I.Characteristics of Paper-Plastic Composite Packaging Materials

Paper-Plastic Composite Packaging Materials

As end-consumers increasingly demand visual impact and packaging perfection from various brands, as well as a focus on environmental sustainability, many brands are incorporating more paper elements into their packaging designs. Fruit gift boxes, snack bags, and medicine boxes all feature paper-plastic composite packaging structures. The novel and unique design of paper-plastic composite packaging materials has generated strong sales for brand owners.

Paper-plastic composite packaging materials are a composite material composed of a paper-based material and one or more other film materials, combined through one or more lamination processes to create a functional composite material. Paper-plastic composite materials overcome the shortcomings of plastic film's poor printability and water-induced deformation, offering high strength, excellent water resistance, and a beautiful appearance. They are currently one of the most popular packaging materials and are widely used in the food, daily chemical, and hygiene product industries. They are also widely used in beverage and food packaging, pharmaceutical packaging, high-end paper boxes, and paper bags for clothing.

Materials and Equipment for Paper-Plastic Lamination

The materials involved in the paper-plastic lamination process include: adhesive for laminating paper-plastic lamination, laminating machines, laminating paper, and laminating films.

Laminating Adhesives: Single-component acrylic adhesives are generally used for paper-plastic lamination.

Laminating Machines: Paper-plastic lamination generally falls into two categories: thin paper and film lamination, and thick paper and film lamination. The tension and roll diameter of the former are similar to those of conventional flexible packaging laminating machines, and they typically utilize dual-purpose machines for both paper-plastic and plastic-plastic lamination. The technical parameters and structure of the latter differ significantly from those of flexible packaging laminating machines, and they typically utilize dedicated laminating machines for paper-plastic lamination.

Precautions for Using Paper-Based Materials for Paper-Plastic Lamination

A wide variety of paper materials can be used for laminating, including coated paper, white kraft paper, yellow kraft paper, offset paper, writing paper, light-weight coated paper, pearlescent paper, soft tissue paper, and base paper. Depending on the packaging design requirements, different composite structures can be created, such as OPP/paper, PET/paper, CPP/paper, PE/paper, and AL/paper. There are dozens of different types of paper, depending on the end use and process. Commonly used in paper-plastic packaging include kraft paper, white kraft paper, soft tissue paper, base paper, and pearlescent paper, with basis weights ranging from 25g to 80g.

Due to the wide variety of paper types and their diverse uses, the following points should be considered when using different papers:

① Generally speaking, relatively glossy papers bond easily to film, while matte papers are more difficult to bond to film. This is primarily due to the uneven texture of the matte surface. To achieve a tight bond, increase the amount of adhesive applied and fill these small pores with adhesive.

② Pay attention to paper density. Some papers have loose fibers. While this may bond well to the film during lamination, delamination is likely to occur after heat sealing.

③ The moisture content of the paper also affects the bonding effect. The moisture content of paper should generally not exceed 0.4%.

④ Pay attention to the cleanliness of the paper surface. Delamination is also more likely to occur when the paper surface is contaminated with dust or other debris. Performance Requirements for Paper-Plastic Composite Products

Paper-plastic composite products

Paper-plastic composite products generally require strong adhesion, a clean, flat, and smooth surface, and no blistering, curling, film loss, or film release. The quality of paper-plastic composites is influenced by many factors, both objective and subjective. These factors include environmental factors in the operating environment (temperature, humidity, and dust levels), factors within the product itself (ink, paper type, and printing method), and factors such as the adhesive and film used in the composite, as well as the composite process conditions, operating methods, and the structure and technical performance of the laminating machine. Therefore, a comprehensive analysis is required.

II. Analysis of the Causes of Major Problems in Paper-Plastic Composites

Common problems in paper-plastic composites include poor composite fastness, blistering or flakes in the center of the composite, uneven shelling, uneven curling, streaking in the center of the composite, and discoloration when laminating gold or silver inks. The most significant issue is poor composite fastness. Some finished paper-plastic composites peel easily off the machine, while others may initially show good adhesion but experience separation after a period of time. In summary, the factors that influence the lamination strength of composite products generally include: the adhesive strength of the laminating adhesive itself, the compatibility of the adhesive with the film, paper, and ink layer, the condition of the adhesive layer, and the processing technique.

Water-based laminating adhesives must not only exhibit strong adhesion to the film, but also to the paper and ink layers. The adhesive must be compatible with these three materials, ensuring a good bond between the film, paper, and ink layers. A detailed analysis is as follows:

1. The Impact of Paper-Plastic Laminating Adhesives

The adhesive's inherent bonding performance depends on factors such as the raw materials, formulation, production process, and solid content used by each manufacturer.

