1. Flexible packaging composites
    Flexible packaging composites were mainly used in food packaging but now they were widely introduced to medical packaging, industrial packaging, building materials packaging and cosmetic packaging. The applications range of flexible packing composites is widely increase and the usage amount is also significantly growing. In order to meet the quality and efficiency in different packaging products, environmentally friendly, lightweight and highly functional composite materials are constantly being developed. The composite plastic film is thinner, stronger, and has better performance in barrier and temperature resistance than single-layer plastic film in the same thickness.
    Most of the flexible packaging composites are made by multi-layer fitting or laminating. Recently, they have been developed many kinds of composites packaging materials: plastic film/plastic film, paper/plastic film, paper/aluminum foil/plastic film, plastic film/inorganic oxide/plastic film, and others.

2. Solvent-free packaging composite technology 
    Common compound flexible packaging composite technologies are solvent-free laminating technology, dry and wet lamination, and extrusion composite. Solvent-free laminating technology, flexography, and coextruded composite are the top three eco-friendly technology in international flexible packaging field, and solvent-free laminating technology is the environmentally friendly technology, which obtains new function by bonding polyurethane adhesive with different substrates by curing reaction. Pushed by many flexible packaging raw materials manufacturers which see environmental protection and sustainable development as their mission, solvent-free laminating technology in Europe and the American market has occupied a very important position, accounting for new complex machine 80% to 90% at present. The solvent-free adhesive is truly environment-friendly adhesive, which has good characteristics as follows:

(1) Non solvent emission:
No solvent volatilization and zero emissions in the whole compound process.
(2) Non solvent residue:
No residual solvent in composite products. To ensure the safety of the contents is more in line with food safety requirements.
(3) No explosion, fire or other safety issues:
Adhesive is not inflammable, explosive, corrosive and dangerous goods, which explosion- proof considerations are not required in transportation and storage process.
(4) High production efficiency:
Solvent-free composite machine production speed is generally up to 250 ~ 350m/min or faster up to 450~600m/min, which efficiency is double than the dry composite machine.
(5) Low energy consumption:
Solvent-free composite machine production temperature is low so there is no any drying process for energy consumption.
(6) Glue characteristics:
Relative to the same level of solvent-based adhesives, the molecular weight of solvent-free adhesives is much smaller, so under the suitable temperature, the adhesive does not require the involvement ofsolvent conditions in the state of the substrate surface infiltration, diffusion, and osmosis.

3. Characteristics of solvent-free composite machine
    Solvent-based and solvent-free composite machine is different in coating system and glue system. The solvent-free composite machine doesn’t need to  have drying tunnel, making a compact structure with low energy consumption, low noise, and small space needed. Compared to the solvent-based composite machine, the solvent-free composite machine has the following advantages:

(1) No drying tunnel, low energy consumption:
The general capacity of dryingin the solvent-based composite machine is 150~250KW, and average power consumption up to 50~70 million RMB per year, even used steam drying process will also cause lots of energy consumption, according to the information provided by  Nordmeccanica(Italian), solvent-free energy consumption is almost one-twentieth than solvent-based composite machine.
(2) Fast composite rate: 
Solvent-free composites do not require drying and no residual solvent-dependent, that make high production efficiency, average speed 250m/min and speed up to 600m/min, which meets two machines for production of one composite machine.
(3) High degree of automation:
Solvent-free composite machine using the automatic proprietary automatic gate, automatic control the ratio of two  components and temperature, that also allows to on-line coating weight adjustment, compact machine saving manpower.
   
