As an important material, corrugated cardboard is used in the production of corrugated paperboard packaging for both domestic and export products. Corrugated paperboards are excellent in performance, light, strong, and easy to handle, and are widely used in the packaging of household appliances, textiles, and foodstuffs. Is an essential packaging container. Judging from its development, the sales volume of cartons reached 8 billion square meters in 1996, and it increased to 12.5 billion square meters in 2000. By the middle of six years in 2002, it has increased by 1.5 times to 20 billion square meters. It is expected that by 2004 the output of corrugators used for transport in China will reach 16.5 billion square meters, making it Asia's largest producer of corrugated paperboards. Adhesives used to bond core paper and face paper can be said to have a decisive influence on their properties, so it is of great value to study adhesives with good properties. Our country has more than 20 years of history in its development.
1. Overview of the development of adhesives for corrugated board
The earliest adhesive used in the carton industry in China was NaSiO3 (also known as water glass, babocyanine) used in the 1930s. Although it has a fast film formation and a low price, the carton bonded with it has a large alkaline content, easily absorbs moisture, and is corrosive. As a result, the packaged goods are damaged and the environment is also polluted. In view of this, China developed starch adhesives in the late 1970s. In the mid-1980s, in 1984, starch adhesives began to be promoted as a substitute for silicates. Although starch has the advantages of wide sources, abundant resources, low price, non-toxicity, and no pollution, it uses its binders with large water content, poor initial viscosity, unstable viscosity, slow curing speed, and poor film rigidity. Therefore, there are many studies on modification of it, but these modifications mainly focus on the oxidation modification, and the oxidation-modified adhesives are mainly targeted at the majority of SMEs in China that lack the automatic forming and drying equipment, and the high-speed assembly line. There are few studies on the production of starch binders. At present, there are a lot of deficiencies in the small amounts that have been developed.
In the foreign countries, from the 1950s, starch binders were used instead of babbitin. In 1935, Stein-Hull of the United States successfully developed corn starch adhesives, and the adhesives can be applied separately according to different preparation methods. For small businesses and large corrugated cartons. Abroad, the deficiency of the initial starch adhesive was also modified, such as improving the moisture resistance, initial viscosity, drying speed, etc. The modification was relatively successful. At present, the starch glue developed in the United States can generally be used for dry pulverization, and can also be used now. With the use of cash, most of the targeted high-speed automated assembly line glue.
From this point of view, research and development of excellent performance and automated assembly line adhesives are urgent issues in China.
2. The development and role of various common additives in adhesives
The most basic raw material in starch adhesives is starch, which includes corn starch, potato starch, potato starch, etc. Different starches will be used when formulating the rubber species due to different origins, but the use of corn starch is currently the largest . The most commonly used additives are pasting agents, cross-linking agents, and oxidizing agents. During the reaction process, a suitable amount of catalyst and defoamer can be added as appropriate. In order to formulate adhesives with specific functions, various functional auxiliaries such as driers, moisture-proofing agents, preservatives, plasticizers and the like are added. The following describes the commonly used additives.
2.1 pasting agent
Gelatin is an additive used in starch adhesives. It has been used in NaOH, and it has been used abroad as well. The main role of NaOH is to lower the gelatinization temperature, and this is achieved by combining with the hydroxyl groups in the starch, breaking some hydrogen bonds, and weakening the intermolecular forces. Once the oxidizing agent is added to the starch to oxidize it, the effect of NaOH will be shown to adjust the pH value and ensure basic oxidizing conditions. The carboxyl group of the oxidized starch will also be changed into sodium salt to increase hydrophilicity and solubility. Of course, the amount of NaOH should be appropriate, the amount is too small, can not reach the gelatinization effect and the glue liquid is not good, and is not easy to store; the amount is too large, the viscosity is too low, the flow is large, not easy to glue, the corrosion of paper products Sex will increase.
2.2 Crosslinker
The basic formula for starch binders is starch, water, and caustic soda. The cross-linking agent is an additive used along with the oxidative modification of the starch. The main cross-linking agent that has been used for a long time is borax (Na2B4O7·10H2O). Glyoxal, formaldehyde, and epichlorohydrin are used in small amounts. Recently, a polymer resin was used together with borax as a cross-linking agent, such as vinyl. Polymer resin, the main component of the molecular formula is (CH2)nNO. The main role of borax is to improve adhesion and initial tack because borax forms a network-like multinuclear complex with fully swollen starch molecules, aids cross-linking of the starch, and renders the resulting film firm. The amount of borax also has certain requirements, too much amount, too much cohesion, so that the liquidity of the glue is poor, easy to gel; the amount is too little, the complex is not enough, the adhesive force is poor, the glue is too thin, so that the corrugated cardboard degumming, easy storage Layered.
2.3 Oxidants and Catalysts
Since the oxidation modification, the oxidants used by most manufacturers at home and abroad are KMnO4, NaClO, and H2O2, but they are selected by comparison for their own production. Of course, these three types of oxidizers have their own advantages and disadvantages. However, as far as the development is concerned, H2O2 is a favorite at home and abroad.
KMnO4 as an oxidant oxidation speed, the amount of less, from the color change can determine the extent of the reaction, easy to operate. But the adhesive made was dark brown and used on corrugated boxes to affect the appearance.
NaClO is a low-cost oxidant, but it is unstable, and its concentration in storage varies greatly, and chlorine gas is emitted in the reaction to pollute the environment.
