to swell at least part of the starch until a given viscosity is reached. Upon reaching the target, the remainder of the pearl starch is added under further mixing to form the final adhesive. In the so-called “No-carrier” process, all the starch, water, 50% caustic solution and boric acid are mixed together and used to “swell” the starch granule under well-controlled heat input. This causes a partial gelatinization of the starch present in the mixture, a process which needs to be tightly controlled to provide the desired viscosity of the formed adhesive composition.
All these processes suffer from various disadvantages. Most importantly, only the pearl starch in the adhesive is available for bonding the paper or cardboard. The amount of primary starch used for raising the viscosity, such as in producing the carrier, and which usually amounts to about 15% of the total starch, is already gelatinized before the adhesive is applied. It is thus not anymore available to participate in the desired bonding function. This means that typically about 15% of the total starch used is not used as adhesive, but only as a viscosity modifier.
Furthermore, the gel point and viscosity of gelatinized starch in water vary from crop to crop, even of the same plant. This means that the process using the starch-based adhesive needs to be adjusted with any variation in the origin of the supplied starch.
Viscosity of the adhesive is however also a very important process parameter in the corrugating industry. With too low viscosity, the water tends to “wick” into the medium, i.e. diffuse away from the glue line and the pearl starch may not have sufficient water left to gelatinize completely, and hence not reach its full bonding properties. This may produce dry and brittle board coming off the corrugator, and which may fail the adhesion test. With too high viscosity, on the other hand, its penetration into the medium becomes too slow, leading to wide glue lines and thick bonds, thus to adhesive overconsumption, and also to overflow of the glue pans.
Another problem is that the viscosity of the adhesive composition, as primarily provided by the gelatinized starch in combination with the boron compound, is not stable. Viscosity is very temperature dependent. Tight temperature control of the adhesive composition is therefore required during its application and also in storage. The adhesive inventory may have to be reheated after a period of downtime, which has to be done carefully and thus is time consuming. Shear, such as it occurs at mixers, pumps, pipes, elbows or valves, at the corrugator and in the return lines to the adhesive inventory tank, breaks down the structure of the adhesive, and/or even more importantly of the gelatinised starch in its carrier, and this reduces the viscosity of the adhesive irreversibly. Therefore, where shear is inevitably applied, this shear must be applied and administered carefully. Production lines may be hundreds of meters long, and the adhesive may therefore need to be pumped, from a centralised “Glue kitchen” where it is prepared, over a distance of hundreds of meters to the point in the production line where it is to be applied. These transfer systems often comprise return lines for the adhesive as well. Pumps should be carefully selected and properly designed. The viscosity of a starch-based adhesive is also not constant over time, as the raw portion of the adhesive will begin to separate from the cooked portion, causing the viscosity to drop. Also gel temperature will rise the longer the adhesive is held in storage, because of a slow adsorption of free caustic into the starch. Fast turnover of the adhesive and limited inventory during down periods are therefore advised.
Another problem is a phenomenon called “spinning”. When the adhesive is applied to the tops of the corrugations or flutes and the corrugated substrate separates from the applicator roll, the adhesive forms threads before breaking up. When these threads break and the adhesive jumps back to the substrate, a part of it typically ends up away from the desired location, called the “glue line”. This part does not participate in the desired bonding function. More adhesive must therefore be applied, which increases the consumption and also the energy requirements of the process, as well as the risk for undesired temperature effects. The extra adhesive also increases the risk for malformation, in particular with thin substrates.
WO 2005/007765 A2 discloses Stein-Hall starch-based adhesive compositions further comprising small amounts of a cross-linked polyacrylate polymer additive with the prime purpose to improve the rheology of the adhesive composition, such that the viscosity of the adhesive changes with shear forces applied to it. In one example, an amount of primary starch and an amount of primary water are first mixed to obtain a primary starch mixture containing 10.7% wt of starch. This mixture needs to be stirred in order to avoid settling of the starch. Into this mixture the additive, as well as sodium hydroxide, secondary water, secondary starch and borax are added, and the result is again stirred. The resulting mixture contains 31.14% by weight of starch, based on the total adhesive composition. In another example, the additive is first mixed with water. Subsequently corn starch is added to a concentration of about 6.2% wt, The mixture is heated to 40° C. and a significant amount of caustic soda is added and mixed again, which causes the available starch to fully gelatinize. Thereafter water, pearl starch and borax are mixed in, providing an adhesive composition in which only 90.8% of all the starch present is non-gelatinized. WO 2005/007765 A2 further discloses solid pre-mixes comprising the additive, to which an amount of water may be added to prepare an adhesive. In these solid pre-mixes, at most 90.9% of all starch present is non-gelatinized. Apart from the addition of the additive, WO 2005/007765 A2 teaches to keep the nature of the adhesive composition virtually unaffected, as well as its solids content and gelling properties, due to the importance thereof when processing the composition.
FR 2890394 discloses the preparation of a Stein-Hall adhesive whereby first an adjuvant is prepared having a boron content of about 10% wt, and further comprising 0.1% by weight of sodium polyacrylate (Norasol LMW 45N). The adjuvant is added to the Stein-Hall adhesive in an amount of 0.345% wt, leading to a concentration of less than 3.5 ppm wt of the sodium polyacrylate in the adhesive. In FR2890394, 10.98% of all the starch present in the adhesive composition is gelatinized.
