Abstract: Paper-aluminum-plastic composite films are widely used in the field of drug packs. This paper focuses on the analysis of quality problems that are easy to occur on paper-aluminum-plastic composite films produced by gravure and solvent-free composite processes. Residual solvents exceeded standards, discoloration of heat seal, odor, bad heat seal and other problems were proposed.
First, the characteristics and main technical indicators of paper aluminum plastic composite film
The paper-aluminum-plastic composite film has the following advantages in the practical application of medical packaging powder and granule packaging products than the plastic-aluminum-plastic packaging commonly used in the market today: ① paper has high mechanical strength, good rigidity, and has a relatively strong holding capacity after bag-making Strong, improve the stiffness of the product appearance; ② no need to make easy tearing during automatic packaging, has a good easy tearing effect; ③ because the paper has high mechanical strength, when filling and sealing on the automatic packaging machine, the sealing The shearing external force received at the same time has a good resistance effect, protecting the aluminum foil from heat sealing, pressing and breaking, and further ensuring that the barrier performance of the intermediate layer aluminum foil is assured.
The paper-aluminum-plastic composite film for pharmaceutical packaging is a type IV paper, aluminum, and plastic in YBB20062012 (draft). The main technical indicators are:
There shall be no defects such as perforation, foreign body, odor, adhesion, separation between composite layers, obvious damage, air bubbles, wrinkles, and dirt. The heat-sealed part of the compound bag should be flat and free of false seals.
2. Water vapor transmission
The test temperature is 38 ℃ ± 0.6 ℃, the relative humidity is 90% ± 2%, and the water vapor transmission rate is ≤1.5g / (m2 · 24h).
The heat cover faces the low-pressure side of oxygen. The test is at a temperature of 23 ° C ± 2 ° C, and the oxygen permeability is ≤ 3.0cm3 / (m2 · 24h · 0.1MPa)
4. Peel strength of inner layer and sub inner layer: ≥2.5 N / 15 mm
5. Heat sealing strength: ≥12 N / 15 mm
The total residual solvent shall not exceed 5.0 mg / m2, among which the residual amount of benzene solvents shall not be detected, and the detection limit of each single benzene solvent shall be 0.01 mg / m2.
7. Leachables test
Easy oxide: not more than 1.5ml.
Non-volatile matter: Water non-volatile matter residue does not exceed 15.0 mg; 65% ethanol non-volatile matter residue does not exceed 30.0 mg; n-hexane non-volatile residue does not exceed 30.0 mg.
Second, the key technology of paper aluminum plastic composite film production
The structural difference between paper aluminum and plastic aluminum is more manifested as the difference between the paper layer and the plastic layer, which has a very significant impact on the printing, compounding process, and use process. We first analyze the characteristics of paper:
① The surface has a certain roughness
GB / T 10335.2 stipulates: smoothness ≥200S (first-class). The detection principle is to place paper and cardboard on a glass plate and apply a specific pressure to generate a semi-vacuum, so as to suck in air and let the air pass through the contact surface, and measure the time required for the vacuum to change within a specified range.
② Has a certain water content
GB / T 10335.2 stipulates that the moisture content for delivery is (5.5 ± 1.0)%.
③ Strong absorption to liquid
GB / T 10335.2 stipulates that the ink absorbency is (15-28)%.
④ Moisture absorption (or loss of water) will cause dimensional changes
GB / T 10335.2 stipulates that moisture-proof materials such as moisture-proof paper or plastic film should be used.
⑤ Poor thermal conductivity of paper, not as good as plastic
(A) the choice of printing technology
Paper roll paper mainly has two methods: gravure and flexo. Each has its own characteristics. Flexographic printing has a rough surface. The advantages of paper are obvious, and gravure is prone to solid color block blooms. ), But the number of screen lines for flexographic printing is currently rarely 150L / IN, and the number of screen lines for gravure printing is generally 150 ~ 170L / IN, so the screen fineness of gravure printing is high. In addition, the installation capacity of flexographic printing equipment in flexible packaging production enterprises is very small, and the vast majority use gravure printing to print paper.
Gravure of paper needs to pay attention to the following aspects:
1. Avoid the layout of shallow dots
Because the paper surface has a certain degree of roughness, shallow nets less than 30% are difficult to transfer normally (related to the surface of the paper and the degree of roughness), and the lines should be solid and deepened to ensure sufficient ink volume.
2.Avoid designs with high overprint accuracy
Due to the influence of hot air drying of gravure printing, after the first color printing is dried, the paper shrinks due to water loss, which is mainly manifested by poor lateral overprinting. The usual method is to dry the paper before printing to reduce overprinting problems caused by changes in the water content of the paper.
