New UV-Curable Coatings for the Resilient Flooring Industry

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					Technical Paper




                  New UV-Curable Coatings
                  for the Resilient Flooring
                  Industry
                                                    I
                  By Dirk Bontinck and                   n many industries, growing                 The aim of this paper is to give an
                  Mike Idacavage                         concerns on ecological issues have      insight in the present and future
                                                         lead to the development of              developments of high performance, yet
                                                    technologies having a lesser chemical        environmentally and economically
                                                    impact on occupational health and            viable resilient flooring coating
                                                    environment. Although the chemical           systems. These developments may
                                                    industry has acted upon these con-           vary geographically; cultural differ-
                                                    cerns by allocating a growing amount         ences will put emphasis on different
                                                    of resources to projects with an             functional and esthetical aspects of the
                                                    ecological background, the switch from       end product, as perceived by the
                                                    existing to these new technologies is        consumer. Although the chemical
                                                    difficult and slow. Among the reasons        industry has been globalizing to a high
                                                    why industries are somewhat reluctant        extent, it will have to find ways to meet
                                                    to make this switch, a higher initial        local demands.
                                                    cost and a potential loss of perfor-
                                                    mance can be regarded as the most            Resilient Flooring
                                                    important drawbacks.                            Next to other flooring materials
                                                                                                 such as hardwood, ceramic tiles,
                  During the last decade, solvents, hazardous                                    carpets and fabrics, the global resilient
                                                                                                 flooring market is estimated at 800
                  crosslinkers, coalescents and free monomers have
                                                                                                 million m2—it covers vinyl, polyolefin,
                  been questioned; the development of waterborne,                                rubber and linoleum. These resilient
                                                                                                 floorings have their specific advantages
                  ultraviolet-curable coatings can offer a way out.
                                                                                                 and drawbacks, and vary in popularity
                                                        This paper will focus on UV-curable      in different parts of the world. Their
                                                    polyurethane dispersions used by the         general characteristics are shown in
                                                    resilient flooring industry, their chemis-   Table 1. Resilient flooring is often
                                                    try, performance and relative cost.          preferred because it is easy to install
                                                        During the last decade, solvents,        and to clean, comes in many different
                                                    hazardous crosslinkers, coalescents          colors and designs, and is relatively
                                                    and free monomers have been ques-            inexpensive. However, two major
                                                    tioned; the development of water-            drawbacks are that many household
                                                    borne, ultraviolet-curable coatings can      materials and dyes can create severe
                                                    offer a way out. The way these               stains, and that many shoe sole types
                                                    waterborne UV-systems can overcome           can leave scuff marks.
                                                    cost and performance drawbacks will             To overcome these drawbacks,
                                                    be addressed, as well as how this            resilient flooring is protected with an
                                                    chemistry could develop in the future.       overcoat. The first coating systems

                  50 RADTECH REPORT MAY/JUNE 2003
                                                                                                                                          Technical Paper
 Table 1
    Different types of resilient flooring
       Type                          Composition                 Advantage                       Disadvantage
       Vinyl                         PVC                         • Low cost                      • Contains 10-30% CI
                                     • Heterogeneous             • Flame retardant               • Shrinkage
                                     • Homogeneous
       Polyester                     PE/PP blends                • Cl-free                       • Primer needed
                                                                 • No shrinkage                  • Overall higher cost
       Rubber                        • Vulcanized rubber         • Low cost                      • Low performance
                                     • SBR                       • No plasticisers               • Poor design freedom
                                                                 • No shrinkage
       Linoleum                      Linseed oil (1/3)           • ‘Natural’ product             •   Not flame retardant
                                     Limestone (1/3)             • No shrinkage                  •   Long term flexibility
                                     Wood/Cork flour (1/3)                                       •   Maintenance
                                     On jute backing                                             •   Linseed monopoly




