1. Field of the Invention
The present invention relates to a corrugated cardboard, an adhesive for corrugated cardboards as well as a method for making the adhesive. In particular, the present invention relates to a moisture-absorptive corrugated cardboard, an adhesive to enhance the properties of corrugated cardboards as well as a method for making the moisture-absorptive adhesive.
2. Description of the Prior Art
The buffer, or shock-absorptive ability of a normal corrugated cardboard, which is mainly for use in the manufacture of package boxes, is usually enhanced by gluing cardboards to both sides tips of corrugated medium with an adhesive. Because the adhesive, usually a starch glue, breaks loose easily, and the strength of the corrugated cardboard boxes drop dramatically after the corrugated cardboard boxes absorb water, they are not suitable for the packaging of moisture-proof items, fresh vegetables and fruits. As shown in FIG. 1, in order to make the corrugated cardboards more moisture-proof, one conventional method is to spray the moisture-proof agent on the surface of inside or outside linerboard of a box. Normally, the moisture-proof agent is a chemical and it is usually toxic. In addition, spraying moisture-proof agent as required increases the cost of production due to extra material cost and procedure.
It is known that addition of zeolite to the outer liner of the corrugated cardboard may make it more moisture-proof and helps the items inside last longer by means of the naturally absorptive zeolite. Zeolite is a common name for the zeolite group, which are crystals of water-containing alkaline metals or alkaline earth metals aluminosilicate. Zeolite is a porous substance with nano-scale pores and holes; pore diameter less than 2 nm is called micropore, 2-50 nm is called mesopore and larger than 50 nm is called macropore. Different types of zeolite has various pore sizes. Molecules smaller than the critical pore size are absorbed but molecules larger than the critical pore size are excluded. This phenomenon is called “molecular sieve.” Due to this phenomenon, zeolite may serve as an absorbent with specific surface area up to 1000 m2/g.
It is known that a corrugated cardboard that helps the items inside last longer has a zeolite absorbent film laminated on the inner surface of the corrugated cardboard. The composite film may absorb ethylene gas and keep the moisture inside from evaporation so the items inside may last longer. However, the drawbacks of the product are that it costs extra and takes additional procedures to manufacture the corrugated cardboard so the corrugated cardboard is not compatible with the traditional process. Besides, the product is not popular because of its expensive price.
It is also known that there is another corrugated cardboard with nano-scale gas absorbent powder added during the manufacturing process of the linerboard for the corrugated cardboard to be specifically absorptive. This kind of corrugated cardboard of special specification is beyond the ability of a normal corrugated cardboard factory. Additionally, a layer of glue is needed to be applied on the surface of the outer liner to make the surface much more even and to enhance the quality, which makes it less absorptive. Actually, the corrugated cardboard of special specification can only be optionally produced in smaller batches.
Traditionally, the main ingredient of the adhesive for the corrugated cardboard is corn starch (main starch) and processed starch as a carrier (carrier starch). Two of which are mixed to form the starch adhesive. Usually, caustic soda (NaOH, also known as lye, or sodium hydrate) serves as an auxiliary component for use in gelation to lower the gelation temperature. Short chain starch uses its hydroxyl groups or carbonyl groups to readily form complex with borax so as to increase the initial adhesive strength. Generally speaking, caustic soda is added in an amount of 1-3% of the total weight of the corn starch and, and borax is added in an amount of 0.2-5.0% of the total weight of the corn starch. When the corn starch is mixed with zeolite powder to be the zeolite starch adhesive, the blended adhesive is less adhesive/sticky because of the lower percentage of the corn starch. Simultaneously, the added zeolite powder would make the drying time longer then deteriorate the initial adhesive strength which may increase the initial bonding time. This slows down the production line for corrugated cardboards. Accordingly, the adhesive with corn starch and zeolite powder has lower adhesive speed and simultaneously adhesive failure occurs due to the lower initial adhesive strength during the production of the corrugated cardboard.
After reacting with water, heat and caustic soda, starch reveals the initial adhesion and the permanent adhesive strength of the gel-like starch. The initial adhesion plays a crucial role during the production process because it determines the adhesive speed. Generally, in order to increase the adhesive speed, more caustic soda is added to lower the gelation temperature. However, although the addition of caustic soda may lower the gelation temperature in short term, it also adversely makes the adhesion unstable in the long run.
By lowering down the production line speed and increasing the amount of adhesive applied on the corrugated cardboard, the adhesive with zeolite powder as an additive may in one aspect increase the initial adhesive strength but in another aspect it also slows down the production, which fails to meet the demands of industrial interests. Moreover, if the production line speed is too much slower, the overly high temperature rather makes the adhesive pre-maturely dry out so the adhesive fails to be adhesive or sticky enough and even the layers of corrugated cardboard are parted by giving a simple pat.
Furthermore, the price or the adhesive speed, which may influence the production of the corrugated cardboard, are still issues to be considered because different kinds of starch have different physical properties when using zeolite powder as an additive in the mixed adhesive for bonding the corrugated cardboard. Practically speaking, neither should starch with good adhesive speed be replaced nor can starch with high amylose content be used because they are much more expensive than raw starch.
It is expected that corrugated cardboard may be even more widely useful if normal materials rather than particular materials are used in a production line to obtain a satisfying product.
Although in the recent years the production speed of the corrugator is greatly improved, the adhesive speed is not optimal because the adhesive is added with zeolite powder. An adhesive containing zeolite powder with better adhesive speed is still needed for industrial application.