**Process Specification for**

**Adhesive Bonding**

+-----------------------------------------------------------------------+
| **Engineering Directorate**                                           |
|                                                                       |
| **Structural Engineering Division**                                   |
|                                                                       |
| **April 2005**                                                        |
|                                                                       |
| ![](media/image1.png){width="1.3069444444444445in"                    |
| height="1.0784722222222223in"}                                        |
|                                                                       |
| National Aeronautics and                                              |
|                                                                       |
| Space Administration                                                  |
|                                                                       |
| **Lyndon B. Johnson Space Center**                                    |
|                                                                       |
| Houston, Texas                                                        |
+-----------------------------------------------------------------------+

**Process Specification for**

**Adhesive Bonding**

+------------+--------------------------+---+---------------------------+
| **Prepared | ***Signature on File***  |   | ***Signature on File***   |
| by:**      |                          |   |                           |
+------------+--------------------------+---+---------------------------+
|            | Michael E. Fowler,       |   | Thomas S. Reese,          |
|            |                          |   |                           |
|            | Materials and Processes  |   | S.R. & Q.A./Quality       |
|            | Branch/ES4               |   | Engineering /Jacobs       |
|            |                          |   | Sverdrup Engineering -    |
|            |                          |   | ESCG                      |
+------------+--------------------------+---+---------------------------+
|            |                          |   |                           |
+------------+--------------------------+---+---------------------------+
|            |                          |   |                           |
+------------+--------------------------+---+---------------------------+

  ------------- -------------------------- --- ---------------------------
  **Approved    ***Signature on File***        
  by:**                                        

                Brad Files, Branch Chief,      
                Materials and Processes        
                Branch/ES4                     
  ------------- -------------------------- --- ---------------------------

  ----------- ---------------------------------------------- --------------
  REVISIONS                                                  

  VERSION     CHANGES                                        DATE

  \--         Original version                               12/95

  A           Reformatted entire document to conform to      9/16/97
              current PRC format. Changed "Class A" and      
              "Class B" to "Level 1" and "Level 2"           
              respectively in accordance with JSC 8500.      
              Removed 1,1,1 Trichloroethane as a cleaning    
              solvent. Removed specific pressure call-outs   
              and referred to material specifications for    
              the correct pressure call-outs.                

  B           Reviewed per ISO requirements. Removed         1/26/2001
              sentence referring to manufacture of composite 
              parts (Section 6.1)                            

  C           Removed references to old division name. Added 11/5/02
              reference document for surface preparation of  
              plastics that describes corona discharge.      

  D           Reviewed per QMS requirements. Updated         4/15/2005
              reference document numbers. Added bondline     
              thicknesses.                                   
  ----------- ---------------------------------------------- --------------

**1.0 SCOPE**

This document covers the requirements for the adhesive bonding of
metals, plastics, and composites in any combination.

# **2.0 APPLICABILITY**

This specification applies to adhesive bonding associated with space
flight or space flight-related hardware fabricated under the authority
of the NASA/Johnson Space Center (JSC).

**3.0 USAGE**

This section gives the requirements for the proper use of this process
specification.

In accordance with the drawing and part definition requirements of JSC
8500, "Engineering Drawing System Requirements", the standard adhesive
bonding process shall be invoked by providing a process note in the
applicable drawing or CAD model as exemplified in Figure 1.

Figure 1. Example of a process note for adhesive bonding.

**3.1 LEVEL**

The \"Level\" designator governs the extent to which quality assurance
provisions are applied and shall be specified in the process note on the
basis of the following definitions:

a\. [Level 1]{.underline} --- Level 1 processes shall include the
practice of the quality assurance provisions as required by Section 7.1.
Whenever invoking these Level 1 provisions, the designer should also
consider calling out an NDE process specification on the drawing or CAD
model.

b\. [Level 2]{.underline} --- Level 2 processes shall include the
practice of the quality assurance provisions as required by Section 7.2.

**3.2 CURE AND POST-CURE SCHEDULES**

Unless otherwise stated on the drawing or CAD model, standard cure and
post-cure schedules will be applied to the material in accordance with
Sections 6.11 and 6.12. Add special cure or post-cure instructions, if
necessary, to the process note on the applicable drawing or CAD model.

**3.3 NON-DESTRUCTIVE EVALUATION (NDE)**

This specification does not address the application of non-destructive
evaluation (NDE) methods. However, when calling for Level 1 processing,
the designer should also consider the use of NDE inspection by calling
out a separate NDE process specification on the drawing or CAD model.

