Solvent-free two-component Polyurethane Adhesive technology
The key to the technology of solvent-free two-component polyurethane adhesives is to synthesize the appropriate isocyanato-terminated main agent and must meet the following requirements:
1. To achieve the required bond strength;
2. Has good coating performance;
3. Low viscosity to ensure good process performance.
In terms of synthetic route selection, polyesters are first synthesized using dihydric and dibasic acids, and the molecular weight of the polyester is adjusted by the choice of the chemical structure of the dihydric alcohol and the dibasic acid and the ratio of the amounts to meet The requirements of the isocyanate-terminated prepolymer are then reacted with polyethers and isocyanates of different molecular weights, functional groups, to synthesize isocyanate-terminated prepolymers.
The isocyanate-terminated prepolymer is an oligomer of a polyester diol and MDI and a mixture of a polyether diol and an oligomer of MDI; the hydroxyl-terminated prepolymer is a polyester diol and a polyether divers Mixtures of alcohols should have a ratio of hydroxyl groups to isocyanate groups of about 1:1.6 (molar ratio).
In the choice of polyester, the molecular structure of polyester has a great influence on the performance of polyurethane. In order to obtain a film with high strength and certain flexibility, various components in polyester should be properly matched, among them, aliphatic Dibasic acids, such as adipic acid and sebacic acid, have linear, hydrocarbon chains in their molecules that increase the soft segment of the polyester, and thus increase the flexibility of the final polyurethane product and lower the freezing point. The introduction of an aromatic ring in the chain increases the rigidity and the ability to resist oxidation, and because of the steric hindrance, it is difficult for the hydrolysis agent to approach the ester bond, thereby improving the hydrolysis resistance. Suitable aromatic diacids are isophthalic acid, terephthalic acid, phthalic acid or anhydride.
Simple diols, such as ethylene glycol, 1,3-propanediol, or 1,4-butanediol, can form linear polyester chains with excellent flexibility and have a strong tendency toward crystallization. Polyesters formed from short chain diols have a high -COO-bond density and therefore have a strong intermolecular attraction, which can explain their strong tendency to crystallize, as well as their poor resistance to polar solvent and carbon Reasons for poor miscibility of hydrogen compounds. Branched diols, such as 1,2-propanediol, interfere with the intermolecular association due to the presence of side chains, and at the same time provide a steric protective effect on the ester bond, thus improving heat, oxidation, and hydrolysis resistance. Performance, and have a weak tendency to crystallize and better solvent resistance.
In two-component solvent-free adhesives, the use of polyester alone is far from meeting all of the required properties and requires the use of polyethers for adjustment. First, polyether polyurethanes are flexible and can reduce the viscosity of products. Second, due to the weaker interaction between polyether chains and polyesters, the cohesive strength of the product can be adjusted to improve the adhesive properties of the product. Third, the hydrolysis resistance of polyether elastomers is much better than that of polyester elastomers, which reflects the superior hydrolytic stability of ether groups over the ester groups. Fourth, the use of polyethers can reduce costs.
In the selection of polyethers, suitable polyethers include propylene oxide polyethers and propylene oxide-ethylene oxide copolymer polyethers, and the viscosity and adhesive strength of the products can be adjusted by changing the molecular weight of the polyether used.
While keeping the molecular weight of the polyester constant, the molecular weight of the polyether is changed and the viscosity of the product decreases as the molecular weight of the polyether increases. This is because the smaller the molecular weight of the polyether, the shorter the molecular chain, the greater the density of urethane bonds in the polymer, the greater the intermolecular attraction and the higher the viscosity of the product.
Application Prospect Analysis
At present, due to cost reasons in China, solvent-free two-component polyurethane adhesives are still far from universal, and solvent-based adhesives are used in large quantities. This is mainly due to the fact that solvent-free composite equipment is expensive on the one hand and requirements on operators. On the other hand, solvent-free products are more prone to complex quality problems when compounded, and the applicable structures are limited and the main agent itself still has certain toxicity. These factors restrict its promotion and use in our country. However, from a long-term perspective, the proportion of solvent-free and alcohol-soluble products will continue to rise as environmental regulations become more stringent and economic development continues.
Since more than 20 years ago, German Herberts has used solvent-free polyurethane adhesive composite packaging materials, due to its obvious economic, safety and environmental protection advantages, as well as the continuous development of laminating technology and equipment, it has made it in Europe and other developed countries. The country has rapidly increased from 10% of its use 20 years ago to 60% of the present.
In recent years, with the vigorous development of the food industry, the industry for making two-component adhesives for polyurethane composite films has been continuously expanding. The current market capacity has reached an average of 3 to 50,000 tons per year according to incomplete statistics. At present, there are more than 100 manufacturers of polyurethane laminating adhesives in China, all of which are solvent-based adhesives. In the future, we should gradually focus on the development of solvent-free adhesives, and we should adopt a combination of technology introduction and independent research and development to develop products that meet the requirements of use as soon as possible.
(Reproduced from: China Packaging News)
