Die Angewandte Makromolekulare Chemie 137 (1985) 57-64 (2199)

Rohm GmbH, D-6100 Oarmstadt, West Germany

COATING POLYCARBONATE WITH ACRYLICS WILL IMPROVE ITS WEATHERABILITY

JURGEN HENNIG

SUMMARY

A problem o f protecting polymers against weather influences is to concentrate the protective agents (UV-absorbers, antioxidants etc.) in the adequate place: the surface layer where photooxidative reactions primarily set in. The outstanding properties o f poly- carbonate (PC) are high levels of toughness and heat resistance whereas polymethylmethacrylate (PMMA) exhibits brilliant trans- parency and excellent weatherability. Mainly because o f its higher spectral energy absorption from global radiation in the critical range from 3 0 0 to 4 0 0 nm wavelength PC is inferior to PMMA in weatherability: yellowing and erosion within a 20 pm surface layer o f PC become visible after 5 years natural weathering. So the idea suggested itself to combine the favourable properties o f these two polymers by coating PC with acrylics. As far as weathering results are available today this idea turned out to be very useful: the improvement in weatherability by coating PC regarding the yellowness index, light transmittance and surface erosion is a factor 2 - 3 in time and even more.

Paper presented at the 17th Colloquium of Danubian Countries f o r

"Natural and artificial ageing of plastics" in Basle (Switzerland) June 4 - 6, 1985.

0 1985 HIithig& Wepf Verlag. Basel 0003-3146/85/$03,00

INTRODUCTION

An old problem of protecting polymers against natural weathering is to concentrate the agents, e . 9. UV-absorbers or antioxidants, in the adequate place: the surface layer where photooxidative reactions primarily will set in. Normally these protective agents are evenly distributed in the whole material, and the very first surface molecules of the polymer are exposed to sun, rain, oxygen etc. So there is a technical as well as an economic demand to improve the situation especially with those polymers which otherwise exhibit outstanding properties except weatherability being only fair. This is e . 9. the case with polycarbonate.

WEATHERING CHARACTERISTICS OF POLYCARBONATE AND POLYMETHYLMETH- ACRYLATE

The outstanding qualities of polycarbonate (PC) are extreme impact strength and high heat distortion resistance. The superiority of polymethylmethacrylate (PMMA) is brilliant transparency and excellent weatherability: there is no troublesome yellowing, no surface attack can be observed even after 10 years or more outdoors application. PC shows the first signs of visible yellowing after 5 years exposure to Central European weather, dull patches form on the weathered surface and after long-term weathering the onset of erosion can be detected under the microscope.

The differences in the weather resistance between PMMA and PC result from the very molecular structure of these two polymers and have mainly nothing to do with the manufacturing know-how. This can easily be understood if it is considered that obviously a polymer can only be attacked by the radiation it absorbs: this can be seen from its absorption spectrum. Obvious is as well that a polymer sample can only be attacked by a radiation which gets to its surface: in outdoor applications this is the natural radiation, s o we have to look for the spectrum of the so called global radiation (radiation of sun and sky).

Finally, if we want to evaluate the sensitivity of a polymer to natural radiation, we must look at the p r o d u c t of its absorption spectrum times the spectrum of the global radiation. This product, the so-called spectral energy absorption HA from global radiation, is plotted in relative units as function o f the wavelength in Fig. 1.

10'

f 10' n,

100

104

10'

1 0 3 200 300 400 500 600 nm 800

A-

Fig. 1. Relative spectral energy absorption Ha

from global radiation for P C and PMMA as function of wavelength h .

A s a result of the higher absorption, it is exactly in the range of the most effective radiation getting to the surface of earth between 300 and 450 nm, that P C is more sensitive than PMMA

( H A = 0 for < 300 nm, because natural radiation below 300 nm does not reach the earth's surface), and this is probably the major reason for the higher resistance of PMMA to photooxidative degradation. In this regard P C behaves similar to an UV absorber what can be deducted from the existence of the benzene rings within its chain molecules. A s a consequence e . 9. the essential yellowing, degradation and erosion of P C during weathering occurs within the uppermost surface layers of 10 to 20 pm, whereas the remaining part of the material keeps less affected for a certain period, because it is protected from the aggressive radiation by the surface layers.

What counts moreover for the higher weather-resistance of PMMA

is that its macromolecules are by far less susceptible to photo- oxidation processes than the molecular chains of P C .

In view of hese facts the idea suggested itself to combine two materia s PMMA and P C in order to add up their favourab properties: making a sheet with the high toughness and heat resistivity of P C which exhibits the outstanding weatherabi of P M M A !

the e

ity

59

COATING

If a material required to be transparent shall be protected from the critical part of natural radiation, this protection should be conveniently provided where it is most effective: at the radiation exposed surface! So this surface has to be covered with a protective layer, which must, however, meet some requirements. Obvious requirements are for example:

o The protective layer itself must consist of a transparent highly weather-resistant material that does not turn yellow, hazy o r

brittle, nor roughens up o r erodes at the surface when exposed to the weather for many years: acrylic polymers will meet this requirements.

o The protective layer must absorb the most critical radiation received by the earth's surface between abt. 300 and 380 nm in order to afford maximum protection to the substrate PC against yellowing and embrittlement.

o The protective layer must adhere well to the substrate for a long weathering period.