H2O2 as an oxidant oxidation effect is good, non-toxic and odorless, with less use, do not give the reaction into the impurity ions in the oxidation process. But its price is the highest among these three oxidants.
The use of catalysts can be said to be less costly in terms of selection, because catalysts are used to speed up the reaction. This is closely linked to chemistry, and a chemical basis can be chosen more easily. When KMnO4 is used as oxidant, no catalyst is needed; when NaClO is used as oxidant, metal ions such as Ni2+, Fe2+ and Cu2+ have good catalytic effect, and the amount of Fe2+ is the highest; H2O2 is used as oxidant, Fe2+ has better catalytic activity, and US has Mn2+ substitution. Fe2+ ​​is a catalyst.
3. Development of Functional Adhesives
Functional adhesives refer to adhesives that modify one aspect of an adhesive in the presence of a deficiency in its use. At present, there are many researches on high-strength, fast-drying, water-proof and moisture-proof, especially for the shortage of high water content and slow drying speed of starch binders, and the most researches on fast-drying. In this respect, there is a big gap between China and foreign countries. According to Dalian's two more famous corrugated box factories, China's three-layer corrugated board car speed is 70 ~ 80m/min, five-storey can only run 30m/min, cardboard dry very slowly, and the United States of starch adhesive The five-story speed reached 200m/min. Therefore, we still have a long way to go. The focus of this foreign modification is on waterproofing.
3.1 Fast Drying Starch Adhesive
The starch binder has a solid content of about 16%, which is less than half of the solid content of the babulolen, which makes its drying rate very slow, thereby affecting the use, so improving the drying speed of the starch binder becomes a modification. The main job. Up to now, there are two main approaches for quick-drying modification: adding fillers to dry and adding catalyst to dry.
Starch adhesives are solvent-based adhesives, the solvent is water, and the host material is starch. The water evaporates or volatilizes out of the binder and enters the surrounding environment. The process of forming a starch film is a drying process. The evaporation of water is only a form, and the formation of film is the connotation and essential content of drying.
1 The filler driers bentonite, kaolin, light calcium carbonate, calcium chloride and alum are the inorganic filler driers used today, of which bentonite and light calcium carbonate are the most used at home and abroad. Its role is to increase the solid content on the one hand, fill the pores of paper fibers to reduce water infiltration, and on the other hand absorb water to form a film when the moisture is reduced, thereby increasing the drying rate.
The drying principle is that the filler converts part of the solvent water in the adhesive into a part of the film, thereby increasing the drying speed. In the glue, the filler does not chemically interact with water and other components of the adhesive, but When the water is reduced to a certain extent, the filler concentration increases, and quickly interacts with water. A molecular packing can “cure†water several times or even hundreds of times its own weight, together forming an integral part of the film, and thus greatly Increase the drying speed of the adhesive.
2 Drier The driers here mainly refer to organic macromolecules. At present, the amount of PVA (polyvinyl alcohol), urea-formaldehyde resin and benzene-acrylic emulsion is more. Some people classify the reaction of these substances with starch binders into two types: graft reaction type and polymer emulsion blend type. Grafting reaction type is for PVA and urea-formaldehyde resin. After adding these two substances into corn starch adhesive, grafting reaction occurs under certain conditions, and the hydrophilic hydroxyl groups in starch molecules can be masked to make water. The main component volatilized in the air. Polymer emulsion blends are based on styrene-acrylic emulsions. Some have also considered PVA and urea-formaldehyde resins in this class, adding a styrene-acrylic emulsion, and drying it with cornstarch binder molecules. Intertwined to form a larger network, it can increase the bonding strength. Benzene-acrylic emulsions have hydrophobic groups that are conducive to the volatilization of water and speed up drying. However, styrene-acrylic emulsions do not dry as well as other desiccants (including fillers).
3.2 water resistance (moisture resistance) starch adhesive
There is very little research in this area. Northwestern University for Nationalities has discussed the effect of process conditions on the water resistance of starch binders and provided a set of cornstarch adhesives for packaging in a wet environment under its own laboratory conditions. Agent production process. However, the only deficiency of its formula is the addition of formaldehyde. There are many foreign studies in this area, especially the modification of the United States has been relatively successful.
The US patent number 5,405,437, filed in 1995, is a patent of All Natural, Starch-based, and Water Resistant Corrugating Adhesive. It has made significant progress in moisture resistance. The preparation of the starch binder in this patent is similar to the Stein-Hull two-step process, in which the main and carrier starches are prepared respectively. The carrier starch can be either dry powdered or can be used immediately. Alternatively, the starch can be prepared in one step. Regardless of the formulation, the desired objectives are achieved by using commonly used additives and raw materials (corn starch, NaOH, H2O2, Mn2+ catalysis, sodium thiosulfate or sodium sulfite terminator), and the process conditions are very subtle. Its other patent in 1995 Patent No. 5,393,336 is also a study of water resistance modification, and is also achieved by controlling process conditions.
4. Conclusion
Starch adhesives have become more and more important with the large-scale use of corrugated boxes, but due to the late start of research in China, there are still many deficiencies in use, such as: poor initial viscosity, plastic The membrane rigidity is small, and the type of glue used in the assembly line is too small. In short, this still requires the untiring efforts of our researchers and the development of better performance adhesives.