involved solventborne nitrocellulose,       neutralizing agent for the carboxylate           enables the manufacturing of
followed by polyaziridine crosslinked       functions. Prepolymer chain length and           low-irritation coating systems.
waterborne polyurethane dispersions,        functionality of the hydroxy acrylate               A comparison of UV-PUD with PUD
and 100% solids UV-cured systems.           can be varied in order to come up with           and 100% UV systems is shown in
Radiation-curable systems are wide-         the desired crosslink density. The use of        Table 2 (following page). It is clear that
spread in the United States, because        non-irritating acrylating oligomers              waterborne UV benefits from the
their ability to reach very high-gloss
values, whereas the polyurethane
dispersions are used primarily in
                                              Figure 1
Europe because of their low-gloss
                                                 Chemistry of UV-PUD
possibilities.
    The performance, as well as the
design demands for this type of                                                                      OH
flooring, has been increasing over the
past decade. An increasing fraction of
                                                    HO           OH   +   OCN    NCO    +            COOH     +   HO         OH

the resilient flooring production is now                                                       OH
being lacquered for improved stain and
abrasion resistance.
                                                           OCN                                                    NCO
UV-Curable Polyurethane                                                                       COOH
                                                                      COOH
Dispersions
    The combination of polyurethane
                                                                                     1. HO-acrylates
dispersions and radiation-curable
                                                                                     2. water/triethylamine
technology has led to the development
of waterborne UV-curable polyurethane
dispersions (UV-PUD). The chemistry
                                                                      COONHEt3               COONHEt 3
is outlined in Figure 1. Anionic,
isocyanate-terminated polyurethane
prepolymer is reacted with a hydroxy-
lated acrylate, and is then dispersed
into water using a tertiary amine as a

                                                                                             MAY/JUNE 2003 RADTECH REPORT 51
Technical Paper



                   Table 2
                      Comparison of technologies
                                                              100 % UV                     UV-PUD               PUD
                          Organic co-solvent-free             OK                           OK                   3-14%NMP
                          Monomer/Oligomers                   Reactive diluents            Water                Water
                          96% EtOH resistance                 OK                           OK                   Limited
                          Stain resistance                    OK                           OK                   OK
                          Hard yet flexible                   Limited                      OK                   OK
                          Sprayable                           Limited                      OK                   OK
                          Thickness control                   Limited                      OK                   OK
                          Tack free before cure               Limited                      OK                   OK
                          60° gloss range                     20-90%                       5-80%                5-70%
                          Monomer deep-cure                   Problematic                  OK                   NA
                          Adhesion to PVC                     Problematic                  OK                   OK




                  advantages of both 100% UV and PUD               enables the design of UV-PUD that is      The use of UV-PUD results in a higher
                  systems, without suffering from any of           tack-free before cure, meaning that       degree of freedom in both process and
                  their obvious disadvantages. Water-              they are non-blocking after water         design as gloss, thickness and surface
                  borne UV-PUD exhibits a superior                 flash-off and before UV curing. This      embossing can be arranged on the
                  balance in chemical resistance and cold          feature is very interesting for the       same coating equipment.
                  flexibility. The improved flexibility can        resilient flooring manufacturer—the          The lack of crosslink density,
                  be explained by the fact that their              surface can be embossed thermally         necessary to obtain good solvent and
                  UV-crosslink density is lower than for           before curing, without sticking to the    chemical resistance, is balanced by the
                  100% UV systems, meaning that the                embossing rolls. If necessary, the        presence of polyurethane. The typical
                  average molecular weight between                 flooring material can even be wound       urethane hard domains provide
                  crosslinks is higher (Figure 2). This            up at another time or at another place.   excellent chemical resistance.



                   Figure 2
                      Relative crosslinking density representation of a 100% UV system and a UV-PUD




