In recent years, the packaging and printing industry, known as the "urban industrial type," has achieved continuous growth at a rate of 10% to 12%, becoming one of the industries with growth potential. With the continuous development of printing technology, various industries have increasingly high requirements for packaging appearance. However, paper box packaging often suffers from scratches and dirt during production on gluing machines or during handling, which severely affects the value of the goods inside. Therefore, it is required that the surface of paper box packaging should have good wear resistance, scratch resistance, water resistance, and dirt resistance. At the same time, in order to enhance product quality, surface finishing processing of paper box packaging is often necessary. There are various methods for surface finishing processing of paper box packaging, including UV coating, calendering, and lamination. Among them, UV coating is relatively common and cost-effective. UV varnish not only meets environmental protection requirements but also has advantages such as fast curing speed, stable application on machines, easy cleaning, and can shorten post-processing waiting time. Moreover, after UV coating, paper box packaging exhibits good wear resistance and scratch resistance along with high gloss. This article will focus on introducing the application of UV varnish in coating paper box packaging for industry reference.

Due to different performances in surface hardness and flexibility under different light curing conditions and temperatures for UV varnish, it is necessary to choose suitable UV varnish in conjunction with printing processes.

Selection of UV Varnish

There are many types of UV varnish; based on product quality and functional requirements they can be divided into flexible UV varnish, wear-resistant UV varnish, re-coatable UV varnish, yellowing-resistant UV varnish, high-adhesion UV varnish and low-volatile UV varnish. Each type of UV varnish has applicable products but also certain application limitations. Generally speaking, printing companies often have a misconception that they require a single type of UV varnish to possess all properties: both flexibility and wear resistance; capable of achieving surface re-coating while also being scratch resistant. Of course all ink suppliers hope to find a product that possesses all properties; however due to the structure of raw materials determining that flexibility conflicts with wear resistance while re-coatability conflicts with scratch resistance; thus there cannot be an all-purpose UV varnish but only the most suitable one.

Therefore it is necessary for printing companies to reasonably select their UV varnishes. Some products undergo complex post-processing after printing; if printed items require die-cutting or embossing after being coated with UV then flexible UV varnishes should be chosen at this time. However it should be noted that flexible UV varnishes use long-chain monomers and prepolymers which have low polymer crosslink density; therefore while they exhibit good flexibility in cured film layers their curing reaction speed is relatively slow. As their polymer crosslink density increases both mechanical strength and hardness will increase leading to loss of flexibility resulting in brittleness over time. Thus generally speaking: better flexibility means slower curing speed and lower surface hardness which may lead to risks such as poor curing after printing or adhesion issues causing scratching during boxing.

Additionally some printed items may require more delicate post-processing steps such as hot stamping or boxing after being coated with UV; at this point re-coatable UV varnishes can be selected because they use resins compatible with hot melt adhesives found in foil stamping which react slowly allowing for low shrinkage rates upon curing while maintaining high surface tension making them soft on top layer surfaces. Therefore better hot stamping effects correlate with better flexibility but this comes at a cost: poorer surface wear-resistance/scratch-resistance performance along with slower curing speeds leading again to risks like poor post-printing curing or adhesion issues causing scratching during boxing.

Wear-resistant UV varnishes utilize resins characterized by high crosslink density along with fast reaction speeds thus providing advantages such as high surface hardness alongside good wear-resistance properties; however these types also tend towards fast curing speeds coupled with large shrinkage rates upon film formation alongside lower surface tension resulting in higher top layer hardness levels overall meaning that better wear-resistance correlates negatively against both re-coatability & flexibility potentially leading risks like color explosion or poor adhesion during boxing.

Precautions when using UV Varnishes

(1) Viscosity: Generally controlled around or below 20 seconds (3# Zahn cup) yields better printing results. However there are no low-viscosity diluents like solvents or water present within any given batch so adjusting viscosity during operation becomes difficult typically requiring temperature control within containers holding said materials instead.

(2) Storage: During usage avoid direct sunlight exposure keeping stored below temperatures around 30°C preferably shaded areas when adjusting viscosity through heating take care avoiding excessive heat/time durations preventing gelation/quality degradation occurring within materials themselves.

(3) Labor protection aspects: Since skin irritation occurs from contact due its non-volatile nature especially under higher summer temperatures if any gets onto skin/clothing it could mix sweat entering skin causing inflammation hence immediate washing off followed by changing work clothes becomes essential whenever contact happens.

Issues needing resolution regarding UV Varnishes

(1) Odor: Although efforts are underway towards reducing odors associated still remain prevalent hence future developments must focus on increasing molecular weights/reactivity levels among prepolymers/monomers including odorless raw materials involving photoinitiators too.

(2) Safety: Compared against water-based coatings regarding odor/food hygiene safety standards fall short additionally FDA approval hasn’t been granted allowing direct food contact either.

(3) Cost reduction: In recent years costs associated have decreased significantly yet still remain comparatively higher than conventional inks necessitating further reductions primarily concerning operational costs linked directly back towards equipment used within processes involving both applications & cures themselves respectively.

Overall despite aforementioned challenges faced by current formulations significant advantages exist surrounding environmental protection energy savings simplified operations alongside enhanced product value added benefits thus believing continued advancements made throughout ongoing developments surrounding said technologies will yield greater opportunities ahead for future growth potential overall!

The above content is excerpted from Ke Yin Media's "Printing Technology · Digital Printing Art" Issue No.12 in 2016.