Bob Lacovara

CFA Technical Director

Gel coat cracking, along with fading or chalking problems, are the nemesis of product warranties for much of the FRP composites industry. While chalking and color change problems can normally be prevented or forestalled by adequate end user maintenance, gel coat cracking is another story. In most cases, where mild chalking or fading can be remedied simply by compounding and waxing, gel coat cracking involves a repair - a warranty claim. The problem can range from cosmetic hairline cracks to cracks which extend into the laminate and portend a structural defect.

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 couple retires and buys the motorhome they have dreamed about for years - it becomes their baby. It doesn’t matter to them if the worlds best laminate is behind the surface, if the gel coat cracks they are upset. After motoring around for years in an aluminum runabout, a fisherman 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. The independent trucker mortgages the farm to buy a new "big rig". He runs 14 maybe 16 hours a day to make ends meet and really takes care of his truck - it is his living. If the gel coat cracks, it is a big deal to him. Of all potential composites related problems, surface cracking is an up front in-your-face customer concern.

Hairline cracks in a gel coat surface are usually 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 operating environment.

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 which incorporate the best balance of these properties for specific applications. The newer third and fourth generation gel coat formulations have also 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.

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 illustration (figure 1) 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.

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 - 20 mils (thousandths of an inch). This range may float with specially formulated products. For example, certain gel coats may work at down to 12-14 mils in thickness, while others are routinely used in the area of 22-26 mils. Tooling gel coats may sometimes be applied at greater then 30 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 acceptable 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 on a regular and 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 operating temperature changes. The preferred window between applying gel coat and the back-up laminate is 8 hours. This may be stretched to overnight in certain cases (confer with the materials supplier). A greater 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. For example, it is not recommended to gel coat a mold on Friday afternoon and apply the skin coat laminate on Monday morning. A number of cases of gel coat cracking warranty problems have been traced to this practice.

Finally, the state of cure of the gel coat may have an influence on the cracking problem. Undercure, resulting from under-catalyzation, low shop temperatures 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-catalyzation can easily lead to a brittle gel coat which cracks with little provocation.

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 (gliders) operate. These gel coat finished aircraft have the capability of attaining very high altitudes in certain atmospheric conditions (mountain wave). At an altitude of 30,000 feet the temperature may be as low as -30⁰ F, and aircraft may then descend into ground level temperatures of +50-70⁰F in a relatively short time. That could comprise a temperature change of 100⁰F, which is extreme. Even a pick-up with a truckcap in a 60⁰F repair shop removed to a winter temperature of 20⁰F represents a substantial, and 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 then how low the temperature becomes.

There are a number of types of cracks which are evidenced in gel coat, and each type signifies a particular problem or set of problems. Even a rudimentary visual analysis of these crack modes can provide insight into the forces acting on the gel coat surface. Various crack configurations indicate the underlying causes and are vital in troubleshooting the problem. In some cases the root problem has nothing to do with the gel coat and is a manifestation of a structural problem or unanticipated movement of the substrate.

In a simplistic, and general view, it might be said there is only one cause of gel coat cracking - movement. If the GC film or the laminate does not move, cracking can not occur. Movement in one form or another can have a number of causes. Many times the cause of the movement can be determined from the pattern of cracking.

Usually associated with impact, radial cracks are a good indicator of the direction of the impact. The classic "spider" crack is a result of a reverse impact or sharp, localized stress riser. A frontal impact is indicated by a concentric circle pattern, with the diameter of the inner circle having a relationship to the size of the impacting object.

There are two groups of linear cracks, stress field patterns and parallel stress cracks. The primary cause of these cracks is flexural strain. However, in the case of stress field cracking, either structural elements or local stress risers modify the parallel pattern into a more complex structure. Parallel stress cracks indicate flexural movement perpendicular to the direction of the cracks. Parallel curvilinear cracks often indicate a distribution of stress over a supported panel surface. If the surface is restrained in two 90⁰ planes, the flexural strain will "fan out", creating a "palm leaf" effect.

Convergent stress field cracks may result when flexural strain is interrupted by a structural member. Parallel stress cracks radiate from a localized nucleation. The main effect is the laminate is deflected inward toward the restraining member. The parallel stress crack is interrupted by a stress concentration around a point. In the case of a divergent stress field, the laminate is deflected away from the supporting member and the crack propagation is consolidated through a localized lack of movement.

This type of crack is a result of an intervening shape, usually a cut-out, in the surface of a panel. The form or shape serves to concentrate strain into a localized area. In the case of a hard point riser, a low level strain may result in cracking due to high level stress concentration in a very small area. A square shape with sharp corners is a prime candidate for creation of a hard point riser. A radial riser may have a different origin. In this case, often a bolt or hardware fitting exerts a tensile force in the area around a hole. The edge of the hole distends causing a tensile failure of the gel coat in the surrounding area.

In order to develop a uniform characterization for of cracking, the following crack severity scale is offered as a method of standardizing the description of a cracking problem. The level of penetration through the gel coat film or into the laminate effects the method of repair, which can range from cosmetic to structural. This crack severity scale differentiates between two levels of cosmetic, involving only the gel coat film and two levels of structural severity, from minor laminate incursion to serious structural penetration.

Minimizing the possibility of gel coat cracking involves attention to a number of areas. First is specifying the proper gel coat for the application. A high Barcol gel coat (hard/brittle) gel coat formulation may be suitable for a mold component which is not dynamically loaded. Whereas a highly stressed part should use a tough gel coat, formulated for appropriate elongation.

The second item is to consider the effect of a structural design on the gel coat surface. Excessive flexing or a flexible panel with rigid corners may contribute to gel coat cracking. Keep in mind, the only cause of gel coat cracking is movement, although there are many contributing factors. The skill of the laminate designer hopefully determines the amount of acceptable surface movement.

Third, gel coat application is critical. Thick gel coat is major culprit in cracking. Gel coat film thickness should be a primary focus of quality control. Accurately controlling thickness requires mil gauging every part, all of the time. Gel coat curing efficiency is another critical factor. Gel time, catalyst level, shop temperature, spray gun set-up and spraying technique are all critical factors in producing high quality crack resistant gel coat.

Laminate sequence timing is another important element. There is an optimal window of timing from gel coat application to laminate application. Pushing the edges or going outside this window increases the chances of interface bonding problems, possibly resulting in cracking. Usually interface bonding problems associated with cracking are sporadic occurring in groups. Optimal timing from GC application to laminate is 1.5 to 4 hours, and 8 hours is acceptable. Overnight is marginal, and letting gel coat cure over a weekend before laminating is not acceptable for crack prevention.

One often overlooked, and very basic principle, is proper mixing of gel coat in the drum. Unmixed gel coat may have a higher styrene content at the top of the drum as compared to the bottom. Styrene is inherently brittle, and an unmixed or inadequately mixed drum of material may effect the properties of the gel coat, causing cracking. Again this problem will appear sporadically and be difficult to diagnose. The lesson is, the basic principles of proper material handling and application prevent a multitude of problems.

Gel coat cracking may be one of the easier problems to diagnose in composites fabrication. The cracks themselves are a roadmap leading to the solution.