Nylon-VARTM, RTM, S-RIM for Composite Applications – Material

and Process

Ed McDade, BruggemannChemical U.S., Inc.

Klaus Gleich, Southern Research Institute

Uday Vaidya, Gregg, Janowski, Brian PillayDepartment of Materials Science & Engineering

The University of Alabama at Birmingham

Overview

• Introduction

• The Chemistry

• Processing

• Properties

Introduction

Reactive Thermoplastic VARTM/RTM/S-RIM

• Similar to the thermoset process

• Reaction of at least two components creates a thermoplastic resin that can be melted, pre-shaped, welded, …

• Low viscosity is required

• Possible materials: Nylon, TPU, C-PBT (Cyclics)

Problems Connected With Thermoplastic RTM

• Reaction can be stopped or made incomplete by– Moisture– Chemicals in fiber sizing

• Most of the thermoplastic compatible sizings are not developed for such type of processes

• Availability of compatible sizings in form of fabric is very limited

– Oxygen

• Only limited support of material manufacturers• Material costs (in case of c-PBT)

Motivation

• Composite structures are currently used by all major industries to reduce weight and thereby increase efficiency.

• Currently thermoset resins are used, associated with long cure times.

• Thermoplastic composites are superior in delamination resistance and impact behavior compared to thermoset composites.

• Thermoplastics are inexpensive and have short cycle times in standard molding operations.

• Vacuum assisted resin transfer molding (VARTM), resin transfer molding (RTM) and structural reaction injection molding (S-RIM) are affordable manufacturing methods for large components with small or medium production volume that is currently limited to thermoset resins.

• High temperature VARTM of Nylon will provide an inexpensive alternative.

The Chemistry

Cast Polyamide 6 vs. Polyamide 6

In many applications polyamide as an engineering material, has replaced casted metals, whenever better abrasion and corrosion resistance, less weight, higher toughness and lower noise levels are requested, along with versatile functional design and commercial machinability.

By using caprolactam monomer casting, stress-free products are producible, especially big, heavy and complicated parts, which may be not produced by the commonly used injection molding or extrusion methods for polyamides. Products manufactured range from various stock shapes (plates, bars, rods, etc.), punching supports, slide plates, rolls, gears, tubes, oil- and gasoline containers and various machinery and industrial parts.

The conversion of caprolactam to pelletized polyamide 6 engineering resin is usually done in industrial scale at temperatures of 250 – 300 °C. At these temperatures an equilibrium between polymer and monomer of 90 : 10 is formed, which means that after granulation the non-reacted caprolactam has to be extracted from the polyamide 6 – chips.

By comparison, caprolactam monomer casting can be done at temperatures of 150°C up to 190°C, so the caprolactam polymerizes completely below the polyamide 6 melting point (225°C) with monomer conversions of up to 98-99.5%. This casting process is relatively easy, with low investment needed and therefore is commercially used around the world, while the production of polyamide 6 – chips requires big production facilities.

Cast Polyamide 6 vs. Polyamide 6

Cast Polyamide 6 vs. Polyamide 6

There are differences between Cast Polyamide 6 and Polyamide 6 chips.

Production:

• Use of simple inexpensive molds possible

• High part weights with various thickness

• Efficient for low quantities

Material:

• Improved mechanical properties

• Better wear resistance

• Better crystalline structure, higher crystallinity

Caprolactam monomer casting is done at temperatures of about 150 °C by the addition of special catalysts and activators to the molten caprolactam. This results in the formation of polyamide 6 by an anionic polymerization reaction mechanism with a monomer conversion rate of about 99 %.

When catalyst and activator are added to the molten caprolactam in a distinct ratio, polymerization occurs quickly with heat evolution. After 5-10 minutes the part is ready for de-molding. For easier processing, catalyst and activator are held molten in separate tanks, in molten caprolactam. These premixes are process stable for many hours (two-pot-system).

Cast Polyamide 6 vs. Polyamide 6

Basic Principles Of Nylon Casting – Raw Materials

ε-Caprolactam: AP-Quality (Anionic Polymerization) water content < 200ppm

Catalyst: Sodium-Caprolactam used in concentration of app. 1.2- 3.0%

Activator: Caprolactam blocked isocyanate or similar used in concentration of app. 1.0-2.5%

Basic Principles Of Nylon CastingMECHANISM OF THE ALKALINE

POLYMERIZATION BY MOTTUS, HEDRICK AND BUTLER

Explanations

1. Reaction 1, between a sodium ion and lactam, e.g. caprolactam, leads to the lactam anion.

2. This lactam anion reacts with a lactam molecule by attack on the carbonyl group. The lactam molecule is split and forms a acyl lactam.

