Scratch resistance of optical polymers

Polyr~r Testing 2 (1981) 199-210

SCRATCH

RESISTANCE

OF

OPTICAL

POLYMERS

MICHEL COULON and WILLIAM LENNE

Research and DevelopmentLaboratory, EssilorInternational Corporation,2his, rue des 2 Communes, 94300 Vincennes, France

SUMMARY The scratch resistance of the lens materials is an important characteristic in the field of spectacle making. This paper describes a unique scratch-testing machine which allows the following parameters to be determined for a given optical material: 1. The 'scratch rupture resistance' which is equal to the critical load Cc (applied on a known penetrator) at which chips begin to be formed. 2. The 'scratchability curve' which expresses the global behaviour of the material below the critical Cc value, and which is obtained by recording the variations of the scratch width as a function of the load. The scratch resistance R then equals Cc where ct = slope of the asymptote of the curve, [3 = ordinate at the origin of the asymptote, C~ = critical load (in grams). 1.

INTRODUCTION

The optical surface quality and its conservation in time are fundamental characteristics in the spectacle-making branch of the optical industry. A spectacle lens is subjected, from the time of its manufacturing, and while it is being used by the wearer, to numerous abrasive conditions, such as those resulting from cleaning, wiping, projection of foreign matter, friction in a spectacle-case, and so forth. 199 Polymer Testing 0142-9418/81/0002-0199/$02.50 © Applied Science Publishers Ltd, England, 1981 Printed in Northern Ireland

200

MICHEL COULON, W/LLIAM LENNE

A great number of well-known tests allow the mechanical strength of a sample surface to be determined. 1.1. Abrasion tests 'Mar Resistance of Plastics' ASTM D 673.70.1 'Resistance of Transparent Plastics to Surface Abrasion' ASTM 1044.56. 2 'Resistance to Abrasion of Plastics Materials' ASTM D 1242.56. 3 These tests are used to determine either the light diffusion or the loss of weight due to abrasion. 1.2. Hardness and scratch-resistance tests Knoop hardness test. 4 Bierbaum scratching test (formerly ASTM D 1526). 5 Methods of testing afocal filters for ocular anti-daylight protectors NF S 77109. Plates of laminated plastic material with decorative surface patterns. Determination of the scratch resistance NF T 54-005. All these tests allow only one of the components of the scratching phenomenon to be determined and thus cannot represent per se the 'wearing test'. 1.3. Wearing test This test consists of providing spectacle wearers, in accordance with the prescriptions, with a lens to be tested, e.g. for the right eye (RE) and a reference lens (CR 39t, ORMA 1000~) for the left eye. These lenses are then examined comparatively, for each pair of spectacles, after one, three and six months wearing, using as visual valuation means a conventional aspect control 'ramp' (white light from two fluorescent lamps plus frosted screen). Our scratching test machine is adapted to simulate a wearing test by defining scratches on the surface of a sample, in accordance with various conditions of load, velocity and tool profile. The variation of any of these parameters allows scratches of various kinds to be obtained--such as scraping, visco-elastic scratching, and fragile scratching--which are similar to those produced on lenses during wearing. These scratches are analysed with a view to determining a scratching strength coefficient of the polymer in question.

f CR 39: Registered trade mark PPG Industries--Diethylenegiycol-bis-allylcarbonate. ORMA 1000: Registered trade mark Essilor International.

SCRATCH RESISTANCE OF OPTICAL POLYMERS

Fig. 1.

201

Scratch resistance testing machine (prototype).

2.

MATERIAL

The prototype scratch resistance testing machine (Fig. 1) allows the scratching p h e n o m e n a on meniscus-shaped lenses to be studied. The variable parameters are load, velocity, and tool geometry. The sample, mounted under a stationary penetrator, effects two rotary motions and is displaced at constant linear speed.

2.1. Obtainment of a constant linear speed (Fig. 2) A cam (2) arranged symmetrically with respect to sample (1) with reference to curvature centre 0 of the lens (convex face) is rotated by frictional engagement with a roller (3).

202

MICHEL COULON, WILLIAM LENNE

! Penetrator

[

Sample

t2

'~"Cam (2)

Fig. 2.

Schematic illustration of the principle of producing a constant linear speed.