Of course, there are further requirements, but more arising from technological aspects rather than from the view on properties. It is obvious that both well-known coating procedures can be applied: laquer coating o r coextrusion. Which process is finally chosen depends not only on whether the aforementioned requirements can be met but also on technological as well as economic points of view.

WEATHERING RESULTS

The desire to improve in particular the weathering behaviour o f

PC by coating with acrylic resins is not brandnew. In the meantime we have obtained more than 3 years results from natural weathering test in Darmstadt (50° north, 9" east, Central European temperate climate, annual average amount of precipitate: 600 mm, of temperature: ll°C, 1890 sunshine hours per year). Most of the test specimens were taken from 4 mm thick extruded sheets.

These results shown in Fig. 2 to 5 are compared with those obtained for straight PMMA on the one hand and uncoated PC o n the other hand. As expected the weathering results of the coated PC lie between those of PMMA and uncoated PC o r are shifted to the behavior o f

PMMA.

60

Light Transmittance

In Fig. 2 t h e decrease o f integral light transmittance (TD6,, DIN 5036) i s plotted a s function o f t h e weathering period t for PMMA, coated and uncoated PC. The measurements were taken from superficially cleaned samples, i. e . from samples freed from adhering dirt particles.

0 I: A T

-1

-6

-8

1 I 3 1-8 5 -10

t-

Fig. 2. Decrease o f light transmittance A T (D o f PMMA, coated and uncoated P C a s function o f

outdoor weathering in Darmstadt.

N 5 0 3 6

The decrease o f light transmittance with t i m e i s diminished a s a result o f coating the surface o f t h e PC: t h e weathering period t o undergo t h e s a m e decrease A T i s extended by a factor 3 in t h e

range o f 5 years. This statement must not be extrapolated for more than 10 years natural weathering, because t h e functions A T = f (t) are not linear.

A more practical approach t o the improvement o f weatherability o f P C by coating the surface with acrylics i s shown in Fig. 3 for the example o f double skin sheets o f PMMA and PC. Double skin sheets are used a s heat insulating transparent glazing and roofing

materials and consist o f extruded profiles with a top and a bottom skin, &r t h e present case in a distance o f 1 6 mm R L E X I G L A S ) or 10 mm (MAKROLON) connected with r i b s spaced at 1 6 or 1 0 mm.

61

Fig. 3. Light transmittance ‘LT (DIN 5036) o f P L E X I G L A S (PMMA) and MAKROLON ( P C ) double skin sheet ( S D P ) a s function o f outdoor weathering in Dsrmstadt; longlife I coated.

The integral light transmittance ( T D 6 5 , DIN 5036) i s plotted against the weathering period t and extrapolated up to 10 years natural weathering in Dsrmstadt (Fig. 3). The benefits o f t h e coating are again clearly manifested: t h e decrease o f light transmittance after 10 years from 80 t o only 77 X with the coated (longlife) material compared to 70 X with the uncoated sheet i s a strong argument in favour o f t h e longlife version.

Yellowing

Figure 4 presents values for the changes o f t h e yellowness index A Y T ( A S T M D-1925) or the yellowing number (DIN 6167) a s function o f the weathering period t for PMMA, coated and uncoated P C .

The barrier effect o f the coating t o yellowing i s obvious, and what must be considered moreover i s that even the skilled e y e needs highly discriminating lighting conditions and t h e direct comparison with a standard in order t o discover differences in the yellowness index A Y I o f at least 2. We expect an increase AYJ o f the yellowness index after 10 years outdoor weathering o f less than 6 with the coated PC compared t o 1 2 and more with uncoated P C .

62

I b rn.

Fig. 4. Increase of Yellowness Index a\rI ( A S T M D-1925, DIN 6167) for PMMA, coated and uncoated PC as function of outdoor weathering in Darmstadt.

Surface Roughness

Since roughening and erosion of the surface o f PC during prolonged natural weathering plays such an important role for the dirt pick up, Fig. 5 shows the surface roughness R Z

(mean peak-to-valley height, DIN 4768) measured with a roughness meter (Perthometer) as function of the weathering period t.

IS wm I2

9

6

3

0 0 I 2 3 4 ymn 5

t -

Fig. 5 . Surface roughness R (mean peak-to-valley height, DIN 4768) of PMMA, coated and uncoated PC as function of outdoor weathering in Darmstadt.

63

As expected the surface of the coated PC behaves like that of PMMA. It stays as smooth and brilliant during weathering as we have experienced it with PMMA, whereas the roughness of the uncoated material increase remarkably and may thus promote the adhesion of dirt and dust which for their part would reduce light trans- mi ttance.

Impact Strength

The usual impact strength test (DIN 53453) with standard small bars 60 x 6 x 4 mm is unable to distinguish between coated and uncoated PC after a weathering period of 5 and more years: the specimens will not break. We are convinced, however, that the surface coating has a favourable effect with regard to shock and impact resistance during long term outdoor exposure, so that e . g hail resistance of the MAKROLON SDP longlife which these sheets show in the original state is retained even after many years in outdoor u s e . Dart tests with artificially weathered samples give reason for this statement.

8 = Registered Trademark

64