VARTM / RTM MATRIX RESINS

Thermal Cure

 

 

VARTM VS RTM

Matrix viscosity and working time are the two major differences between VARTM and RTM. For RTM, the matrix must travel through the "X" and "Y" direction of a compact reinforcement, which requires using less then 200 cps for several hours. VARTM travels on top with transport media and only needs to impregnate the "T" or "Z" direction. This requires shorter time and can take advantage of the lower temp and faster vitrification/cure time combined with reduced thermal stress.

Because RTM requires lower viscosity, higher injection temperature (typically 160F - 250F range) is required for high-end matrix resins. The slow reaction rate at injection temperature translates into long latency at ambient and below, creating a single component system. For smaller parts, faster, two component RTM resins should be used and would overlap with the slowest of VARTM matrix resins.

 

CONVERTERS & MODIFIERS

Epoxy based RTM and VARTM matrix technology is dominated by the converters and modifiers. All vendors use similar epoxy resins. Converters can be divided into three categories and the amine class further divided. Processing, cost, kinetics, viscosity and end composite requirements dictate the converter class.

The three chemical classes are:

Amines, Anhydrides, and Cyanates

ANHYDRIDES
Advantages:

Pot Life

Cost

Tox

Viscosity

Wetting

Long process window

Industrial pultrusion history

Disadvantages:

Long Cure Cycle

Long to vitrify

React with water

Hard to toughen

Multiple slow cure mechanisms,

Time and process dependent.

No cure end point.

No durability or fracture toughness history

Never used for high-end composites.

AMINES
Advantages:

A. Cycloaliphatic

Mechanical/toughness balance

Low temp cure /Post cure

Easy to toughen

B. Aromatic

Mechanicals

Cure cycle

Easy to toughen

30+ years pre-preg history

Disadvantages:

A. Cycloaliphatic

High toxicity

Can carbonate with H2O & CO2

Moderate Stiffness

B. Aromatic

High Toxicity

Could crystallize

Viscosity

CYANATES
Advantages:

Wet mechanical retention

Large process window

Dielectric properties

Disadvantages:

Cost

Could crystallize

Cure rate is catalyst sensitive 

Sensitive to trace metal ions, H2O, Lewis Acid & Lewis BASE catalysts.

Viscosity

Fracture toughness- hard to toughen

Single raw material source

 

AMINES

The three classes: aromatic, cycloaliphatic, and aliphatic, result in different cure rates and end properties but cure mechanism is the same. One amine hydrogen is available for one epoxy, which is known as stoichiometric i.e.: 1:1. This mechanism is very process forgiving as there are essentially no side reactions. (without catalysts)

 

ANHYDRYDES

The anhydride functionality does not react with an epoxy ring and must first be broken with another functional group, usually a hydroxyl or water. In contrast to amines, anhydride reactions are multiple. Water reacts directly with anhydrides to form a di acid, which lowers Tg, and slows time and temp to cure. After anhydride ring is opened, all reactions are very slow and there is no equilibrium end point, i.e.: different time and temps will give different properties, (the significance of which is T.B.D.)

 

CYANATES

Cyanates are well known for their database in space and low dielectrics for radomes, but not as well known as a converter for epoxies. Cyanates will react directly with the epoxy ring to form a 5 membered oxazolidone ring along with some cyanate/cyanate to form a triazine. This epoxy/cyanate reaction to an oxazolidone is the only epoxy cure mechanism where a heterocyclic ring is formed and a residual hydroxyl is not formed which translates into greater wet strength retention and higher wet and dry mods, especially at elevated temperatures. This mechanism is most probable for achieving high-end hot wet pre-preg type properties with Vartm processing, but toughness is an issue.

 

 

CATALYTIC Epoxy/Epoxy

Small amounts of catalyst, either basic like a tertiary amine, Dicy, or imidazole family, or acidic such as a or a BF3 or BCL3 complex can be added to any epoxy to initiate or catalyze the epoxy functionality to react with itself i.e.: homopolymerize. Excellent combination of latency and cure rate can be achieved, but high temp wet and dry mods and fracture toughness are a problem

 

 

MODIFIERS AND MORPHOLOGY

Modifiers are added for viscosity reduction and greater toughness, with as little reduction in mechanicals as possible. The low viscosity requirements eliminate most modifiers used in pre-pregs, which include thermoplastic oligomers and pre-precipitated particles.

