|
|
|
|
|
News from
Island Coast Boat Works |
|
|
|
Why gel coats
crack
A number of factors contribute to gel coat cracking,
an in-your-face customer concern.
By Bob Lacovara
Marina Business Today, May 2002 |
|
|
|
This
article was reproduced by Island Coast Boat Works
with the permission of Marina Business Today |
| |
|
|
Gel coat
cracking, along with fading or chalking problems are the nemesis of
product warranties for much of the fiber reinforced plastics (FRP)
composites industry. While chalking and color change problems normally can
be prevented or forestalled by adequate end-user maintenance, gel coat
cracking is another story. Inmost cases where mild chalking or fading can
be remedied simply by compounding and waxing, gel coat cracking involves a
repair -- a warranty claim.
While gel coat
cracking may not be viewed as a major factor in the big picture of
designing, building, tooling and producing a composite component, it is a
big deal to the customer. For example, a fisherman, who after motoring
around for years in an aluminum runabout, comes to the place where he can
afford a new bass boat. It's his toy -- it shines, it goes fast, it makes
him a real pro. If the gel coat cracks, he is upset. Of all potential
composites-related problems, surface cracking is an up front, in-your-face
customer concern.
Hairline cracks
in a gel coat surface usually are considered a cosmetic problem, and are
treated as such. However, on occasion, gel coat cracking is an indication
of underlying structural problems or a result of environmental operating
conditions. There are a number of contributing factors to gel coat
cracking which include formulation, product design, application and
factory operating environment.
Gel coat formulation
Gel
coat manufacturers walk a fine line in balancing high gloss properties and
toughness. Generally speaking, it is easier to produce higher gloss in a
harder gel coat, while a tougher (and softer) material tends to have less
initial gloss. the trade-off is the harder formulations tend to be more
brittle, while the tougher formulations tend to exhibit less gloss.
Formulators have developed gel coats that incorporate the best balance of
these properties for specific applications.
The newer third
and fourth generation gel coat formulations also have made strides in
enhancing properties which provide a wider window of both toughness and
gloss. However, the fabricator must choose carefully in light of the
intended use of a product. For example, a high gloss gel coat system
suitable for restaurant seating components may not be suitable for a
canoe, which requires flexibility. A careful choice of gel coat type and
formulation is required to provide the best fit for each application.
|
|
Product design
Gel
coat, by nature of being on the outer surface of a structure, is subject
to the highest strain of the entire laminate. The tensile or compressive
strain in a loaded laminate increases with distance from the neutral axis
of the load. In the accompanying illustration of a typical laminate placed
under a flexural load, the highest tensile strain is recorded at the top
surface, while the highest compressive strain is at the bottom surface.
There is no strain at the interior of the laminate, at the neutral axis.
Because of the critical positioning of the gel coat film in a laminate
structure, both the laminate and the supporting structure must take into
account the strain imposed by anticipated operating loads.
|
|
|
| Factory gel
coat application
The primary source of control the fabricator has in influencing gel
coat cracking is in the application. the method of application, and
conditions surrounding the process, are a major influence on the integrity
of the gel coat film. Gel coat film thickness may be the single most
important control point in the process. For most gel coats, the range of
acceptable thickness is between 16 mils (1 mil = 0.001 inch) to 20 mils.
There is a specific optimum thickness range for each formulation of gel
coat required by the manufacturer of the product. The fabricator is
well-advised to heed these recommendations. |
|
Out-of-spec gel coat thickness can cause a variety of problems from
undercure for a thin gel coat, to cracking for a thick gel coat. Another
point to note is that the average thickness of gel coat on a part may not
prevent cracking. For example, if a part averages 18 mils thick, but the
corner areas are 26 mils, localized cracking may occur in the thick areas.
It is important to achieve the proper thickness in the most highly
stressed areas of a part. |
|
Because thickness is a critical control point for crack prevention, it is
necessary to use the spray process for gel coat application. Although the
practice of brushing or rolling gel coat is common in Europe, this method
does not produce gel coat performance acceptance for the highly cosmetic
products produced in the United states. High-quality gel coating
procedures call for mil gauging (measuring film thickness) on every part
produced. In addition, critical areas such as corners should be mil gauged
regularly on an ongoing basis. |
| Gel
coat adhesion to the substrate laminate is another factor which influences
cracking. The interface bond between the gel coat film and the laminate is
responsible for preventing long-term cracking due to cyclic loading of a
panel or from thermal stress due to changes in operating temperature. the
preferred window between applying gel coat and the back laminate is eight
hours. This may be stretched to overnight in certain cases; confer with
the materials supplier. A great time period is not recommended due to the
state of cure of the gel coat film and the possibility of surface dust or
other contamination. |
|
Finally the state of cure of the gel coat may have an influence on the
cracking problem. Undercure, resulting from under-catalization, low shop
temperature or too thin a film, will usually produce a flexible gel coat.
while this flexible gel coat is not prone to cracking, it may be inclined
to premature color degradation, loss of gloss, chalking or chemical
attack. On the other hand, over-catalization can easily lead to a brittle
gel coat which cracks with little provocation. |
| Factory
operating environment
One of the major operating factors involved with gel coat cracking is
coefficient of thermal expansion (CTE), or more simply put, expansion and
contraction. As temperature changes so does the size of an object. Small
temperature changes yield small, imperceptible changes in dimensions.
Large temperature changes or rapid transitions may produce more dramatic
effects. |
| A
classic example of extremes is the environment in which high performance
sailplanes (glides) operate. These gel coat finished aircraft have the
capability of attaining very high altitudes in certain atmospheric
conditions (mountain wave). Aircraft flying at an altitude of 30,000 feet
may be in temperatures as low as -30F, then in a relatively short time may
descend into ground-level temperatures of 50 F to 70 F. That could
comprise a temperature change of 100 F, which is extreme. Even a boat in
60 F repair shop removed to a winter temperature of 20 F represents a
substantial, rapid temperature change. |
|
While gel coat which is properly formulated and applied usually performs
well, even under these temperature extremes, potential cracking problems
can be evidenced if improper materials or techniques are used. The rate of
temperature change seems to be a greater concern than how low the
temperature becomes. |
|
Bob Lacovara is director of technical services for the Arlington, VA-based
Composites Fabricators Association (CFA). He is a recipient of the CFA
President's Award for outstanding industry service, as well as an author
and frequent lecturer. Having been in the composites industry for more
than 30 years, he maintains an industrywide international consultancy. He
can be reached at the group's technical office in Harleysville, PA, at
(215) 513-7546 or via e-mail at
blacovara@aol.com. The CFA's Web site can be accessed at
www.cfa-hq.org. |
|
|
|
|
|
|