                                            100% UV system                                                   UV-PUD




                  52 RADTECH REPORT MAY/JUNE 2003
                                                                                                                                             Technical Paper
                                                                                                 dyes, dissolved in various carriers
 Table 3                                                                                         (mineral oil, absolute ethanol and
                                                                                                 water) for 16 hours under a cover
    Formulation for waterborne UV-PUD                                                            glass. Then, the excess dye is re-
        UV-PUD                            UCB                       100.0                        moved, the sample is cleaned with
        TS100                             Degussa                   1.0                          water and isopropanol. The remaining
                                                                                                 stain is reported on a scale from 1 to 5.
        Byk 346                           Byk                       0.4
                                                                                                 (1 meaning a severe stain, 5 means
        Aquamat 266                       Byk                       1.0
                                                                                                 that no trace of the dye can be
        Irgacure 500                      Ciba                      1.5                          observed.) An average of several
        Ucecoat XE430                     UCB                       0.7                          stains is reported per carrier.
                                                                                                    The results clearly indicate that
                                                                                                 solvent and stain resistance improve
                                                                                                 dramatically as the crosslink density
                                                                                                 increases. The alcohol-based stains
   A typical formulation for waterborne          Influence of Crosslink Density                  are very hard to pass; 96% ethanol
UV-PUD is shown in Table 3. Next to                 It is obvious that a UV-cured coating        softens the coatings considerably,
the binder itself, a matting agent, a            becomes harder and more rigid as the            making it a vehicle for the dyes. Only
wetting agent, a wax, a photoinitiator           number of acrylic double bonds                  the UV-PUD with the highest crosslink
and a rheological modifier are being             increases. The chemical and solvent             density can withstand these alcohol-
used. This formulation results in a              resistance should be expected to                based stains. Although the crosslink
Brookfield viscosity of 1,000 mPa.s and          increase accordingly. A range of UV-PUD         density increases drastically in this
a 60° gloss of 25%.                              with an increasing level of crosslink           range, surface hardness does not seem
   Application on resilient flooring is          density was synthesized and is presented        to increase accordingly. However,
preferably done by roller coating                in Table 4. For comparison, the crosslink       the flexibility is lost rapidly as the
in reverse mode. Flash-off for a                 density for a 100% UV system can be             UV network becomes denser. The
23 g/m2-wet coating is achieved in               8 milli-equivalents of acrylic double           performance of uncoated PVC flooring
30 seconds at 100°C. Curing is carried           bonds per g of solid and higher.                is included as a reference. Because of
out with two Hg-lamps at 80 W/cm, at                The stain results were obtained              the presence of plasticizers, PVC itself
a typical line speed of 10 m per minute.         after exposing the coating to a range of        acts as an excellent carrier for dyes.


 Table 4
     Performance of 10 µm UV-PUD coatings on PVC resilient flooring with increasing
     crosslink density
                                                 Uncoated PVC       UV-PUD # 1      UV-PUD # 2      UV-PUD # 3      UV-PUD # 4
      Meq. acrylic per g of solid coating                                 1.00           2.00            2.75            3.50
      König hardness (s)                               NA                 120            145             170             180
      Taber CS17/1 kg (mg) @ 1000 cyc.                48.6                28.8           26.1            22.5            19.5
      Cold flexibility (impact test) (°C)             <-20                <-20           -10              0               20
      MEK double rubs                                  10                 60                85           125             160
      Stain resistance (5 = best)
                        Oil based stains                1                  3                4             5               5
                   Alcohol based stains                 1                  1                1             1               3
                    Water based stains                  2                  3                4             5               5
                             Shoe polish                1                  2                3             4               5
                               Liquid tar               1                  2                3             4               5




                                                                                                 MAY/JUNE 2003 RADTECH REPORT 53
Technical Paper



                   Table 5
                      Performance of 1O µm UV-PUD coatings on PVC resilient flooring with increasing Mw
                      between crosslinks
                                                                       Uncoated PVC      UV-PUD # 5       UV-PUD # 6       UV-PUD # 7


                        Meq. acrylic per g of solid coating                                    2.56          2.49              2.43
                        Molecular weight between crosslinks                                   2,000         4,000             8,000
                        König hardness (s)                                  NA                 170           165               160
                        Taber CS17/1 kg (mg) @ 1,000 cyc.                  48.6                24.1          25.6              25.0
                        Cold flexibility (impact test) (°C)                <-20                 5             0                -10
                        MEK double rubs                                     10                 130           125               125
                        Stain resistance (5 = best)
                                               Oil based stains             1                   5              5                 5
                                          Alcohol based stains              1                   1              2                 1
                                           Water based stains               2                   5              5                 5
                                                    Shoe polish             1                   3              3                 4
                                                      Liquid tar            1                   4              4                 4




