On the Road to Improved Scratch Resistance Engl

On the road toimproved scratch resistance

Desmodur Desmophen

On the road toimproved scratch resistance

Dr. U. Meier-Westhues, Dr. Markus Mechtel,

Thomas Klimmasch, Dr. Jrg Tillack

PUR clearcoats excel by their high resistance againstacid rain (etching) and other chemicals. The appropriateselection of polyol and polyisocyanate allows deliberatecontrol of hardness, glass transition temperature and re-sistance features. With increasing hardness e.g. the etchresistance rises and the scratch resistance reduces.Overcoming this difficulty of diverging scratch resist-ance and etch resistance will only be possible throughincreasing the cross-linking density while at the sametime keeping a low Tg. The use of tailor-made polyolscould lead to car clear coatings with increased scratchresistance in a relatively short time.

Polyurethane clearcoats were used for the first time inautomotive OEM (Original Equipment Manufacturing)around the mid nineteen eighties. Due to their extraor-dinarily high quality level they quickly spread and to-day, around 20% of all vehicles produced world-wideare coated with 1Component- and 2Component-PURclearcoats.

Today, 2C-PUR clearcoats lead to the highest qualitylevels, 1C-PUR systems on the basis of blocked poly-isocyanates do not quite reach this level, however,they are often a good compromise between relativelyeasy processability and quality demands.

2

80 140 CPolymer

2C-PUR

Polymer

130 150 C

HBL

1C-PUR

Polymer Polymer

With increasing hardness the etch resistance rises and the scratch resistanceby contrast falls PUR clearcoats excel by their high resistance againstacid rain (etching) and other chemicals. Here, they areclearly better than customary polyacrylate/melamine-formaldehyde resin systems (Thermo Setting Acrylics =TSA). This is attributable to the urethane bonding whichis extremely stable towards acids. The careful selectionof polyol and polyisocyanate in 2C-PUR formulations al-lows deliberate control of the hardness, the glass tran-sition temperature and the resistance characteristics.Test results have shown that with increasing hardnessthe etch resistance rises (not due to chemical changesbut rather as a consequence of decreasing soakability)however, the scratch resistance by contrast falls. Whenimproving the scratch resistance we must overcome thisreversal effect without negative influence on the etchresistance.

The etch resistance is tested by laying out coated pan-els in Jacksonville, Florida, for 14 weeks. The extent ofetching spots is visually evaluated. The followingscratch resistance results (Table 1 and Table 2 nextsides) originate from practical data from a car wash testseries after brush cleaning. The practical behavior of the clearcoats can be simulat-ed by certain laboratory test methods. The gradientoven method to define the etch resistance as well asthe Amtec Kistler laboratory car wash have become es-tablished mainly in Europe.

The gradient oven method determines the minimumtemperature necessary for the acid to cause coatingdamage after a defined exposure time. Depending onthe specification, sulfuric acid or special acid mixturesare used.

The Amtec Kistler laboratory car wash (Figure 2) workslike a real car wash with polyethylene brushes that ro-tate over the test panels. Quartz is normally used asscratching medium today. The method is described inthe DIN standard 55668.

The correlation between car wash and Amtec Kistler isunderstandable, yet in both cases relatively hard partssuch as sand or quartz affect the coating.

In America the Crockmeter method has become stan-dard to determine the scratch resistance. In thismethod the test panel is scratched with woolfelt follow-ing the ASTM D 6279-98. And non-silicate materialssuch as feldspar or calcite are used as abrasive media.The results of the Crockmeter method make obviousthat this method doesnt correlate with the practical re-sults from the car wash and consequently even not withthe Amtec Kistler method. In contrast to the practicaltest where the TSA clearcoat is scratched most heavily,this clearcoat shows the best results in the Crockmetermethod.

Desmodur Desmophen

Figure 1: Reaction mechanisms of 2C-PUR and 1C-PUR systems

Figure 2: Amtec Kistler laboratory car wash

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Table 1: Characterization of the clearcoat systems

Figure 3: Plastic deformation and fracture

Maximum reflow

Plastic deformation T > Tg

Fracture T > Tg

Minimum reflow

System [1] 2C-PUR flexible [2] 2C-PUR semi hard [3] 2C-PUR hard [4] TSA

Desmophen A 870 Desmophen A 870 Desmophen A 870 Polyacrylate/Desmophen VP LS 2971 Desmodur N 3390 Desmodur N 3390 Melamine resinDesmodur N 3390 Desmodur Z 4470

Pendulum hardness 87 130 170 160(Knig) [s]

Glass transitiontemperature Tg 30 55 80 30 70

max E1(DMA) [ C]

Etching testJacksonville/FL 4 3 2 10

(14 weeks summer) 0 = very good/10 = bad

Car Wash (10 cycles) 89 87 84 85Gloss retention [%]

4

Nano scratch method allowsdifferentiation between plasticdeformation and fractureTo clarify the apparently contradictory results the nanoscratch method is used. This method allows the differen-tiation between plastic deformation and irreversible frac-ture as possible scratching causes (Figure 3).

