Adhesives. We use adhesives to hold things together. For optical applications, we can define several classes of adhesives:

Transcription

1 We use adhesives o hold hings ogeher. dhesives For opical applicaions, we can define several classes of adhesives: Opical adhesives Transparen. Opical qualiies are imporan. Srucural adhesives Srengh is mos imporan Elasomers Use rubbery properies for sealing and o provide compliance Cyanoacrylaes Superglue Quick saking, Beware: ougassing can ruin coaings Issues: Choice of maerials, bond hickness, bond area Srengh of he bond Sabiliy Siffness Thermal sresses o Can hermal effecs cause he bond o fail? o Can hermal effecs cause disorion in he opics Ease of assembly o Surface preparaion o primers Ease of disassembly o Solvens o Mechanical cuing J. H. Burge Universiy of rizona 1

2 Elasomeric bonds Rubber and RTV (ν ~= 0.5) behave srangely as an elasic maerial. G (Shear modulus) ~ 100 psi (~1 MPa) E 0 (Young's modulus) ~ 3 G = 300 psi (~3 MPa) E B (Bulk modulus) ~ 100,000 psi (~1000 MPa) Shear siffness, where hickness < widh Shear siffness s is s δ y shear G Very complian xial siffness, for ensile elongaion where axial lengh >> widh δ z E L axial 0 xial siffness 1 1 L Very complian In compression, his buckles and behaves non-linearly J. H. Burge Universiy of rizona 2

3 For hin rubber, hickness << widh, ransverse srain is consrained, use he bulk modulus 2 δ z axial EB Very siff! Wha abou compression for a more general case? The dominan axial compliance for his case is due o shape change, no maerial srain. xial siffness depends on shape xial siffness z is z δ z axial EC Where E C is he compression modulus, which depends on geomery C 2 ( φ ) E = E + S 0 1 E 0 = Young s modulus S = Shape facor φ = maerial compressibiliy coefficien (=0.64 for RTV) J. H. Burge Universiy of rizona 3

4 The shape facor S is defined as S Load area l w = Bulge area perimeer hickness (2w + 2 l) For small S, E C = E 0 as above For large S, E c would blow up according o he relaionship above. Take he maximum value of E c o be he bulk modulus E b For 2 mm hick RTV pad 20 x 20 mm E 0 = 3MPa G = 1 MPa φ =0.64 S = (20x20)/4x(2x20) = 2.5 E C = 3 MPa ( *2.5 2 ) = 15 MPa Z = E c / = 3000 N/mm S = G/ = 200 N/mm z / s = 15 : 15 imes siffer axially han in shear. Reference: P. M. Sheridan, F. O. James, and T. S. Miller, Design of componens, in Engineering wih Rubber (. N. Gen, ed.), pp. 209{Munich:Hanser, 1992) J. H. Burge Universiy of rizona 4

5 Hardness Defined as resisance o indenaion, according o specific ess: (McMas Carr) Viscosiy fluid s resisance o flow. The hickness of he fluid. 10 poise = 1 Pa s MTERIL PPROXIMTE VISCOSITY (in cenipoise) 70 F 1 Blood or erosene 10 ni-freeze or Ehylene Glycol 15 Moor Oil SE10 or Corn Oil 50 o 100 Moor Oil SE30 or Maple Syrup 150 o 200 Moor Oil SE40 or Casor Oil 250 o 500 Moor Oil SE60 or Glycerin 1,000 o 2,000 Corn Syrup or Honey 2,000 o 3,000 Molasses 5,000 o 10,000 Chocolae Syrup 10,000 o 25,000 Heinz echup or French's Musard 50,000 o 70,000 Tomao Pase or Peanu Buer 150,000 o 250,000 Crisco Shorening or Lard 1,000,000 o 2,000,000 Caulking Compound 5,000,000 o 10,000,000 Window Puy 100,000,000 J. H. Burge Universiy of rizona 5

6 Thermal sress Usually, hermal mismach is accommodaed by he adhesive, because i has mos compliance. lso, he hermal expansion of he adhesive is usually less imporan han ha of he subsraes For bond wih dimension a, hickness - a - T α 1 G, α 2 γ dhesive goes ino pure shear. Maximum shear srain, γ, a disance a/2 from cener: a a α1 T α2 T 2 2 a γ ( α1 α2) T 2 Shear sress τ a disance a/2 from cener: Ga τ = Gγ = ( α1 α2) T 2 This approximaion will work mos of he ime. I is conservaive. cual sress is reduces by addiional compliance of subsraes. For cases where he subsraes are hick compared o he glue: Subsrae compliance a G 0.1 Epoxy compliance E epoxy subsrae For epoxy, G ~= 150 ksi. Bonding meal o glass. pproximaion is only valid for size/hickness: a/ < 100 more complee relaionship is given in Yoder p.802 and Vukabraovich p 123. (Noe L is he bond radius, no he overall size. (Chen and Nelson 1979). J. H. Burge Universiy of rizona 6

7 Example Bond Glass o luminum For 3M 2216, G = 342 MPa a 24 C, 1500 MPa a 0 C CTE = 100 ppm/ C Shear srengh of 2216 = 2 ksi = 14 MPa Nominal bond is 0.1 mm hick. For 5 mm bond, 14 MPa is 274 N (62 lbs) for T = 20 C, a/2 is (20 C)(5 mm)/2*(23 7)ppm/ C = 0.8 µm γ = a/2 / )=0.8/100 γ = Using G@24 C of 342 MPa τ = 2.8 MPa ( safey facor = 14/2.8 = 5 If cooled 20 C, use G = 1500 MPa τ = 12 MPa This is close o he srengh of he epoxy Taking ino accoun deflecion of 6 mm hick aluminum, 25 mm hick glass. Use relaion on p 125 of Vukabraovich (L is he bond RDIUS) he sress is reduced from 12 MP o 11 MPa. To survive his emperaure: 1. Use an epoxy wih lower modulus a emperaure 2. Use hicker adhesive layer 3. Prepare he glass surface (fine ground and acid eched) o provide > 2000 psi (14 MPa) srengh. his poin he epoxy is he weakes link. 4. Use smaller bond area 5. Use mechanical design wih flexures o allow hermal expansion. J. H. Burge Universiy of rizona 7