3. The sodium ion is replaced with a proton and a refreshed lactam anion is again available

4. The acyl lactam is now the initiator for the rapid polymerization at high temperatures.

The whole reaction is extremely sensitive to moisture and oxygen.

Mechanism

Cast Polyamide 6 vs. Injection Molded Polyamide 6

Examples of mechanical properties

TENSILE STRENGTH

0

10

20

30

40

50

60

70

Nylon 6 Cast Nylon 6

N/m

m²

MODULUS OF ELASTICITY

0

1000

2000

3000

4000

Nylon 6 Cast Nylon 6

N/m

m²

Invention Of Cast Polyamides

• Mottus, Hendrick and Butler postulated in 1956 a chemical mechanism for the alkaline polymerization of lactams in the absence of water.

• Industrial importance started in the late 60´s whenε-caprolactam became affordable for producers and was offered in a ‘moisture – free’ quality, e.g. with a water content < 200 ppm (0.02%).

• Today the worldwide consumption of Cast Polyamide 6 is approx. 30,000 mt / yr.

Thermodynamics

• A normal procedure for Polyamide 6 Casting is the ‘Two-Pot-System’. One pot contains the catalyst, the other the activator, both held molten in caprolactam. After mixing the two melts, the polymerization starts with an exotherm of 37 kcal / kg. This heat catalyzes the speed of polymerization, which ends only after a few minutes at conversions of 98 to 99.5 %.

• Too fast of a polymerization may cause problems with the uneven loss of reaction heat during crystallization , which causes internal stress.

• Parts, even with uneven wall thickness, can be produced with low internal stress, if the polymerization is kept homogeneous by taking the right measures, like using the right kind and quantity of catalyst and activator, and the right temperatures of melt and mold, where the difference should not exceed 40°C.

• After de-molding the casted part should be cooled down in a controlled environment (tempering).

Thermodynamics: Progress Of The Reaction

Thermodynamics

Comparison Of DSC For Nylon

Heat Flow Versus Temperature for Nylon 6 (600 HS)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 50 100 150 200 250 300 350

Temperature (°C)

He

at

Flo

w (

mc

al/s

ec)

DSC-1 (02dsc044_002 Run)

Pellet Specimen

Heating Rate: 3°C/minEnvironment: Helium

Heat Flow Versus Temperature for Nylon 6 (X)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 50 100 150 200 250 300 350

Temperature (°C)

He

at

Flo

w (

mc

al/s

ec

)

DSC-1 (02dsc044_003 Run)

Sliced Specimen From Bulk Piece

Heating Rate: 3°C/minEnvironment: Helium

• DSC of standard Nylon 6 granules

• DSC of reactive Nylon 6– sharp peak– higher degree of

crystallinity

Processing

Process Technology Of The Anionic Polymerization Of Caprolactam

Explanations

1. Storage vessel for caprolactam

2. Reactor for caprolactam with activator

3. Reactor for caprolactam with catalyst

4. Mixing head

5. Mold, heated

6. Flexible tube

7. Mixing head, valve

Flowchart

VARTM / RTM / S-RIM

Process VARTM RTM S-RIM

Typical Injection Pressure

≤1 bar ≤ 5 bar ≤ 50 bar

Tooling Single sided tool

Double sided tool

Double sided steel tool

Injection Unit Mixing vessel Pressure vessel, most cases no mixing heat

Separate tanks for each component, mixing head

Typical Achievable Fiber Volume Content

40% 40 % 55%

VARTM Challenges

• High temperature sealant (tacky) tape, which does not adversely affect the polymerization process.

• High temperature bagging material.

• High temperature infusion and vacuum lines.

• Infusing the resin through the preform before polymerization takes place.

• Heating the preform to the required polymerization temperature (150oC), at a high rate, after infusion.

Challenges – Nylon Infusion

• Minimize moisture, polymerization will not take place under high moisture conditions.

• Water deactivates Bruggolen C10 (sodium caprolactamate) by forming sodium hydroxide and caprolactam.

• Maintain a constant temperature of the mixed, molten caprolactam, catalyst and activator at which the reaction rate is at a minimum and maintaining a very low viscosity for infusion.

• Ramp to 150oC in minimum time for polymerization.

VARTM System

Glove box < 10% RH

High temp processing table Reaction kettle w/ mech stirrer

High Temperature VARTM

GS 2-650 high temp tacky tape

Securlon L-2000 vacuum bagging

Aluminum infusion & vacuum spirals

Teflon infusion & vacuum lines

High Temperature VARTM

Pilot Run

• The first preform tried was glass fiber. Regular woven E glass, sized for thermosets was used.