Scratches caused by plastic deformation are producedby relatively low loads. The coating deforms only plas-tically under the effect of the scratching medium.When heating above glass temperature the plastic de-formation largely recovers due to its viscoelasticity.This phenomenon of self-healing or recovering iswell known and is especially observed with coatingson PUR basis in the field.

Stronger forces eventually lead even to an irreversibledamage of the coating, the so called fracture. A self-heal-ing or recovering of this scratch type is possible to avery minimal extent only.

Table 2: Etch and scratch resistance

Desmodur Desmophen



Etching testJacksonville/FL 4 3 2 10

(14 weeks summer) 0 = very good/10 = bad

Gradient oven test 41 45 49 3630 min, 1% H2SO4 [C]

Car Wash (10 cycles) 89 87 84 85Gloss retention [%]

Amtec Kistler10 cycles 71 68 60 51

Gloss retention [%]

Crock meter 90 83 77 95Gloss retention [%]

5

The nano scratch method (Figure 4) produces a singlescratch in the coating using an indentor with increasingnormal load. The method is accompanied by atomicforce microscopy. The critical load is ascertained atwhich a fracture occurs in the film for the first time. Ad-ditionally the residual depth, so the plastic deforma-tion, is measured under a load which is clearly belowthe critical load (here 5 mN).

All of the PUR systems 1 to 3 show a higher plastic de-formation (residual depth) than the TSA system 4(Table 3).

This has an effect under Crockmeter scratching, whichdue to its working method using a relatively low nor-mal load and relatively soft abrasion medium essen-tially causes plastic deformation.

The recovery behavior of the coating after 1h 70Ctreatments can be used as proof for this. The selfheal-ing behavior of the PUR systems is so strongly devel-oped that all the films recover almost to the initial val-ue. A scratching is visually no longer detectable. Incontrast the TSA system 4 was partly irreversiblyscratched already under relatively careful conditions ofthe Crockmeter method, the systems recovers onlyslightly.

Figure 4: CSEM Nano scratch tester (measuring principle/evaluation criteria)

Cantilever tip (indentor) Coating

Substrate (panel)

? ?

Flat sample recipient (stage)movable in x/y direction

progressive load

Nor

mal

forc

e [m

N]

Scratch length [mm]

5 mN

cracks

Z

Y

X

6

Desmodur Desmophen

Topography at 5 mN Point of fracture

Progressive load

Here, it becomes obvious that systems which recover wellfrom Crockmeter scratching must show a low plastic de-formability. However, with these test parameters theCrockmeter method is not suitable to simulate the rela-tively hard conditions of the car wash.

The critical load values from the nano scratch method im-ply for the PUR systems 1 and 2 a clearly less inclination tofractures of these systems compared with the hard PURsystem 3 and the TSA system 4. Both the practical testand the Amtec Kistler method proved the result from thenano scratch examinations. The PUR systems 1 and 2 areclearly superior to the hard PUR system 3 and the TSA sys-tem 4.

Concept: To increase the cross-linkingdensity in the film and to keep the Tglargely constantIt appears that cross-linking density and glass transitiontemperature [Tg] oppose each other in influence onscratch resistance. High cross-linking density leads tohigher scratch resistance, whereas a high Tg has the op-posite effect. The approach to further increase the scratchresistance of PUR clearcoats starts from the idea to in-crease the crosslinking density in the film and, however,to keep the Tg largely constant (Table 4).

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Table 3: Comparison of scratching methods

Similar to system 2, the system 5 formulation is based onthe combination of Desmodur N 3390 und polyacrylate,however, due to increased concentrations of urethanebondings it shows a higher cross-linking density. Throughtargeted setting of the polyacrylate combination, the Tg ofthe coating could be increased only from 55C to 65C.However the crosslinking density could be increased from900 g/mol to 680 g/mol.

In comparison to system 2 system 5 shows a reducedresidual depth. After Crockmeter scratching the system isclearly less scratched than system 2. After recovery thesystem recovered nearly completely. Obviously, the Crock-meter method works so carefully that it doesnt simulatethe realistic scratching conditions e.g. in a car wash.

Conspicuous is the significantly increased value of thecritical load. So, the highly cross-linked system 5 is irre-versibly scratched at relatively high loads only. This be-comes clear in the examinations following Amtec Kistler.This system behaves clearly more favorably than system2 with a conventional cross-linking density. Etch resist-ance, however, remains on the same high level.



