Polymer emulsions are generally a high molecular weight polymer dispersion synthesized by the emulsion polymer method. It often uses water as a dispersion medium, and the polymer is dispersed in water in the presence of an emulsifier in the form of fine particles (particle size of about 0.05 to 1 μm). Milk-like milky-white appearance is usually present, and small-particle emulsions have micro-blue light, so they are called emulsions or aqueous emulsions. Emulsions are widely used in paints, adhesives, inks, and sealants. The most common polymer emulsion adhesives are polyvinyl acetate emulsions, vinyl acetate-ethylene copolymer emulsions (VAE emulsions), polyacrylic copolymer emulsions, and neoprene emulsions. These polymer emulsions have good adhesion strength to porous materials such as paper products, have strong adaptability to them, and have good adjustability of adhesive properties, so they are increasingly used in paper packaging applications. widely.
In recent years, there are many types of polymer emulsion adhesives used in paper packaging, such as: white plastic for color corrugated box, flat glue for paper box, paper tube glue, paper-plastic compound glue, and carton cover. Plastic, sealing tape with pressure sensitive adhesive and so on. Many of these adhesives are designed for certain packaging products, have good bonding strength, are non-polluting, have high specificity, and increase the usage year by year.
This article does not describe the synthesis of the emulsion, but discusses with the reader some key technical parameters that should be mastered when using these emulsions on paper products. It is hoped that these emulsions will be better used in future applications to improve product quality.
Second, color tile box, flat box with plastic
Coloured corrugated boxes and adhesives for flat-tie boxes are the largest varieties of polymeric binders used in paper packaging. The glue used in many areas is polyvinyl acetal, strictly speaking, should not be attributed to polymer emulsions, it can only be counted as water-soluble adhesives. However, because it incorporates starch (or oxidized starch) and fillers, it looks like an emulsion and is often called white glue.
Polyvinyl alcohol is the key to the adhesive performance of white glue. Its price is relatively high. Therefore, the amount of polyvinyl alcohol in the glue determines the cost of the glue. The price of white glue is generally not high, so its solid content is generally not high, about 17~20-%. The amount of polyvinyl acetal in these solids content of about 20% is usually only 10 to 20%, and the rest is oxidized starch or filler. There is a new type of white glue that does not use oxidized starch, but uses cheaper modified water glass, and its appearance is also milky white, and its adhesive strength is also good. The polyvinyl alcohol is acetalized for the purpose of: improving the water resistance; increasing the viscosity (consistency) of the white glue; and preventing freezing in the winter. However, if the degree of acetalization reaches 45 to 50%, although it is advantageous to the above three items, in particular, it shows that the white glue has a large viscosity, but this will obviously affect the initial tack force. The addition of filler can act as a squeezing effect in a certain range, but excessive filler has a significant effect on the bonding strength. The latex without filler is a completely transparent film after drying on a glass plate, and the filler is translucent or milky white after the film is added. The more the filler is, the worse the transparency is.
The viscosity of the white glue should generally be in the range of 6,000 to 10,000 mPa·s. The high viscosity does not mean that the adhesive strength of the adhesive is large, the solid content does not increase, and the effective adhesive material does not increase, artificially. With thickeners, the viscosity is adjusted very high and the result is not conducive to coating. In use, it is often added some water to lower the viscosity, which reduces the content of the effective adhesive material, detrimental to the adhesive strength, there is a decline in the initial viscosity, warping, shooting and other problems. Therefore, the user should not only pay attention to the viscosity of the adhesive, but should consider the solid content and the viscosity is suitable for coating.
The white glue's PH value should be controlled between 6.4 and 7.0. Low and high levels can sometimes cause ink color changes, especially where gold ink is printed. The PH value is too low and there is an accident that the gold ink is darkened.