3. Plate roller increasing amount
In plastic gravure printing, due to the elongation characteristics of plastic under heat, the plate diameter increases by 0.03 ~ 0.05mm during plate making, but the paper prints dry and loses water, and the paper size is shortened, so it is not necessary to increase. If during printing, the tension of the material segments between the color groups is inconsistent and is increasing, it is caused by an excessive increase in the plate diameter.
4, electrostatic ink absorption device
To increase the ink transfer effect when printing on paper, an electrostatic ink absorbing device can be used.
5.Selection of ink type
Due to the strong absorption of paper, when using solvent-based inks and solvent-based varnishes, a large amount of solvent components will penetrate into the paper fibers and be adsorbed, and it will be difficult to completely evaporate, resulting in excessive smell and solvent residues. At present, the application of paper-based water-based inks and water-based UV varnishes in cigarette packs is very mature. This experience is worthy of reference for gravure printing of flexible packaging paper.
When using solvent-based inks and solvent-based varnishes, sufficient drying is required to ensure the heat resistance of the varnish, but the moisture in the paper has been greatly reduced after drying, the tear strength of the paper is reduced, and separation between paper layers is prone to occur. When using one-component solvent-free compounding, water is exactly the curing reaction medium of the adhesive. Too little water content in the paper obviously affects the crosslinking curing of the adhesive.
(B) the choice of composite technology
In the paper-aluminum structure compounding method, extrusion compounding and solvent-free compounding have obvious environmental protection advantages, while traditional dry compounding can only be paper sizing. Also, because paper has strong absorption, the solvent is greatly absorbed after coating. Adsorbed to the inside of the paper, it is difficult to volatilize, and the coating amount is large. It can be seen that the traditional dry composite has obvious defects on the aluminum structure of the paper.
Extrusion compounding uses molten resin to bond paper and aluminum foil by hot pressing. The adhesion of pure LDPE to aluminum foil is generally only about 2N / 15mm, and resins such as EEA and EAA (blended or blended) are required. Extrusion compound method) to improve the compound fastness with aluminum foil. Usually, the thinnest extruded layer thickness is about 11μ, which has a high cost. However, the extruded PE layer has good softness. It protects the aluminum foil during hot-press sealing and is not easy to be crushed and broken.
Solvent-free compounding is a relatively economical and environmentally friendly compounding method. The following aspects should be noted during operation:
1.Selection of adhesive types
Solvent-free glue is divided into one-component and two-component. The single component requires moisture for cross-linking and curing. The moisture in the paper is exactly the curing reaction medium of the single-component solvent-free glue, and the molecular weight of the single-component glue is large, and the viscosity after coating and cooling is also relatively large. The two-component solvent-free adhesive is not easily absorbed by the paper and there is a lack of glue. The one-component is more suitable for the solvent-free composite of paper and aluminum structure.
The two-component adhesive has a lower viscosity, and is more easily absorbed by the pores on the surface of the paper after lamination, and the surface lacks glue, and the moisture in the paper will consume the curing agent, which is one of the factors that cause the adhesive layer to dry. Therefore, single-component solvent-free glue is used in paper-aluminum solvent-free compounding.
2.Selection of coated film
When the paper is glued, the fiber pores of the paper will absorb more liquid glue, and in severe cases, the surface will lack glue and delaminate. Generally, in order to make up for the amount of glue absorbed by the paper, the coating amount must be increased, and sometimes it needs to reach more than 4gsm.
When the aluminum foil is glued, because the thickness of the aluminum foil is more than 7 μ, unwinding and over-rolling are easy to wrinkle, which requires higher mechanical accuracy, tension adjustment and operating skills of the equipment. Because aluminum foil coating can reduce the amount of paper to absorb glue, the coating amount can be reduced accordingly, such as 2.0gsm.
3.Selection of coating amount
The coating amount is not as large as possible. The thickness of the adhesive layer remaining between the paper surface and the aluminum foil interface after bonding is an effective coating amount. The glue liquid absorbed into the paper does not contribute to the compound fastness, and it is printed and dried. The moisture in the paper after drying is also limited. The higher the coating amount, the more free moisture it requires for crosslinking and curing. The drying temperature during printing is above 80 ° C, and the free moisture in the paper is basically evaporated. The weakest link in the curing process is the adhesive molecules that are in contact with the aluminum foil. If the coating amount is too large, the moisture in the paper It is also very few, so when it matures, it can penetrate through the adhesive layer to reach the interface with the aluminum foil to participate in the reaction. There are fewer free water molecules. Eventually, the crosslinking and curing are incomplete (the curing time is not improved), the heat resistance is poor, and the heat Sealing and delamination.
4.Details to be paid attention to in solvent-free composite operation
① When the paper is glued, the coating amount cannot be detected by the weight difference method, because the deviation of the weight of the paper itself is too large, resulting in measurement errors.