Thus, the following condition prevails at any time: VA =21rRN = VB =27rR'N' = C,e With a view to varying the linear scratching speed, N' is varied. The linear speed is indicated in digital values on the front of the machine. 2.2. Sample movements A rotary motion about its axis (movement 1, Fig. 2). An oscillating rotary motion (movement 2, Fig. 2). The combination of these two movements allows the following two types of test to be performed:

1. Scratching rupture test (Fig. 3a): A spiral extending from the periphery to the centre of the sample is traced. Simultaneously, the load varies from 0 to critical load Co, the latter value corresponds to the beginning of chip formation. 2. Determination of the scratchability curve (Fig. 3b): Concentric scratches are produced under constant load, each scratch extending over a complete

SCRATCH RESISTANCE OF OPTICAL POLYMERS

/ Fig. 3a.

Fig. 3b.

/

203

/

Spiral under increasing load until chips begin to appear.

Concentric scratches under constant load, for tracing L = f(c).

circle or a semi-circle. This second type of testing is used for determining the deformation curves: width ( t ~ m ) = f (load in grams). 2.3. Penetrator-load system The load is equal to a mass of water delivered by a metering pump (syringe type) which may operate either in a continuous manner (the mass of water increases regularly during the entire testing time period), or in a discontinuous (intermittent) m a n n e r (the mass of water is constant during one rotation (or a half rotation), but varies from one circle to the next one). In the latter case the pump delivers, from any given scratch to the following one, a selected load (e.g. 2, 4, 6 or 8 g). The penetrator is a stereophonic reading or tracking head diamond (radius, 15 tzm; angle, 60 °) modified so as to be adapted to support a maximum load of 60 g.

204

MICHEL COULON, WILLIAMLENNE

2.4. Characteristic data of the machine Speed range 50-1000 cm/min Load range 0-60 g Sample geometry, diameter: 5 0 - 5 5 - 6 0 - 6 5 - 7 0 ram; base: + 4 . 0 0 - + 6" 0 0 - + 8.00 diopters 3.

DESCRIPTIONOF THE SCRATCHES

Two different types of scratches may be distinguished, which appear successively as the load increases, for a given tool and a given speed. 1. A visco-elastic scratch resulting from an initially elastic and then plastic deformation of the material. This scratch has the form of a groove bordered by two material shoulders and constitutes the result of a mere displacement of material (Fig. 4). This scratch is more or less completely erased after the penetrator has passed (recovery phenomenon). The remaining trace may also be partially erased under the affect of temperature. 2. A 'brittle scratch' resulting from an irreversible deformation of the material. This scratch has the aspect of a groove with irregular contours and results from tearing off material (chips) (Fig. 5). Thus, the following methods of studying the scratch may be envisaged: 1. Only the beginning of chip formation is taken into account, and a 'scratch rupture load' is thus determined. Thus, a punctual evaluation of the scratch resistance for a critical load, Co, is made, without taking into account the perturbations of the optical surface which occur at loads of less than Co. This is the case of the first test series, reported in Section 4.1.

Fig. 4. Viscoelasticscratch on CR 39 (load, 14.5 g; speed, 3 cm/s). Magnifiedx 150.

205

SCRATCH RESISTANCE OF OPTICAL POLYMERS

Fig. 5. Brittle scratch on CR 39 (load, 37 g; speed, 3 cm/s) Magnified × 150. 2. T h e b e h a v i o u r of t h e m a t e r i a l is s t u d i e d o v e r the e n t i r e r a n g e of l o a d v a l u e s f r o m 0 to Co. U n d e r t h e s e c o n d i t i o n s , it is p o s s i b l e to t a k e into a c c o u n t p a r a m e t e r s such as t h e ' i n s t a n t a n e o u s d e f o r m a t i o n ' , t h e ' d e f o r m a t i o n p e r l o a d u n i t ' , a n d t h e ' m e a n p l a s t i c i t y p r e s s u r e ' . A l l t h e s e p a r a m e t e r s can b e g r o u p e d u n d e r t h e v a l u e s Cc a n d a,/3, 3'. This is t h e case of the s e c o n d test series, r e p o r t e d in S e c t i o n 4.2.

4.