In the processes disclosed in JP 45020598B4, first a primary paste is made, typically containing 4.13% wt gelatinised starch, 0.86% wt of caustic soda and 0.85% borax in water. In example 2, the primary paste is a mixture of 4.72% wt sodium polyacrylate and 0.86% wt NaOH in water. Also prepared are secondary mixtures containing more than 32% wt or more of adhesive components in water. These mixtures are too rich to be suitable by themselves as adhesive. In these secondary mixtures, sodium polyacrylate and starch are used as the adhesive components, in weight ratios which vary from 0/10 to 10/0. The secondary mixtures are subsequently mixed together with the paste, to obtain an adhesive composition in which at least almost 12.5% or more of the starch is gelatinized. In Example 2, the secondary mixture contains starch only. The polyacrylate, in the ratios and concentrations used, was found to bring an effect of water resistancy when used in combination with starch. Without starch, the adhesive force was reported zero. All the pastes in JP 45020598B4 contain 0.86% wt of NaOH as gelatinizing agent and for gel point reduction of the ungelatinized starch, and which leads to the pH of the resulting adhesive compositions being 13.1 or above. The gel points of the adhesives in JP 45020598B4 were not measured.
The gelatinized part of the starch in the state of the art adhesive compositions is also susceptible to microbiological degradation. The gelled starch is prime food for bacteria, and, when attacked, is converted to soluble carbohydrate, which reduces viscosity and may even develop odour. Biocides may be used, but regular cleaning of the installation and change of biocide type is advised, in order to reduce breeding grounds and biocide resistance.
Also polyvinyl alcohols are used as adhesive components in adhesive composition, optionally in combination with other adhesive components such as a starch and/or a dextrin.
Another type of adhesives employ dextrin as one of, or as the sole adhesive components. Dextrins are a group of low molecular weight carbohydrates obtainable by the hydrolysis of starch. Dextrins are polysaccharides formed by heating dry or acid-modified starches in a process called pyrolysis. They can be used at higher solids levels than native or modified starches, creating stronger bonds, more tack and faster-drying properties than pastes made from unmodified starch. Dextrin pastes provide excellent machineability and also can be used in adhesives and coatings which come in contact with food products.
Frequently dextrin adhesives, as starch based adhesives are formulated with boron compounds such as borax (short for 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 or dextrin. 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.
Laminated corrugated board is a multilayer material in which one or both of the top and bottom side of a corrugated cardboard core are adhered to a sheet of flat board or paper, usually called the liner. An adhesive layer is applied to the top of the corrugations or flutes, following which the tops of the corrugations are adhered to the paper sheet.
The first addition of a flat paper liner to a corrugated medium of flutes is called the “single face” step, occurring at the “single facer” and producing the “single facer web”. Providing a flat liner to the opposite side of the medium is called the “double backer” step, occurring at the “double backing” station. Adding a second corrugated medium of flutes as the next layer is called the “double facing” step. This may continue up to 3, 4 and 5 layers, depending on the purpose of the corrugated cardboard. Many weights, thicknesses and combinations of liners and medium are used for different applications. However, the principles of manufacture remain the same.
In an adhesive composition containing at least one polyvinyl alcohols as the adhesive component, boron components such as borax are often added to the composition in order to improve the tack and/or of the so-called spin or spinning behaviour and associated problems, as explained above, of the composition.
A problem with the Stein-Hall process, and its known variants, but thus also with many other adhesive preparation processes which do not use starch, but for instance are based on dextrin or on polyvinyl alcohols, is that the properties of the adhesive as desired in these different corrugating process steps typically differ, and that each step or material used desires an adhesive having its own particular gel point, its own dry matter content, and also its own particular viscosity and overall rheology behaviour. Each of these process steps may be provided with their own adhesive inventory, which is usually kept relatively small in view of the limited shelf life of the adhesive, but the typically common adhesive preparation step, usually performed in a common “Glue kitchen”, needs to adapt its recipe each time to the needs of the particular downstream consuming process part. This adds significant complexity to the overall corrugating process.
Another problem with the known adhesives is that they often contain boron compounds, especially in combination with starch and/or polyvinyl alcohols, but also in combination with dextrins, and in combination with starch and/or dextrins also a significant free amount of a strong base. Because of toxicity concerns, the presence of boron compounds has recently become less desirable, in particular for adhesives which may be used in food contact applications. Also the presence of the strong base, such as caustic soda, has become undesirable. These strong bases are aggressive towards many construction materials. Their presence demands for less common construction materials, which adds complexity and investment costs to the process. Strong bases are also aggressive to skin, and therefore demand more complex procedures for the safety of the personnel operating the processes. In some geographies therefore, the use of strong bases, such as used in the high pH streams of the adhesive and cardboard industry, is becoming more and more regulated, and may even one day become prohibited. There is therefore a growing need for adhesive compositions having a lower boron content, preferably being free of boron and/or having a lower pH, ideally being about pH neutral.
There therefore remains a need for an adhesive composition which is easy to prepare, of which the viscosity may be easy to predict and control, which makes more effective use of the starch employed in its preparation, which exhibits the non-newtonian rheology behaviour preferred in the corrugating process, and which contain less strong base. Preferably, such adhesive composition would also contain less boron, more preferably be free at least one of these less desired ingredients.
The present invention aims to obviate or at least mitigate the above described problem and/or to provide improvements generally.