**4.0 APPLICABLE DOCUMENTS**

The following documents were used in developing this specification:

JPR 8500.4 *Engineering Drawing System Requirements*

JPR 5322.1 *Contamination Control Requirements Manual*

The following document is invoked as part of this specification:

ASTM D 2093 *Standard Recommended Practice for Preparation of Surfaces
of Plastics Prior to Adhesive Bonding*

ASTM D 2651 *Standard Recommended Practice for Preparation of Metal
Surfaces for Adhesive Bonding*

ASTM D 3933 *Standard Guide for Preparation of Aluminum Surfaces for
Structural Adhesives Bonding (Phosphoric Acid Anodizing)*

ASTM D 1002 *Strength Properties of Adhesives in Shear by Tension
Loading (Metal-to-Metal)*

ISO 13895 *Adhesives -- Guidelines for the Surface Preparation of
Plastics*

**5.0 MATERIAL REQUIREMENTS**

**5.1 MATERIAL SPECIFICATIONS**

All materials used in the adhesive bonding process shall conform to
applicable Government or other specifications.

**5.2 MATERIAL OUT-TIME AND SHELF-LIFE**

The out-time and shelf-life of the material shall not exceed the
shelf-life requirements specified by an applicable MDS. Traceable
out-time and shelf-life records shall be kept for materials whose MDS
specifies out-time and shelf-life limits. If the material out-time or
shelf life exceeds the MDS specifications, then a requalification of the
material shall be accomplished in a manner prescribed by the MDS.

**6.0 PROCESS REQUIREMENTS**

**6.1 WRITTEN PROCEDURES AND STANDARDS**

For contracted work, refer to the contract for requirements concerning
the use of written procedures. Contractors shall also obtain applicable
Material Data Sheets (MDS) from JSC Manufacturing before processing.

For work performed at JSC facilities, written procedures shall be used
and they shall consist of Detailed Process Instructions (DPI\'s)
selected for use from the DPI-1001- series of work instructions. MDS's
shall also be used internally.

**6.2 FACILITIES**

All bonding operations shall be performed in a controlled environment
that is free of contaminants to the extent that bonding operations are
not affected. The bonding environment shall have controlled temperature
and humidity as required to prevent degradation of the adhesive bond.

## **6.3 MASKING**

Parts and assemblies shall be masked as necessary to protect surfaces
not to be bonded from damage, contamination from adhesives, cleaning
solutions, or abrasives.

## **6.4 PREFIT OF PARTS**

All detail parts shall be free from burrs, waves, and other
imperfections. The detail parts shall provide adequate contact over the
entire bonding area.

**6.4.1 LEVEL 1 REQUIREMENTS**

Where practicable, the detail parts shall be prefit to determine
mismatch. Surfaces not prefitted shall conform to the applicable
drawings*.* Bonding surfaces which conform to all of the following
requirements are exempt from prefit:

a\. Both surfaces are nominally flat.

b\. Smallest dimension is 2.00 inches (5.00 cm) or less.

c\. Total bonding surface area is less than 8.00 square inches (51.6
cm2).

Mismatch between detail parts shall not exceed the requirements of Table
I.

Table I. Mismatch Requirements

+----------+---+---+---------------------+---------------------+----------+
| **Ad     |   |   | **Mismatch**        | **% of Bonding**    | **       |
| hesive** |   |   |                     |                     | Layers** |
|          |   |   |                     | **Area**            |          |
| **Type** |   |   |                     |                     | **to     |
|          |   |   |                     | **Allowed**         | Use**    |
+----------+---+---+---------------------+---------------------+----------+
| **Film** |   |   | 1 Thickness Layer   | NA                  | 1        |
+----------+---+---+---------------------+---------------------+----------+
|          |   |   | Between 1 and 2     | 5                   | 2a       |
|          |   |   |                     |                     |          |
|          |   |   | Thickness Layers    |                     |          |
+----------+---+---+---------------------+---------------------+----------+
|          |   |   | Between 2 and 3     | 5                   | 3a,b     |
|          |   |   |                     |                     |          |
|          |   |   | Thickness Layers    |                     |          |
+----------+---+---+---------------------+---------------------+----------+
|          |   |   | 4 Thickness Layers  | 0                   | Rework   |
|          |   |   |                     |                     |          |
|          |   |   |                     |                     | Parts    |
+----------+---+---+---------------------+---------------------+----------+
| *        |   |   | 0.000\" - 0.005\"   | 100                 | NA       |
| *Paste** |   |   |                     |                     |          |
+----------+---+---+---------------------+---------------------+----------+
|          |   |   | 0.005\" - 0.015\"   | 5                   | NA       |
+----------+---+---+---------------------+---------------------+----------+
|          |   |   | \>0.015\"           | 0                   | NA       |
+----------+---+---+---------------------+---------------------+----------+
| NOTES:   | a | G |                     |                     |          |
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+----------+---+---+---------------------+---------------------+----------+
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+----------+---+---+---------------------+---------------------+----------+
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+----------+---+---+---------------------+---------------------+----------+

**6.4.2 LEVEL 2 REQUIREMENTS**

The detail parts shall be prefitted prior to bonding to assure
non-interference conditions.