• The sizing proved to be a major problem.

• Full wet-out and polymerization was not achieved.

Modifications

• An aluminum block with cartridge heaters was used for processing.

• Satin weave carbon fiber.

• The preform was washed with acetone to remove all lubricants and impurities.

• Stainless steel tubing replaced the aluminum spiral.

• Non-porous Teflon was used on both sides of the preform.

• An aluminum plate was used on the top surface to assist flow.

Modifications

Cartridge heaters

Heating tape on infusion line

Silicon infusion & vacuum lines

Aluminum top plate

Modified high temperature VARTM processing

Path to dry nitrogen gas

Reaction kettle with nitrogen blanket

Preform Lay-Up

Carbon preform lay-up Partially molten caprolactam

Nylon Infusion

Nylon Infusion Profile

Properties

Micrographs Carbon/Nylon

Carbon Nylon Panel

Micrographs and SEM showing wet-out

Results - DSC

Heat Flow Versus Temperature for Nylon C

-0.1

0.07

0.24

0.41

0.58

0.75

0.92

1.09

1.26

1.43

1.6

0 35 70 105 140 175 210 245 280 315 350

Temperature (°C)

Hea

t F

low

(m

cal/

sec)

Nylon C, DSC-1(03dsc002_005)

Heat Flow Versus Temperature for Nylon B

0

0.23

0.46

0.69

0.92

1.15

1.38

1.61

1.84

2.07

2.3

0 35 70 105 140 175 210 245 280 315 350

Temperature (°C)

Hea

t F

low

(m

cal/

sec)

Nylon B, DSC-1(03dsc002_004)

Heat Flow Versus Temperature for Nylon/Carbon A

-0.3

-0.13

0.04

0.21

0.38

0.55

0.72

0.89

1.06

1.23

1.4

0 35 70 105 140 175 210 245 280 315 350

Temperature (°C)

Hea

t F

low

(m

cal/

sec)

Nylon/Carbon A,DSC-1(03dsc002_003)

Nylon/Carbon A,DSC-2(03dsc002_006)

Nylon/Carbon A,DSC-3 Run 1(03dsc002_007)Nylon/Carbon A,DSC-3 Run 2(03dsc002_008)

Heat Flow Versus Temperature for Nylon and Nylon/Carbon

-0.3

-0.04

0.22

0.48

0.74

1

1.26

1.52

1.78

2.04

2.3

0 35 70 105 140 175 210 245 280 315 350

Temperature (°C)

Hea

t F

low

(m

cal/

sec)

Nylon/Carbon A,DSC-1(03dsc002_003)

Nylon/Carbon A,DSC-2(03dsc002_006)

Nylon/Carbon A,DSC-3 Run 1(03dsc002_007)

Nylon/Carbon A,DSC-3 Run 2(03dsc002_008)

Nylon B, DSC-1(03dsc002_004)

Nylon C, DSC-1(03dsc002_005)

Nylon peaks

Results – Tensile

Tensile tests were conducted on tabbed 12.5 mm wide samples.

Pure Nylon

Pure Epoxy

nylon/carbon (copped) (30%)

carbon/epoxy (50%)

Modulus (GPa) 64.86 2.76 57 70UTS (MPa) 822 70 - 110 3.17 221 600

reported valuesVARTM

Results – 3 Point Bend

Pure Nylon

Pure Epoxy

nylon/carbon (copped) (30%)

Flex Modulus (GPa) 45 3.45 3.45 16.5Flex Stregth (MPa) 490 110 101 317

VARTM

reported values

Three point bend tests were conducted as per ASTM standard D 790M-93.

Failure mode tensile face fracture and delamination

Results - Impact

The low velocity impact tests were conducted using an instrumented Dynatup 8250 impact testing machine. A hemispherical tup of diameter 19.5 mm and mass 0.12 kg

was used as the indenter. The total impact mass was 3.36 kg

Carbon/epoxy (SC-15)_Pure Nylon

Pure Epoxy

Impact energy (J/m) 16726 13230Maximum load (N/m) 1840 1200

KIC (MPa-m1/2) 2.8 0.3-0.6

reported values

Thickness NormalsiedVARTM

Back face Tensile side

Impact side

VARTM - GLASS

Glass fiber with nylon compatible sizing- only available in roving form.Hand Loom used to manually weave the roving into fabric for infusion

Further work with glass fiber / nylon is necessarySizing has to be optimized

Micrographs – Glass/Nylon

Resin rich area Cross over over tows

Cross section of toes showing wet-out

ACKNOWLEDGEMENTS

The authors wish to express their appreciation to

the Federal Transit Administration

for the support of this work.