EXPERIMENTATION

4.1. Determination of the 'scratch rupture load' (first test series) T h e v a l u e s o b t a i n e d for ' s c r a t c h r u p t u r e l o a d s ' of v a r i o u s m a t e r i a l s a r e listed in T a b l e 1. This classification e x h i b i t s c e r t a i n a n o m a l i e s , e s p e c i a l l y as r e g a r d s t h e

TABLE 1 SCRATCH RUPTURE CRITICAL LOAD, Cc, IN GRAMS, FOR COATED AND UNCOATED MATERIALS. S = 3 c m / s . THE SURFACES WERE CLEANED WITH ETHYL ALCOHOL PRIOR TO TESTING

Material Diethyleneglycol-bis-allylcarbonate CR 39 (ORMA 1000) Injection-moulded polycarbonate PC Injection-moulded polymethylmethacrylate PMMA Multi-layer anti-reflection coating TSV on CR 39 Anti-scratch coating on PC Single layer anti-reflection coating+ anti-scratch coating on CR 39 Type A anti-scratch coating on CR 39 Type B anti-scratch coating on CR 39 Single layer anti-reflection coating TSV on CR 39

Cc

Cc

(g)

Cc(CR39) (ORMA 1000)

33 53 16

1000 1610 480

29-5 30

890 910

30.5 53 41

920 1610 1240

30

910

× 10 3

206

MICHEL COULON, WILLIAMLENNE

polycarbonate which has a very high coefficient (1610), although its low resistance to abrasion is well known. Consequently, a second test series was carried out. 4.2. Determination of the scratchability curve (second test series) Concentric scratches were produced. Each scratch was produced at constant load between 0 and Co. After measuring the width of the scratches by phase-contrast microscopy, the curve scratch width (in/~m) = f load (in grams) is established. Several representative curves are shown in Fig. 6. T h e general tracing of these curves is indicated in Fig. 7. The shape of these curves corresponds to L = aC x g(1 - e-W). A material exhibits a higher scratch resistance the lower the values of ct, /3 and V and the higher the value of Co. T h e following equation m a y be considered as defining the scratch resistance R

=

-

Cc -

a~

a = the slope of the asymptote of the curve /3 = the ordinate at the origin of the a s y m p t o t e of the curve Cc = the critical load. The" values obtained are listed in Table 2.

L H.m) Q

9C 80 70 6C 50 40,

b

e

30 20 10 I

I

I

I

I

10

20

30

40

50

IC 60

Fig. 6. Scratch width (~m) as a function of load (g). (a) Injection-moulded PC, (b) anti-scratch coating on PC, (c) injection-moulded PMMA, (d) CR 39 (ORMA 1000), (¢) type A anti-scratch coating on CR 39.

207

SCRATCH RESISTANCE OF OPTICAL POLYMERS /_

!

tl C C~

Fig. 7. Tracing of curve 'scratch width' (p,m) = f (load in grams).

TABLE 2 CHARACTERISTIC PARAMETERS OF T H E CURVE REPRESENTING T H E SCRATCH WIDTH (IN b t m ) AS A FUNCTION OF LOAD (IN GRAMS), FOR COATED AND UNCOATED MATERIALS.

S = 3 cm/s. THESURFACESWERECLEANEDWITHETHYLALCOHOL,PRIORTOTESTING Material

f3

c~

3, / Q.CR 39

Diethyleneglycol-bis-allyl carbonate CR39 (ORMA 1000) Injection-moulded polycarbonate PC Injection-moulded polymethylmethacrylate PMMA Multi-layer anti-reflection coating TSV on CR39 Anti-scratch coating on PC Single layer anti-reflection coating TSV + anti-scratch coating on CR39 Type A anti-scratch coating onCR39 Type B anti-scratch coating on CR39 Single layer anti-reflection coating TSV on CR39

14.2 42.3

0.5 0.97

0,44 0,09

33 53

1000 280

6"8

1 . 0 8 0,64

16

460

11 22.1

0.7 0.26 0-46 0.20

29.5 30

810 630

16.4

0.55

30.5

730

16'2

0"39 0-51 53

0.17

1790

13.9 0.46

0.30

41

1380

14,8

0.24

30

760

0.57

4.3. W e a r i n g test R e l a t i v e a s s e s s m e n t e f f e c t e d a f te r t h e w e a r i n g test a l l o w e d the m a t e r i a l s to be classified as 1st, 2nd, 3rd, etc. T h e f o l l o w i n g scale is o b t a i n e d o n t h e basis 1000 f o r C R 39 ( O R M A 1000), v a r i o u s r e l a t i v e v a l u e s b e i n g a t t r i b u t e d to t h e i n t e r v a l s ( l s t - 2 n d , 2 n d - 3 r d ) . F o r this p u r p o s e , t h e n u m b e r of scratches an d t h e i r i n t e n s i t y d u e , f o r e x a m p l e , to light r e f r a c t i o n o r diffusion by t h e scratches, w e r e t a k e n into a c c o u n t ( T a b l e 3).