## **6.5 SURFACE CLEANING**

The detail parts shall be precleaned to remove oil, greases, inks, etc.
using either methyl ethyl ketone (MEK) or Isopropanol in the case of
metals. Isopropanol or acetone is acceptable with thermoplastics,
thermosets, paints, coatings, etc. which are not compatible with MEK.
Silicone-based cutting and forming fluids shall not be used when
processing detail parts for bonded assemblies.

## **6.6 SURFACE PREPARATION**

### 6.6.1 PLASTIC SURFACES

Plastic surfaces shall be prepared for adhesive bonding per ASTM D 2093.

### 6.6.2 METALLIC SURFACES

Metallic surfaces shall be prepared for adhesive bonding per
ASTM-D-2651, in general, and per ASTM D 3933 for phosphoric acid
anodizing of aluminum.

The maximum elapsed time between surface priming and the application of
adhesive for metallic parts shall be 12 hours.

### 6.6.3 UNPRIMED SURFACES 

Unprimed surfaces shall not be solvent-wiped subsequent to surface
preparation and before priming or assembly for adhesive bonding.

## **6.7 PRIMER APPLICATION**

When an adhesive primer is specified on the design drawing, the primer
shall be applied to the prepared surfaces to be bonded. The time between
surface preparation and primer application, the primer thickness, the
primer cure, and the maximum time between primer cure and adhesive
application shall be in accordance with the applicable material
specification or the manufacturer\'s recommendation for those materials
when a material specification does not exist.

## **6.8 ADHESIVE APPLICATION**

### 6.8.1 FILM ADHESIVES

Unless otherwise noted on the drawing, or in accordance with mismatch
requirements of Table I, one layer of film adhesive shall be applied
between faying surfaces. Pressurization and cure of the adhesive shall
begin within the out-time of the adhesive.

### 6.8.2 PASTE ADHESIVES

The adhesive components shall be mixed in accordance with the adhesive
material data sheet (MDS), or the manufacturer\'s recommendations for
those materials when a material specification does not exist. The
adhesive shall be applied to both faying surfaces. Unless otherwise
noted, the adhesive thickness shall be 0.003 -- 0.007 inches. Assembly
of the parts and pressure application shall take place before the
adhesive work-life has expired.

### 6.8.3 SCRIM CLOTH

One layer of scrim cloth shall be placed between faying surfaces for
bondline control when called out on the design drawing. This is only
required for paste adhesives.

## **6.9 ASSEMBLY OF PARTS** 

Care shall be taken in assembling parts in the curing fixture or bonding
jig to assure that the adhesive is not disturbed during the cure period,
and that excessive residual stresses in the components are avoided.
Shims, caul plates, and other shop tools shall be used as much as
necessary to ensure that every adhesive bondline has adequate support
and pressure application.

## **6.10 PRESSURE APPLICATION**

Pressure shall be maintained throughout the cure of the adhesive. When
heat is applied to the adhesive to promote cure, pressure may only be
removed after the assembly cools to at least 150°F (66°C).

### 6.10.1 VACUUM BAGS

A minimum of 20 inches Hg (0.068 MPa) is required throughout the cure
when using vacuum bags unless otherwise indicated in the material
specification or on the drawing. The vacuum pressure shall be regularly
monitored and recorded for Level 1 processes. The vacuum gauge shall be
placed at a position remotely away from the inlet to the vacuum pump
line.

### 6.10.2 AUTOCLAVES 

Vacuum is initially needed to seat the diaphragm against the bonded
assembly. The pressure differential shall be maintained according to the
material specification or drawing throughout the cure. The differential
pressure shall be regularly monitored and recorded for Level 1
processes.

### 6.10.3 DEAD WEIGHT

Dead weight pressure is acceptable for relatively flat adherends. The
pressure applied shall be according to the material specification or the
drawing throughout the cure when using dead weight loading.

## **6.11 ADHESIVE CURE**

The cure and/or post-cure cycle (time, temperature, and pressure) shall
be in accordance with the applicable material data sheet, or the
manufacturer\'s recommendation for those materials when a material
specification does not exist. Equivalent cure cycles, when specified on
the engineering drawing, may be used provided cured adhesive properties
are maintained.

