Authors: Ming Junjiang, General Manager, Valence Chemical (Shanghai) Co., Ltd.

Chen Weimin, Technical Director, Valence Chemical (Shanghai) Co., Ltd.

Abstract: In recent years, with the development of the packaging industry, the consumption of adhesives has remained at a high level. The global packaging industry accounts for about 35% (1) of the total adhesive consumption, making it the largest application sector for adhesives. This article reviews the application and hazards of organic solvents in water-based adhesives for post-printing paper products, and based on existing domestic and international laws and regulations, puts forward a comprehensive solvent-free solution for the paper products industry.

Keywords: Paper packaging, organic solvents, water-based adhesives

Printing and processing of paper products is one of the important post-printing processes (2), including children's books, paper box forming, paper bag forming, hardcover book casing, mounting, etc. Due to the numerous processes involved in printing and processing paper products, different processes require adhesives with different functional properties to match different production processes. For example, high-speed equipment requires adhesives with short curing times, while manual production processes require adhesives with longer open times; for processes such as mounting and children's books, the adhesive needs good post-drying deformation resistance, etc. Due to the uneven quality of paper packaging, the functions of various adhesives vary greatly, and the adhesives used in the processing of printed paper products are even more diverse, with solvent adhesives containing ethyl acetate and water-based adhesives containing harmful solvents such as benzene and toluene. Therefore, in the printing and paper products industry, the environmental protection of adhesives is an urgent issue to be addressed.

Overview of the Use of Organic Solvents in the Water-Based Adhesive Industry

The solvents referred to in the adhesive industry usually refer to organic solvents such as benzene, toluene, xylene, and ethyl acetate. Organic solvents are widely used in adhesives, coatings, and other chemical products. These solvents will directly evaporate into the atmosphere during use, polluting the environment and endangering workers' health.

Organic solvents are common components of water-based adhesives. Water-based adhesives (commonly known as "white glue") widely used in the paper products industry usually contain 3-5% solvent components, while the solvent content of adhesives for difficult-to-bond substrates (commonly known as "yellow glue") can be as high as 20%.

The Role of Solvents in Water-Based Adhesive Formulations

Solvents are usually an important component of adhesive formulations. Although solvents are not found in the final adhesive film, they often play a very crucial role in the final properties of the adhesive film. Generally speaking, they mainly have the following functions:

1. Used to dissolve the base material into a component of the adhesive, such as dissolving rosin with toluene to better mix the toluene rosin solution into the emulsion (3).

2. Used to dilute the adhesive to a certain viscosity, which is conducive to wetting and facilitates coating application;

3. Used to adjust the drying speed of the adhesive. In some packaging production processes, the adhesive is required to dry quickly. Since the evaporation rate of the solvent is faster than that of water, adding a solvent can effectively solve this drying speed problem;

4. Used as a penetrant and etchant. For difficult-to-bond substrates such as BOPP films, due to the corrosive nature of the solvent, the surface gloss of the film can be destroyed, improving the mutual penetration between the BOPP surface and the adhesive to promote the adhesion of the adhesive to the difficult-to-bond substrate;

5. Used to reduce the cost of adhesives and improve efficiency. As a large-scale industrial raw material, its cost is relatively low. For adhesives, which are fine chemicals, increasing the amount of solvent will significantly improve the cost of the adhesive;

6. Used as a medium for polymerization reactions, making the reaction milder and the temperature easier to control;

Potential Hazards of Solvents

Toxicity: Organic solvents are liposoluble, so in addition to entering the body through the respiratory and digestive tracts, they can also be rapidly absorbed through intact skin. After organic solvents are absorbed into the human body, they will act on the nervous and blood systems rich in lipids, as well as parenchymal organs such as the liver and kidneys, and also have some irritation to the skin and mucous membranes. The main organs affected and the intensity of the effects of different organic solvents vary, depending on the chemical structure, solubility, contact concentration and time of each organic solvent, as well as the sensitivity of the organism. The table below shows the basic parameters and toxicological information (4) of some commonly used solvents in water-based adhesives:

Solvent Name

Benzene

Toluene

Xylene

Ethyl Acetate

Appearance

Colorless and transparent liquid with a strong aromatic odor; sparingly soluble in water.

Colorless and transparent liquid with a strong aromatic odor; insoluble in water.

Colorless and transparent, easily volatile liquid with a strong aromatic fragrance, insoluble in water.

Colorless and transparent flammable liquid with a special fruity odor, partially soluble in water.

Boiling Point

80.1℃

110.6℃

144℃

77℃

Flash Point

-11℃ (closed cup)

4.4℃ (closed cup)

17℃ (closed cup)

-5℃ (closed cup)

Toxicity

Benzene vapor is toxic and can not only damage the human body through the respiratory tract, but also be absorbed directly through the skin. The toxicity to the human body is most significant in the hematopoietic organs and nervous system.

Toluene is highly irritating to skin and mucous membranes, and its vapor has a strong effect on the central nervous system. Inhaling toluene vapor at a concentration of 100-200 ppm for 8 hours can cause fatigue, nausea, general weakness, drowsiness, loss of motor function, and hallucinations.

Its toxicity is lower than that of benzene and toluene; it can damage skin and mucous membranes. High concentrations of xylene vapor have anesthetic effects.

High concentrations of ethyl acetate vapor irritate the eyes, nose, and throat, but symptoms subside after leaving the vapor. Extremely high concentrations can cause conjunctivitis and anesthesia.