208

MICHEL COULON, WILLIAM LENNE TABLE 3 CLASSIFICATION OF WEARING TEST RESULTS OBTAINED WITH COATED AND

UNCOATEDMA~RIALS,w r m REFE~NCE TO CR 39 ( O ~ A 1000) Classifu:ation score

Material

Diethyleneglycol-bis-allyl carbonate CR 39 (ORMA 1000) Injection-moulded polycarbonate PC Injection-moulded polymethylmethacrylate PMMA Multi-layer anti-reflection coating TSV on CR 39 Anti-scratch coating on PC Single layer anti-scratch coating on CR 39 + antireflection coating TSV Type A anti-scratch coating on CR 39 Type B anti-scratch coating on CR 39 Single-layer anti-reflection coating TSV on CR 39

5.

1000 300 500 800-1100 600- 800 800-1100 1400-1800 1400-1800 800-1100

DISCUSSION

When comparing the classifications resulting from the two scratch test series to the classifications resulting from the wearing test (Fig. 8), it is seen that the classification on the basis of R = (Cc)/(a/3) shows a satisfactory correlation with the wearing test. Thus, it appears that an assessment of the scratch resistance reflecting the behaviour of the material over an extensive load range (from 0 to Co) is representative of the wearing test. The proposed formula R = (C¢)/(a/3) appears to reflect this behaviour satisfactorily. The study of the scratching behaviour of optical polymers was completed by examining the following points: 1.

2. 3.

Verifying that the scratches produced on the lens' surfaces have a size (dimensions) and are of a nature (visco-elastic; brittle) similar to the size and nature of scratches present on lenses worn by a user Influence of tool geometry Influence of scratching speed

6.

CONCLUSION

A scratch testing machine has been developed, which allows the following tests to be carried out:

209

SCRATCH RESISTANCE OF OPTICAL POLYMERS

18oo

-D

1700 1600

D,E

PC, D

1500

1500

1500

14oo

-E

1500 1200 1100 1000 900

i CR 59

1000

CR)

%9)

1000

)A,C,F ) ) )

B,C A,F

8o0

CR 59

~A

;-F mC

7oo B

600 500

PMMA

500

PMMA

500 PMMA

4o0 500

PC

~-pc

200 lOO

Comparison of classifications: Co, wearing test, Cc/ct/3. A: Multi-layer anti-reflection coating TSV on C R 39. B: Anti-scratch coating on PC. C: Anti-scratch coating on C R 3 9 + s i n g l e layer anti-reflection coating on TSV. D: Type A anti-scratch coating on C R 39. E: Type B anti-scratch coating on C R 39. F: Single layer anti-reflection coating on C R 39.

Fig. 8.

1. 2.

determination of the 'scratch rupture critical load', Co, of a material establishment of the scratchability curve of a material for loads varying from 0 to Cc

This latter test allows a material to be subjected to loads varying from 0 to Cc critical load, and the proposed scratch resistance coefficient (for which characteristic a physical interpretation is sought) defines a scratch resistance satisfactorily correlated to the wearing test. REFERENCES t.

A S T M (1973). Standard Method of Test for Mar Resistance of Plastics, ASTM, pp. 2 2 5 - 8 ( A S T M D 673.70).

210 2. 3. 4. 5.

MICHEL COULON, W I I J J ~ / ~ LENNE

ASTM (1973). Standard Method of Test for Mar Resistance of Transparent Plastics to Surface Abrasion, ASTM, pp. 407-8 (ASTMD 1044.56). ASTM (1973). Standard Method of Test for Resistance to Abrasion of Plastics Materials, ASTM, pp. 439--46 (ASTM D 1242.56). I ~ o o p F., PETERS G. and EMERSON W. B. (1939). J. Res. Bur. Stand., 23, 39--61. BmanAUM C. H. (1930). Trans. Am. Soc. Steel Treating, 18, 1009.