2. Poor compatibility between the adhesive and the film.

The chemical structure of BOPP film, commonly used for lamination, lacks reactive groups and is non-polar. Its surface free energy is low, resulting in poor compatibility with the adhesive, making surface bonding difficult. Therefore, the film used for lamination must be corona treated.

3. The Impact of the Compatibility between the Adhesive and the Paper on Lamination Strength.

Composite paper is generally a porous material made from interwoven plant fibers. It undergoes pulping, conditioning, papermaking, and a coating to compensate for surface irregularities, by calendering or supercalendering. Composite printed products utilize a wide variety of paper types, with whiteboard and coated paper commonly used for lamination. Coated paper has a denser structure and a smoother surface, while whiteboard has a looser structure and a slightly rougher surface. When the adhesive layer and paper are compressed, the composite bond strength decreases for papers with less smooth surfaces and increases for papers with smoother surfaces. During use, the amount of adhesive applied should be adjusted according to the paper type to compensate for surface irregularities and improve adhesion.

4. The Impact of Adhesive-Ink Affinity on Composite Bond Strength

The affinity between the adhesive and ink layer is more complex than that between the adhesive and film or between the adhesive and paper. Currently, the inks used are primarily resin-based. Ink is a complex mixture composed of pigments, resin binders, and auxiliary materials. Flat-printed products are generally printed with spot-color inks. The colors of the printed products are juxtaposed and overlapped, and the ink layer is generally thinner, while the ink layer of embossed products is slightly thicker. From the perspective of the ink layer structure, embossed printing is more complex than flat printing. The internal structure of the ink layer of the printed product, that is, the chemical properties of the ink, is also related to the blistering phenomenon during lamination. The composite condition of the black ink surface is related to the affinity of the glue, which is related to the bonding strength.

(1) The pigment particles in the ink used are large, and the bonding strength between the glue and the ink layer will be slightly poor. For example, the aluminum powder and copper powder particles in silver ink and gold ink are larger, much larger than the particles of ordinary ink. These metal particles adhere to the surface of the ink layer and emit silver or gold light when exposed to light. If the amount of glue is too little, the glue layer and the silver ink or gold ink layer are bonded in the form of point contact, resulting in poor bonding strength and easy peeling;

(2) For printed products with poor paper flatness, since a thin layer of ink cannot fill the concave and convex areas of the paper, the surface of the ink layer will also appear uneven. The ink layer of flat printing is thinner than that of embossed printing, and the unevenness of the ink layer surface is more obvious. In this case, the bonding strength between the glue and the ink layer of the composite product will also be poor.

(3) The smoothness of the ink layer surface is too high, which is not conducive to the strong bonding between the glue and the ink layer. Some printed products have been placed for too long, and the ink layer surface has crystallized, which is both hard and smooth. If the glue and the ink layer cannot be well wetted and affinity with each other, it will also cause weak bonding. In severe cases, it will fall off with a slight peel.

(4) For images with a large amount of ink, the affinity between the glue layer and the ink layer is slightly worse than that of paper, which will also affect the bonding strength. III. Performance characteristics of Sinograce Chemical Paper Plastic Laminating Adhesive SA-228 paper plastic composite adhesive is a water-based acrylic emulsion that can be used for laminating corona-treated BOPP film with printed paper products. It has the characteristics of strong bonding strength, high brightness, embossing resistance, good leveling on the machine, and low foaming.

How to use SA-228 laminating adhesive

The laminating machine should be completely cleaned before use. Raise the laminating machine's hot roller temperature to 60-90°C and the drying oven temperature to 60-130°C.

Pour the laminating adhesive into the adhesive tank and start the coating roller to spread and mix thoroughly before laminating. No solvents are required for mixing.

When laminating BOPP film coated with white paper, the adhesive does not need to be completely dried; it should pass through the laminating rollers without squeezing out of the film. When laminating gold or silver cardboard or products printed with gold or silver ink, complete drying is essential.

Laminated products can be cut directly; peel strength reaches its peak after 24 hours.

Machine speed, drying oven, hot roller temperature, and adhesive application amount should be flexibly controlled by the user based on the equipment, product type, and actual conditions. Precautions for Using Paper-Plastic Composite Adhesive

1. Water-based laminating adhesive should generally not be mixed with other types of composite adhesives.

2. Clean the adhesive roller with water immediately during downtime.

3. Before processing gold and silver ink printed surfaces, it is recommended to make a sample to confirm color retention and bond strength.

4. Due to the low ambient temperature in winter, avoid prolonged storage of composite products in low temperatures before drying.

5. For printing surfaces with thick ink layers, large dry ink layers, rough paper, deep embossing, or embossing requirements, it is recommended to test a small sample to confirm satisfaction before proceeding with mass production.

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