Third, the paper tube with emulsion adhesive
The paper tube adhesive should be selected according to the use of the paper tube. Ordinary, anti-pressure strength requirements of the paper tube, using the above polyvinyl alcohol glue on it. For paper tubes requiring higher compressive strength, for example, large paper tubes for roll aluminum foil, polyvinyl acetate emulsion must be used. For paper tubes used in wet, higher temperatures, the adhesive can be copolymerized with vinyl acetate. From the current situation of commercially available polyvinyl acetate emulsions, due to the price fluctuations of vinyl acetate and polyvinyl alcohol, the quality of the manufacturing process of each plant is different, and the quality is uneven. Users should pay attention to grasping the points.
The solids content of the emulsion binder should be greater than 30%. The level of solid content often indicates how much of the adhesive is effective to bond. With high solid content and low water content, the time required for the paper tube to reach a certain compressive strength is short.
Fast tack is an important indicator of paper tube adhesives. Polyvinyl acetate emulsions have a certain degree of fast adhesion. In addition to increasing the solids content, polyvinyl alcohol as a protective colloid is preferably not subjected to acetal treatment. It has been observed experimentally that, in the case of the same solid content, the polyvinyl alcohol aqueous solution not subjected to the acetal treatment has a quicker tack than the polyvinyl alcohol aqueous solution subjected to the acetal treatment. In addition, polyvinyl acetate emulsions modified by acrylic or rosin derivatives also have good fast tack. In addition to its own characteristics of latex, to improve the fast viscosity, latex coating process is also particular about the glue should be appropriate, not too thick. The simple determination of quick stickiness can be as follows: Prepare two kraft papers with a width of 20mm and a length of 250mm, apply a thin layer of adhesive on one of them, and immediately press the other one to fit it well and spread it after one minute. If there is paper fiber stuck, or if the paper surface is napped and the marks are pulled, the adhesive will have good tackiness.
For paper tubes used in hot and humid environments, cross-linked polyvinyl acetate modified emulsions can be used for this purpose.
IV. Emulsion adhesive for lapping and laminating
The best type of adhesive for cartons or cartons that are glued directly with tape using flat tapes is the hot melt adhesive. Due to the use of hot melt adhesives, investment in specialized equipment is required. Therefore, solvent-based or emulsion-based adhesives are currently used as zippers. In the past, the use of solvent-based adhesives for laminating paper-based adhesives has resulted in strong adhesion and fast adhesion, but it is harmful to the environment and the human body. At present, both styrofoam and laminating adhesives have begun to use emulsion adhesives to eliminate public hazards. These two types of adhesives share some common features, which are all high solids, low viscosity polymer emulsions.
The preparation of these two types of polymer emulsion adhesives 0 is usually based on vinyl acetate-ethylene copolymer emulsions, ie, VAE emulsions or vinyl acetate-acrylic copolymer emulsions, supplemented with tackifiers and other additives, and VAE emulsions. The cost price is in Acrylic Emulsion, and the adhesive properties vary. As adhesives, there are some rubber-type emulsion adhesives that are commercially available. They are usually made by dissolving synthetic rubber in toluene or ester solvents, and then adding a certain amount of tackifiers, emulsifiers, and water to emulsify them. This kind of adhesive obviously contains a certain amount of solvent and has a large viscosity. It is not as easy to use as the above two types of emulsion adhesives, and the creep resistance is worse.
The solids content of the emulsions for zippers and overlaminates is generally above 50%, mostly 55 to 61%. The high solid content is good for quick drying, and the small amount of water penetration will increase the bonding strength rapidly, so as to achieve the purpose of quick adhesion. The viscosity of these two types of adhesives is generally around 2000 mPas, which is advantageous for thin layer coating. The residual unit content of the adhesive should also be a control index. In particular, for acrylic copolymer emulsion adhesives, the presence of a small amount of acrylic monomers tends to give the emulsion a special odor. Cosmetic packaging is extremely unfavorable. Its residual unit content should be less than 1%, preferably less than 0.5%.
With the expansion of the application of paper packaging containers, there are more and more special requirements for adhesives. In addition to these requirements for adhesive producers, users should also scientifically understand the physical and chemical properties of these adhesives. Performance can be better applied in production.
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