                  Influence of Molecular Weight                Economical Considerations                   suffer from its “emerging technology”
                  between Crosslinks                              In addition to the technical             status. The price for aliphatic
                      In Table 5, a range of UV-PUD with       advantages, waterborne UV-PUD               diisocyanates, a key raw material for
                  increasing molecular weight between          systems have a clear advantage over         UV-PUD, remains a serious threat to
                  crosslinks, is presented. The overall        many competing technologies in              the development of competitive
                  crosslink density was kept as a constant     relation to occupational health and         polyurethane systems. However, when
                  by increasing the functionality of the       environmental issues. However, their        taking into consideration the outstand-
                  hydroxylated acrylate with increasing        penetration into the resilient flooring     ing stain performance of UV-PUD, it
                  chain length between crosslinks.             market will only happen when the            can be argued that a higher coating
                      Increasing chain length does not         “price is right.” As process and design     weight of 100% UV is needed to catch
                  have a significant effect on most            benefits are hard to break down in          up with UV-PUD. As shown in Table 6,
                  properties. The crosslink density has        dollars and cents, the cost of the          this puts the waterborne technology in
                  been shown to have a greater influ-          lacquer itself was analyzed and what        a whole different perspective; the cost
                  ence. However, the cold flexibility does     the raw material cost per m2 of             difference is not at all what it appeared
                  change—the UV network becomes less           produced flooring could be. Table 6         to be at first sight.
                  rigid with increasing chain length           shows a number of cost simulations.
                  between crosslinks.                          At identical coating weight, UV-PUD will    Future Developments in
                                                                                                           UV-PUD
                                                                                                               Further improvements in the
                   Table 6                                                                                 balance of flexibility vs. chemical
                                                                                                           resistance are to be expected. The
                      Raw material coating cost per m2 of resilient flooring                               incorporation of mixed isocyanates and
                                                                                                           extenders looks promising at this
                                                             100% UV      UV-PUD        PUD
                                                                                                           point. A lot of effort is also being made
                        Cost per kg of solid (USD)            6.98          11.75       11.2
                                                                                                           in the development of dendrimer
                            2
                        g/m (dry coating weight)               12            12          12                technology—a more efficient use of
                                    2
                        Cost per m (USD cents)                     8         14          13                acrylic double bonds and secondary
                                                                                                           crosslinking mechanisms should also
                                                                                                           be beneficial.

                  54 RADTECH REPORT MAY/JUNE 2003
                                                                                                                                     Technical Paper
   The development of acrylated                 Water-based UV may never fully           reluctant to move into an oligomer
acrylic resins can bring the raw            replace 100% UV systems. Neverthe-           related technology as 100% UV.
material cost down to an acceptable         less, they could well be an interesting         As the resilient flooring industry
level. Acrylic resins as such do bring a    alternative in terms of improved gloss       suffers globally from overcapacity, those
cost reduction, but are extremely           control and process freedom with only        manufacturers with the most versatile
susceptible to stains. Any technology       one type of UV lamps and one coating         and flexible operation might well have
that can reduce or eliminate the level      roll. The tack-free before cure proper-      the best chance for survival. Clearly,
of aliphatic isocyanate needed in           ties of UV-PUD allow for the separation      UV-PUD can offer those options. ◗
waterborne UV-systems should be             of coating and curing. Since the
examined with great interest.               bottleneck obviously is the PVC              —Dirk Bontinck is Radcure research
                                            calandering or foaming, all UV curing        manager at Surface Specialties UCB,
   Another interesting topic is
                                                                                         Drogenbos, Belgium. Mike Idacavage
the development of self-curing              could be reduced to one dedicated line
                                                                                             is vice president of research and
UV-systems, omitting the rather             at higher speeds, avoiding the installa-        technology at Surface Specialties
expensive photoinitiator.                   tion of UV lamps on all production lines.                       UCB, Smyrna, Ga.
                                                UV-PUD should be regarded as
Conclusion                                  complimentary to 100% UV systems.
   It has been shown that UV-PUD            Depending on technological and market
brings performance, process and             demands, one or the other will prevail.
design benefits. The relationship               In Europe, many resilient flooring
between crosslink density, chain length     producers have switched or are now
between crosslinks, chemical resis-         switching to waterborne UV systems.
tance and cold flexibility was studied.     They are moving away from solvents
A raw material cost comparison was          such as N-methyl-pyrrolidone and
made between existing technologies          hazardous crosslinkers such as
for the resilient flooring application.     polyaziridines, present in PUD, and are




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                                                                                         MAY/JUNE 2003 RADTECH REPORT 55

				
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