4. The types of solvent-free polyurethane adhesive and development history
    The solvent-free composite process, equipment, and adhesives developed in the 1970s, olvent-free adhesives are divided into the following types:

(1) Two component polyurethane adhesive:
One component plyurethane adhesive is used with A-agent ( polymers consistence of isocyanate capped ) and B-agent (typically composed of polyols ) in a proper proportion. The main reaction mechanism of two component polyurethane adhesive is the mixture of Hydroxyl (OH) in the polyol component and the polymers consistence with isocyanate capped, curing with bridging crosslinking reaction and produce adhesive strength that bonded different substrates together.
(2) One component wet-curing polyurethane adhesive:
The chemical structure of one component wet-curing polyurethane adhesive (PUR) contains a relatively short chain of polyester or polyether that consistency of isocyanate capped, the isocyanate groups (-NCO) react chemically with moisture in the substrate or environment, releasing CO2 for curing.
    
    For the two-component solvent-free polyurethane adhesive, the use of A and B agent causes inconvenience. In addition, the low viscosity will result in excessive adhesive because of the inaccurate operation parameter setting while applying to paper//plastic film, plastic film//paper//plastic film lamination. As a result, this may lead to the paper color changing, paper hardening and the initial strength becomes lower then PUR. Coating P. Materials Co., Ltd. can provide the PUR for flexible packaging.

5. PUR adhesive used in the lamination of film and paper

(1) Characteristics of PUR adhesive
    PUR adhesive is 100% solid content, solvent-free and isocyanate (NCO) capped resin, which called as reactive hot-melt Polyurethane. PUR is present as a viscous colloid or crystallized blocks at room temperature, heated to make it become flowable to wet the substrate surface and easy to process. The isocyanate groups (-NCO) in PUR is strong chemical reactivity, apart from the bridging crosslinking reaction with water vapor in the environment, it can also have a bridging crosslinking reaction with the active hydrogen functional groups, and the resin can’t be hot-melted again after bridging crosslinking reaction. The transformation between chemical states during the whole reaction is shown in Figure 2. The reaction product has a certain degree of temperature resistance and good flexibility.

(2) Applications of the lamination of film and paper
    PUR mainly for paper//plastic film, plastic film//paper//plastic film or plastic film//plastic film laminating in flexible packaging composite processing technology, the laminating without solvents used in the process doesn't need drying , which make fast speed, less used and more accurately. The Adhesion, functionality and physical properties of PUR can be easily developed to meet all the specific requirements of paper and plastic film products, which are characterized as follows:

a. PUR is 100% solid content, only heating to hot-melt and solvent free, so no environmental pollution and no harmful substances in the finished product residue.
b. PUR processing applications in the flexible packaging materials processing temperature are about 70~90℃ ,the color of the material will not be damaged in the processing.
c. Heated rubber roller can control the gap precisely to avoid the uneven of adhesive.
d. PUR hot melt adhesive can form the bridging crosslinking reaction with water vapor in the environment, no   bridging agent used.
e. When applied to the plastic film//plastic film composite, it is recommended to install the humidifier on the complex machine, PUR melt machine at 70 ~ 90 ℃ melt adhesive in glue tank, the adhesive will transfer to     the substrate by the transfer roll. Adhesive will be cooled and condensed to produce theinitial stickiness so that the adhesive will not easily slide, and then moisture adhering to the surface of the substrate reacts with the PUR, and the chain is gradually hardened and high cohesion of the adhesive layer, the schematic view shown in Figure 3.
f. Adhesive viscosity is relatively important in use of paper and plastic film lamination, due to the expected amount of cloth and the consideration of permeation degree to avoid over-infiltration of paper substrates and   the lacking of adhesive in adhesive layer that caused hard handle and the dark appearance of complex products. The relationship between the processing time and the bonding strength of PUR is shown in Figure   4, properly adjusting of adhesive temperature can change its viscosity and make the composite processing more easily. In the application of coating PUR, with the processing temperature of 85℃, theoretically, the amount of adhesive can be 3 ~ 4g / m² (according to the paper roughness of different adjustments), with the composite structure of CPP//paper//PET can easily processing into bags. The common flexible packaging of paper and plastic composite finished as shown in Figure 5. This kind of finished product are basically focus on the performance of heat sealing strength, friction coefficient and transparency, which according to market demand standards to develop applicable products.