In daily industrial production, however, harm caused by the use of solvents is frequently observed:

(a) Risks to enterprises using solvent-containing adhesives: Printing and packaging companies using solvent-based adhesives face mandatory rectification by environmental protection departments; severe cases lead to fines and factory shutdowns. According to news reports, a packaging and printing company in South China was forced to shut down by environmental protection authorities after inspections revealed that the benzene content in the air of its workshop exceeded the standard due to the solvents in the adhesives used. This resulted in significant economic losses for the company.

(b) Hazards to operators using solvent-containing adhesives: Cases of workers developing allergic reactions due to solvents in adhesives are frequently reported. A Dongguan client of Huawai Adhesives previously used water-based adhesives containing solvents from other manufacturers, causing skin redness and itching on the hands of some employees. After switching to Huawai's solvent-free water-based adhesives, the problem was completely resolved.

(c) Impact on brand suppliers using solvent-containing adhesives: Major brand suppliers, facing a large number of end consumers, must consider that the presence of solvents affects their brand value and consumer trust. Therefore, leading international brands are beginning to mandate that their packaging and printing suppliers do not use solvent-containing raw materials, including adhesives.

Domestic and international legal regulations on solvent use

Volatile organic solvents such as benzene and toluene directly evaporate into the atmosphere during use, polluting the environment and harming workers' health. Therefore, many government departments abroad have issued regulations to restrict the use of volatile organic compounds (VOCs).

1. North America: The well-known North American environmental organization GreenSeal has clear limits on VOCs in various commercial adhesives, currently using the version of October 19, 2000. This standard classifies adhesives based on the substrate used; for porous substrates, the VOC limit is 120 g/L. (6)

2. European Union: EU regulations closely related to VOCs include EU Directive 2004/42/EC and the European Eco-label. The EU directive sets a minimum VOC limit of 30 g/L for water-based products, while the European Eco-label sets a minimum VOC limit of 15 g/L for water-based products.

3. China: GB18583-2001 clearly stipulates that benzene cannot be used as a solvent in adhesives, and its maximum content as an impurity must not exceed 5 g/kg. Toluene is only subject to a quantity limit; the limit is that the total amount of toluene + xylene must not exceed 200 g/kg.

Application of water-based adhesives in the packaging and printing industry

Water-based adhesives are widely used in the packaging and printing industry, mainly including: 1. Water-based sealing adhesives made from EVA emulsion, rosin, toluene, SBS, SIS tackifying resin, and emulsifier, widely used in the sealing of laminated cartons and the finishing of fine-bound boxes (5); 2. PVAc emulsion (white glue), widely used in printing plate making, splicing of medium-density fiberboard fine boxes, and children's book making, etc.; 3. Starch and modified starch, used in cardboard lamination and the preparation of corrugated packaging; 4. Bone glue and gelatin, widely used in fine food packaging.

Huawai Adhesives' solvent-free solutions in the paper industry

Huawai Chemical is one of the earliest suppliers to advocate and promote solvent-free adhesives in the paper industry. After nearly four years of research and development and market promotion, Huawai Chemical has become one of the first adhesive suppliers to provide comprehensive solvent-free solutions in the paper industry. Huawai Chemical's solvent-free products can be used not only for ordinary paper but also for bonding paper with PET film, PVC film, and PP film, and are particularly suitable for high-end wine boxes, food boxes, cosmetic boxes, fine cigarette boxes, and electronic product packaging boxes. The following are Huawai Adhesives' solvent-free products:

PB8028NT series: Suitable for the pasting applications of decorative gift boxes such as high-end wine boxes, food boxes, cosmetic boxes, fine cigarette boxes, and electronic product packaging boxes. Suitable for difficult-to-bond materials (such as BOPP & PET laminated materials), it features low odor, strong wet tack, long open time, good resistance to curling and deformation, and high final bonding strength.

The PB 8028NT series is a new generation of solvent-free water-based adhesives independently developed by Huawai Chemical, which can replace most "yellow glues" in the paper industry. It has been selected by many large paper packaging companies and has solved long-standing problems for these companies.

PB8621H series: Suitable for gluing and edge bonding of medicine boxes, toothpaste boxes, etc., with or without lamination. It features low odor, fast drying speed, high initial adhesion, and high final bonding strength, suitable for high-speed gluing machines.

PB8362H series: Suitable for bonding PP, PE, and PET films to paper, especially paper with UV coating, with excellent bonding performance, suitable for medium-speed gluing machines.

PB8246H & PB8797 series: Used for paper bonding, such as the lamination of paper and cardboard, book lining, and the production of children's hardboard books. The products have high solid content, fast drying speed, and small deformation of the finished product. The products do not contain any solvents and fully meet the EU testing standards for children's toys.

PB8329A series: Widely used in paper bonding, such as the lamination of paper and aluminum foil, PET film to produce gold and silver cards, etc. It features low odor, low coating amount, high peel strength, long open time, good anti-deformation performance, good water resistance, and high temperature resistance.

PB8018 Series: Water-based adhesive, suitable for envelope sealing, resealable adhesive, used for sealing ordinary envelopes and envelopes with heavy ink; suitable for high-speed envelope machines.

Huawai Chemical's above solvent-free adhesive products have passed the following environmental protection tests:

1, FDA 21 CFR 176.170

2, More than eighty substances of very high concern as stipulated by Reach.

3, Six harmful substances as stipulated by Rohs.

4, EN71 Part 3:1994+A1:2000+AC:2002

5, ASTM F963—11

6, 16 kinds of phthalate tests