With modifier addition, the resulting morphology is critical for mechanical/toughness performance. Two phase, bi modal particle size, creates a synergistic toughening mechanism that results in enhanced fracture toughness for best toughness balance. Two converter types: anhydride and cyanates, are intrinsically difficult to develop desired bi-modal morphology.

Applied Poleramic has developed desired two-phase modifiers for all of the converter classes. With 3D stitching the second phase may not be advantageous, as modulus suffers linearly with volume of second phase.

The material engineer has choice of Matrix property balances.

 

 

TWO COMPONENT

 

User-friendly VARTM resins should be impregnated at 140F or below, with ambient the goal. The higher the required balance of mechanical and toughness properties, the less likely ambient impregnation can be achieved. 140F and below requires a two component system.

API supplies all converter choices, single and two-component, together with a complete balance of mechanical and toughness properties. In addition to the product line, custom matrix resins are developed to meet required specs such as BMS 8-256, 276, 8-79 and 8-168.

 

CONVERTER TYPE

Cycloaliphatic Amine

SC-1:

Clear, single phase, low cost matrix resin.

Part B: two types available with different speeds.

Post cure optional.

SC-10:

Higher Tg and performance version of SC-1.

Part B: two types available with different speeds.

SC-779:

Toughest VARTM in the industry.

Tg = 200F

SC-13:

Used successfully in marine applications.

SC-15:

Two-phase epoxy cycloaliphatic amine.

Most widely data based Vartm/Scrimp matrix resin which includes United Defense, Army, University of Delaware, and several Phase II SBIRs for ballistic panels.

SC-79:

Meets requirements of BMS 8-79 (also 8-219 and 8-168).

Pre preg is Hexcel F-155 (developed by API personnel).

Tg 280 F, 250 F post cure; 320 F with 350 F post cure.

 

 

CONVERTER TYPE

Aromatic Amine

SC-31:

Lowest cost for this high performance category.

SC-31T:

Two phase toughened version of SC-31.

SC-32:

Developed for general aviation

RTM applied.

SC-35:

Highest performance of this family.

Tg 200 C with 2.4% water pick up.

Long infusion time at 110 F

Anhydride

SC-22: Epoxy Anhydride

High end of anhydride converted epoxies.

Tg 204 C. Excellent wet Tg and modulus retention.

Viscosity at 75 F is 650 cps and at 85 F 300cps. Very long working time.

2.6% wt. gain in water boil.

SC-23:

Two phase toughened version of SC-22.

Cyanate

SC-36:

Two-phase toughened.

Wet Tg 179 C.

Five hundred pounds VARTMed by Boeing

SC-37: Cyanate/Acrylate

Tg greater than 180 C. Very attractive for radomes where maximum electricals required.

Dielectric Constant 2.7, Loss Tangent .01, with excellent mechanicals.

User-friendly alternate to pure cyanate. Its less than one half the price of pure cyanate with only very small electrical and mechanical performance reductions.

Five hundred pounds VARTMed, Boeing.

SC-38

Great wet strength retention.

High Tg and modulus.

High performance.

1.7% wt. gain in water boil.

SC-38T

Toughened version of SC-38. Increased damage tolerance with slight modulus sacrifice.

 

 

 

SINGLE COMPONENT

 

All require elevated cure impregnation and frozen storage.

Converter choices: Anhydrides, slow Aromatic Amines, Cyanates and Catalytic. The anhydride and cyanates are catalytically controlled and catalyst choice dictates cure rate and latency.

 

CONVERTER TYPE

Aromatic Amine

VR-6:

Excellent balance of mechanicals and application.

Viscosity: 100 cps at 160 F and less then 200 cps after 6 hours.

VR-6T:

Two-phase toughened version of VR-6.

Same user friendly application time and viscosities.

Approx. x2 the fracture toughness.

Anhydride

SC-24

General purpose single-phase

Tg greater then 200 C

Cyanate

VR-10:

Single phase very stable

Excellent wet mechanicals Tg Dry: 380 F / Tg Wet: 320 F.

VR-10T:

Two-phase toughened version of VR-10.

Tg Dry: 350 F / Tg Wet: 270 F.

 

Note:

Prepreg type matrix resins have been VARTMed, if working time is adequate at injection temp, in order to preserve database. Examples are PR500 & 520 as well as 5250-4 BMI. Injection from 180-250 F is required.

 

 

SPECIALTY PRODUCTS

SC-11 and SC11M:

Developed for United Defense (C.A.V.) exclusively for ballistic application.

First product of any material form that would pass ballistic shock.