### 6.11.1 APPLICATION OF HEAT

Heat, if needed, shall be applied by the use of either a circulating air
oven, an autoclave, heater blankets or heat lamps.

### 6.11.2 TEMPERATURE MEASUREMENTS

Temperature measurements shall be recorded by means of thermocouples.
The thermocouple shall be placed in the flash area of the adhesive if
possible. Otherwise, a thermocouple placed on the part on the outer mold
line of the bond is acceptable.

## **6.12 IN-PROCESS VERIFICATION TESTING**

### 6.12.1 LAP SHEAR TESTS 

A minimum of five representative lap shear specimens, conforming to
ASTM-D-1002, shall be bonded with each application of adhesive, at the
same time, with the same adhesive, in the same equipment, and cured
under the same conditions for the assembly they represent. The adherend
may be a material other than that specified on the design drawing
provided that the specimen surfaces are prepared as necessary to achieve
bonded specimens that adequately represent the bonding process. If
primer and/or scrim cloth is used in the assembly, then primer and/or
scrim cloth shall be used in the specimens.

### 6.12.2 HARDNESS TESTS

In addition to the lap shear specimens, a hardness specimen shall be
potted for in-process verification of adhesive cure. It shall be cured
at the same time, with the same adhesive, in the same equipment, and
under the same conditions for the assembly it represents. No vacuum or
pressure is required. The specimen shall be removed from the bonding
equipment and tested according to section 7.1 prior to
pressure/temperature removal.

## **6.13 RECORDS**

Pressure, time, and temperature records on each part shall be recorded
at least every 15 minutes. These records shall be retained for permanent
reference on each lot of material and on each production part for the
life of the part for Level 1 processes.

## **6.14 WORKMANSHIP**

Bonded joints shall be of high quality workmanship. Acceptable parts
shall be free of defects that adversely affect intended use. Appearance
shall be uniform and free of defects as specified herein or on the
design drawing. Adhesive shall be evident at the edge of all visibly
accessible bondlines.

# **7.0 PRODUCT ACCEPTANCE TESTING**

## **7.1 LEVEL 1 BONDS**

Product acceptance shall be based on a test evaluation of the process
control test specimens described in section 6.12 that accompany the
product. The lap shear specimens shall be tested per ASTM-D-1002 at room
temperature. Hardness specimens shall undergo Shore D hardness
evaluation for epoxy adhesives or a Shore A hardness evaluation for
silicone adhesives.

For product acceptance, the lap shear strengths and hardness results
shall meet or exceed the design allowables of the adhesive. If design
allowables are not available, vendor-provided allowables (S allowables)
with an applied factor of safety are permissible. This factor of safety
shall be provided by the engineering organization responsible for
process engineering.

The results of product acceptance testing shall be recorded.

## **7.2 LEVEL 2 BONDS**

Product acceptance shall be based upon a visual inspection of the
product and a test evaluation of a potted adhesive hardness specimen.
Visual inspection shall address bondline uniformity and cure of the
adhesive. The hardness specimen shall be as described in section 6.12.2
and shall undergo Shore D hardness evaluation for epoxy adhesives or a
Shore A hardness evaluation for silicone adhesives.

For product acceptance, the hardness results shall meet or exceed an
allowable for the adhesive. If an adequate allowable is not available,
vendor-provided allowables (S allowables) with an applied factor of
safety are permissible. This factor of safety shall be provided by the
engineering organization responsible for process engineering.

Lap shear test specimens and process verification records are not
required.

# **8.0 PROCESS QUALIFICATION**

Detailed process instructions (DPI\'s) for the bonding process shall be
qualified for each combination of materials used prior to acceptance of
finished products. Qualification shall consist of documented test
evidence that the procedure for bonding, when performed by suitably
trained and proficient operators, is capable of meeting all the
requirements of this specification. Any change in procedure or materials
shall require requalification.

The qualification testing shall involve a minimum of ten lap shear
specimens tested at room temperature to qualify the procedure against
strength allowables. If adequate allowables are not available,
vendor-provided allowables (S allowables) with an applied factor of
safety are permissible. This factor of safety shall be provided by the
engineering organization responsible for process qualification.

**9.0 TRAINING AND CERTIFICATION OF PERSONNEL**

Training of all bonding technicians shall be performed according to
written detailed procedures. Proper training shall, at a minimum, be
structured in such a way as to ensure that each trainee is capable of
producing adhesive bonds that pass Level 1 process verification
criteria. Training and certification records shall be kept.

# **10.0 DEFINITIONS**

a\. Faying Surfaces. Pre-bonded, close-fitting surfaces.