② When paper and aluminum are combined, the temperature of the composite roller should be low to reduce the fluidity of the adhesive layer, which also reduces the absorption of the glue by the paper.
③ The air entering the plastic bucket should be dried and filtered.
④The relative humidity of the environment should be less than 65% RH. The coating temperature of the one-component glue is above 80 ° C, and the reaction activity with the moisture in the air is high.
⑤ After paper-aluminum lamination, if the inner layer is laminated on a dry laminating machine, the paper-aluminum layer needs to be completely cured before proceeding.
5.Inner layer composite process
The inner layer is generally compounded with PE heat-seal material, and one-component or two-component solvent-free glue can be selected. Because there is very little moisture adsorption on the aluminum foil and PE surface, the one-component solvent-free glue should be appropriately humidified.
In summary, the solvent-free compounding of paper-aluminum structure, the use of a single-component solvent-free glue, the use of aluminum foil coating method, the coating amount of 2.0gsm, is the ideal process conditions.
Analysis of common problems of paper-aluminum-plastic composite film
Different processing methods have different forms of failure. Below we mainly analyze the common quality problems of paper-aluminum-plastic composite films when using gravure and solvent-free composite processing methods:
1, paper aluminum heat seal delamination
The YBB paper-aluminum-plastic composite film standard does not specify the composite strength index of the paper-aluminum layer in the paper-aluminum-plastic composite film. Generally, we peel the paper layer away and observe the paper fiber transfer. If there is a layer of paper fiber left on the aluminum foil surface, , Indicating that the composite strength at room temperature is good.
Paper aluminum delamination has various manifestations and its mechanism is also different. When paper-aluminum-plastic heat-sealing, due to the poor thermal conductivity of the paper, it is necessary to set a high heat-sealing temperature, sometimes the heat is as high as 200 ° C or more, and the heat resistance of the adhesive layer is a greater test. In addition, under the effect of heat above 200 ° C, the paper will lose water and generate a certain shrinkage stress, which exacerbates the phenomenon of heat seal delamination.
1) Delamination only occurs in the heat-sealed area, and the strength is good in the non-sealed area
The heat of the heat sealing knife causes the deterioration of the adhesion fastness, which is generally separated from the interior of the adhesive layer. We can analyze it with the help of PU stains. The main factors are:
① The heat resistance of the adhesive itself is insufficient. Countermeasure: Need to choose appropriate adhesive model.
② The crosslinking curing of the adhesive is insufficient. The factors that cause ineffective curing are: insufficient curing temperature and time, and too little free moisture in the paper. Countermeasures: Prolong the curing time; humidify the paper appropriately.
2) The composite strength of both the heat-sealed and non-heat-sealed parts is poor
The amount of glue remaining between the paper surface and the aluminum foil interface is too small. Possible factors: the paper is too absorbent and the coating amount is low. Countermeasures: Use aluminum foil to glue, reduce the absorption of glue to the paper; increase the coating amount appropriately.
3) Delamination between paper layers
The bonding strength between paper fiber layers is low, and the delamination is effected by the heat-shearing shearing force (the most obvious performance of the heat-sealing method by round pressing), which is related to the water content of the paper itself and its cohesive strength. In addition, the gap between the heat-sealing rollers should be adjusted to avoid excessive shear stress.
The limit of solvent residue in the YBB standard is 5.0mg / m2, but the paper has a strong absorption. When using solvent-based inks and solvent-based varnishes, the solvent components inevitably penetrate into the paper in large quantities, making it difficult to volatilize and dry. The final solvent residue exceeds the standard, which is a technical defect in solvent-based ink printing paper. The fundamental solution is to use water-based inks and water-based varnishes.
3. Discoloration of heat seal
It is mainly caused by insufficient heat resistance of ink and varnish, or insufficient drying of varnish.
In addition to the odor generated by the solvents remaining during printing, the components in the paper may also cause odors. The detection method is to seal the original paper in a closed glass bottle and bake, and then sensorily detect whether there is a significant abnormal odor, or extract the gas in it for chromatography. Analysis, if there are many unknown small peaks on the baseline, it means that there are more volatile substances in the paper and odor will be generated.
5. Poor heat seal
Due to the poor thermal conductivity of the paper, a high heat sealing knife temperature is required for heat sealing, and the thicker the paper, the more obvious the impact. Therefore, the material of the PE layer should be selected as low temperature heat sealing material as possible.
During the use of paper and aluminum composite film, heat seal delamination, discoloration, and poor heat seal phenomena are obviously related to the material structure design. If the temperature of the heat seal knife during operation can be reduced, the probability of quality accidents can be effectively reduced. Reduce the temperature required for the heat sealing knife by the following methods: control the thickness of the paper, the paper weight should not be too large; the inner layer is selected from ultra-low temperature heat sealing materials.