Very compliant with excellent adhesion.

Toughened Tackifier

PT-1: Solution

Controlled Elastomer particles in thermoplastic epoxy.

50% improvement in anhydrideVARTM C.A.I.

Supplied 30% solids in acetone spray applied.

PT-1: Film

Controlled tack for preform application.

PT-1: Powder

Powder spray applied.

80 C fuse.

PT-3: Solution

Fire retardant version of PT-2.

PT-3: Film or Powder

SCRIMP EPOXY MATRIX RESINS

 

 

SC-1

SC-10

SC-779

SC-13

SC-15

TYPE

Clear

Single Phase

Clear

Single Phase

Toughened

Two Phase

Toughened

Two Phase

Toughened

Two Phase

# of Components

2

2

2

2

2

Mix ratio of A/B (wt)

Faster Conv.

100:22

Slower Conv.

100:22

Faster Conv.

100:22

Slower Conv.

100:22

100:30

100:20

100:20

Viscocity @ ambient cps

295

380

280

300

300

250

350

F Application temp

77

77

77

77

77/300

77

77

*Hours to reach 700 cps @ application temp

1.4

2.1

1.4

1.9

1.4

1.0

3.25

Weight/Gallon

9.25

9.20

9.4

9.35

9.15

9.16

9.15

**Cure Cycle F

12 + @ 77 F

12 + @ 77 F

12 + @ 77 F

12 + @ 77 F

12 + @ 77 F

12 + @ 77 F

1 + @ 140 F

Post Cure Hrs / F

4 @ 160

4 @ 160

4 @ 160

4 @ 160

2 @ 180

4 @ 160

2 @ 200

Tg Dry F

180

176

210

205

195

185

228

Tg Wet F

151

146

168

164

158

 

178

Kic Psi square inch

       

3100

 

1400

Flex Strength/Mod

Ambient psi/ksi

14.9 / 410

15.3 / 400

18.0 / 430

17.8 / 430

16.2 / 390

16.8 / 390

19.1 / 390

Cost Range/ $ per lb

4

4

4.5

4.5

5

5

7

% water P.U.

3.0

3.2

2.6

2.8

2.8

2.9

1.3

*100 Grams at application temp ** Suggested time/temp for demolding with unrestrained post cure cycle.

Cure cycle in mold can be any cure/post cure combination.

Kic = inch lbs 1/2 in2 Wet=48hr H20 Boil Tg=RDS G' Break

COMPOSITE PROPERITIES IN TEST

(will include when available)

 

 

SCRIMP EPOXY MATRIX RESINS

 

SC-12

SC-14

SC-15

SC-22

SC-35

SC-79

TYPE

2 Phase

Acrylate Modified

Two Phase

Two Phase

Epoxy/

Anhydride

Slight

Two Phase

Two Phase

Matrix

# of Components

3

2

2

2

2

2

Mix ratio of A/B/C(wt)

100/80/20

100/30

100/30

1:1

100/20

100/40

h @ ambient cps

450

400

350

650

13,000

320

Application temp/h cps

Ambient/ 450

Ambient/400

Ambient / 350

85 F 300 cps

60 C / 350

Ambient 320

*Hours to reach 1000 cps @ application temp

6.5

4

3.25

>6

4.5

7

Weight/Gallon

9.1

9.1

9.15

 

9.4

9.3

* *Cure Cycle unrestrained

1 hr @ 60 C

3 @ 60 C

2 @ 60 C

6 hr 180 F

12 + @ 65 C

 

Post Cure C

2 hr @ 90 C

5 hr @ 100 C

5 Hrs @ 100 C

2 @ 350 F

2 @ 177 C

1 @ 350 C

2 @ 250 C

Tg Dry C

125

103

114

210

166

165

132

Tg Wet

93

75

82

165

118

127

115

Kic

1275

1100

1400

750

610

1150

Flex Strengh/Mod

Ambient ksi/msi

19.1/450

18.5/400

19.1/390

16.5/480

21.5/480

23-440

Cost Range/ $ per lb

3.5

3.5

7

16

12

9

% water P.U.

1.62

2.1

1.3

2.4

2.7

2.3

*100 Grams at application temp ** Suggested time/temp for demolding with unrestrained post cure cycle.

Cure cycle in mold can be any cure/post cure combination.

Kic = inch lbs 1/2 in2 Wet=48hr H20 Boil Tg=RDS G' Break

COMPOSITE PROPERITIES IN TEST

(will include when available)

 

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