Nano scratch tester 28.7 19.4 15.0 11.9Critical load [mN]

Residual depth 0.59 0.64 0.54 0.38at 5 mN [m]

Crockmeter Direct 90 83 77 95Gloss retention [%]

1 h 70 C 100 99 97 97Gloss retention [%]

Amtec Kistler Direct 71 68 60 51Gloss retention [%]

1 h 70 C 76 72 61 52Gloss retention [%]

Car wash 10 cycles 89 87 84 85Gloss retention [%]

17 cycles 96 93 90 89Gloss retention [%]

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The proof of quality improvement must eventually be pro-vided in the field. Of all systems tested the coating on ba-sis of the highly cross-linked 2C-PUR version (system 5) isthe best. After 10 and 17 car wash cycles the relative glossretention still is 93% and 96%, respectively.

The principle of increasing the cross-linking density withonly a modest increase in glass transition temperature Tgseems to prove successful to further increase the qualityof PUR clearcoats.

Also the water reducible 2C-PUR system 6 shows interest-ing characteristics. The basis is a polyacrylate/poly-urethane dispersion which is crosslinked with the hy-drophobic HDI-trimer Desmodur N 3600. The systems Tgis about 57 C at a crosslinking density of 800 g/mol.

Table 4: Influence of the cross-linking density on the technological characteristics

Desmodur Desmophen

Even in long-term developments to improve scratch re-sistance the principle of increasing the crosslinking den-sity is used. Very promising are the so-called Dual Curesystems. They are 2C-PUR clearcoats which are partlyUV-cross-linked as well as thermally hardened. In partic-ular radiation hardening systems are known for their highcross-linking density. In clever combination with thermalhardening the correct balance regarding flexibility andhardness should be achieved. In addition the thermalhardening through polyol/polyisocyanate reaction whichtakes place at room temperature, provides a sufficientquality even in the shadow areas of the coating.

System [2] 2C-PUR semi hard [5] 2C-PUR semi hard [6] 2C-PUR waterborne

standard cross-linking density increased cross-linking density

Tg [ C] 55 65 57

Nano scratch tester 19.4 28.7 27.0Critical load [mN]

Residual depth 0.64 0.55 0.047at 5 mN [m]

Crockmeter Direct 83 90 89Gloss retention [%]

1 h 70 C 99 99 94Gloss retention [%]

Amtec Kistler Direct 68 81 76Gloss retention [%]

1 h 70 C 72 85 78Gloss retention [%]

Car wash 10 cycles 89 93 91Gloss retention [%]

17 cycles 93 96 94Gloss retention [%]

Gradient oven test 45 44 4530 min, 1% H2SO4 [C]

9

Around 20% of all vehicles produced world-

wide are coated with 1C and 2C polyurethane

based coatings due to their extraordinarily

high quality level.

PUR clearcoats excel by their high resist-

ance against acid rain (etching) and other

chemicals and are clearly superior to the

customary TSA clearcoats, here. The appro-

priate selection of polyol and polyisocyanate

allows deliberate control of hardness, glass

transition temperature and resistance fea-

tures. With increasing hardness e.g. the etch

resistance rises and the scratch resistance

reduces.

Overcoming this difficulty of diverging

scratch resistance and etch resistance will

only be possible through increasing the

cross-linking density while at the same time

keeping a low Tg. The use of tailor-made

polyols could lead to car clear coatings with

increased scratch resistance in a relatively

short time.

The water reducible 2C-PUR clearcoat is an

alternative of equal quality and less VOC to

the high solid 2C-PUR clearcoats.

Results at a glance

10

Another approach to improve the scratch

resistance while keeping the etch resistance

is the use of UV technology. Mainly Dual

Cure systems combine the advantage of

high cross-linking density from the UV tech-

nology with the flexibility and shadow hard-

ening of the 2C-PUR technology.

A comparison of the different scratching

methods shows that the nano scratch

method is the scientifically most substantial

one. When producing single scratches the

parameters such as residual depth and criti-

cal load are ascertained that characterize

the physical behavior of a coating. The

Crockmeter test is a very careful scratching

method which mainly leads to plastic defor-

mations. Hard scratching conditions such as

in a car wash are not simulated with this

method. Field tests and Amtec Kistler labo-

ratory car wash results correlate well. In

both cases there are hard scratching condi-

tions which lead both to plastic deformation

and fracture to an essential portion.

Desmodur Desmophen

11

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This information and our technical advice whether verbal,

in writing or by way of trials are given in good faith but

without warranty, and this also applies where proprietary

rights of third parties are involved. Our advice does not

release you from the obligation to verify the information

currently provided especially that contained in our safety

data and technical information sheets and to test our

products as to their suitability for the intended processes

and uses. The application, use and processing of our pro-

ducts and the products manufactured by you on the

basis of our technical advice are beyond our control and,

therefore, entirely your own responsibility. Our products are

sold in accordance with the current version of our General

Conditions of Sale and Delivery.

Edition: 12.04

Printed in Germany E

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On the Road to Improved Scratch Resistance Engl