Patent Description:
Composite structures generally include a fiber and a resin material. Chemical and/or physical properties of the fiber and/or of the resin material may have a significant impact on the chemical and/or physical properties of the composite structure that is formed therefrom. As examples, the structural integrity, longevity, chemical resistivity, and/or light stability of the composite structure may be dictated by the chemical and/or physical properties of the fiber and/or of the resin material. With this in mind, a specific fiber and/or resin may be utilized to form a specific composite structure with one or more desired physical and/or chemical properties. In some examples, a composite structure that exhibits one or more desired physical and/or chemical properties also may exhibit one or more undesired physical and/or chemical properties. As a specific example, a composite structure that exhibits a desired structural integrity also may exhibit an undesired chemical resistivity and/or light stability.

It is known to coat and/or cover a composite structure, such as to protect the composite structure from environmental factors. In some examples, such coating may be accomplished by painting the composite structure. In other examples, it may be desirable to form a hybrid composite structure that includes a surface composite layer and an underlying composite layer. The surface composite layer may be selected to protect the underlying composite layer from the environmental factors, while the underlying composite structure may be selected to provide a desired structural integrity. However, it is difficult to maintain a desired level of distinction between the surface composite layer and the underlying composite layer during curing of the hybrid composite structure. Thus, there exists a need for improved uncured composite structures, cured composite structures, and/or methods of curing uncured composite structures.

<CIT>, in accordance with its abstract, states that there is an aim to provide a molding material that can yield a fiber-reinforced resin molding with excellent surface smoothness and appearance after curing, and a molding method therefor. The proposed solution is to provide, on an outermost layer of a molding material of a fiber-reinforced resin molding, a layer of a second thermosetting resin material with a lower glass transition temperature than that of a first thermosetting resin material contained in an inside CFRP layer as a matrix.

There is described herein, a method of curing an uncured composite structure that includes a structural layer, which includes an uncured structural resin with a structural resin viscosity, a structural resin gel point temperature, and a structural resin gel time, and a surface layer, which includes an uncured surface resin with a surface resin viscosity that is greater than the structural resin viscosity, a surface resin gel point temperature that is less than the structural resin gel point temperature, and a surface resin gel time that is less than the structural resin gel time, the method comprising: initially heating, for an initial heating time, the uncured composite structure to an initial temperature to generate a partially cured composite structure, wherein the initially heating is sufficient to gel the uncured surface resin but insufficient to gel the uncured structural resin; and subsequently heating, for a subsequent heating time, the partially cured composite structure to a final cure temperature, which is greater than the initial temperature, to generate a cured composite structure, wherein a combination of the initially heating and the subsequently heating is sufficient to fully cure both the uncured surface resin and the uncured structural resin.

There is also described herein an uncured composite structure, comprising: a structural layer that includes an uncured structural resin, wherein the uncured structural resin has a structural resin viscosity, a structural resin gel point temperature, and a structural resin gel time; and a surface layer that includes an uncured surface resin, wherein the uncured surface resin has a surface resin viscosity that is greater than the structural resin viscosity, a surface resin gel point temperature that is less than the structural resin gel point temperature, and a surface resin gel time that is less than the structural resin gel time.

<FIG> provide illustrative, non-exclusive examples of uncured composite structures <NUM>, cured composite structures <NUM>, and/or methods <NUM>, according to the present disclosure. Elements that serve a similar, or at least substantially similar, purpose are labeled with like numbers in each of <FIG>, and these elements may not be discussed in detail herein with reference to each of <FIG>. Similarly, all elements may not be labeled in each of <FIG>, but reference numerals associated therewith may be utilized herein for consistency. Elements, components, and/or features that are discussed herein with reference to one or more of <FIG> may be included in and/or utilized with any of <FIG> without departing from the scope of the present disclosure.

In general, elements that are likely to be included in a given (i.e., a particular) example are illustrated in solid lines, while elements that are optional to a given example are illustrated in dashed lines. However, elements that are shown in solid lines are not essential to all examples, and an element shown in solid lines may be omitted from a particular example without departing from the scope of the present disclosure.

<FIG> is a flowchart illustrating examples of methods <NUM> of curing an uncured composite structure, according to the present disclosure. The uncured composite structure includes a structural layer, which includes an uncured structural resin, and a surface layer, which includes an uncured surface resin. An example of the uncured composite structure is illustrated in <FIG>. More specifically, <FIG> illustrates a composite structure <NUM> in the form of an uncured composite structure <NUM>. Uncured composite structure <NUM> includes a structural layer <NUM>, in the form of an uncured structural layer <NUM>, that includes a structural resin <NUM>, in the form of an uncured structural resin <NUM>. Uncured composite structure <NUM> also includes a surface layer <NUM>, in the form of an uncured surface layer <NUM>, that includes a surface resin <NUM>, in the form of an uncured surface resin <NUM>.

Methods <NUM> may include providing an uncured composite structure at <NUM> and/or applying a pressure at <NUM>. Methods <NUM> include initially heating at <NUM> and subsequently heating at <NUM>.

Providing the uncured composite structure at <NUM> may include providing any suitable uncured composite structure, which includes the structural layer and the surface layer, in any suitable manner. As an example, the providing at <NUM> may include laying up, forming, and/or defining the uncured composite structure. In some examples, and as discussed in more detail herein, the providing at <NUM> includes providing such that the structural resin has a structural resin viscosity, a structural resin gel point temperature, and a structural resin gel time. In some examples, and as also discussed in more detail herein, the providing at <NUM> includes providing such that the surface resin has a surface resin viscosity, a surface resin gel point temperature, and a surface resin gel time. In some examples, the structural resin viscosity is less than the surface resin viscosity, the surface resin viscosity is greater than the structural resin viscosity, the structural resin gel point temperature is greater than the surface resin gel point temperature, the surface resin gel point temperature is less than the structural resin gel point temperature, the structural resin gel time is greater than the surface resin gel time, and/or the surface resin gel time is less than the structural resin gel time.

Applying the pressure at <NUM> may include applying the pressure to the uncured composite structure during the initially heating at <NUM> and/or applying the pressure to the partially cured composite structure during the subsequently heating at <NUM>. In some examples, the applying the pressure at <NUM> includes applying an elevated atmospheric pressure to the uncured composite structure and/or to the partially cured composite structure. In some examples, the applying at <NUM> includes utilizing an autoclave, autoclaving the uncured composite structure during the initially heating at <NUM>, and/or autoclaving the partially cured composite structure during the subsequently heating at <NUM>.

In some examples of methods <NUM>, the initially heating at <NUM> includes hardening the surface layer to generate a hardened surface layer. In some such examples, methods <NUM> further include resisting deformation of the hardened surface layer, which may be caused by the applying at <NUM>, during the subsequently heating at <NUM>. Stated another way, the initially heating at <NUM> may harden the surface layer such that, during the subsequently heating at <NUM>, the hardened surface layer is more resistant to deformation when compared to the structural layer and/or to the uncured surface layer. Such a configuration may permit the hardened surface layer to function as and/or to be a resin shell for the structural layer during the subsequently heating at <NUM>. Such a configuration may cause composite structures, which are formed utilizing methods <NUM>, to have an improved surface quality and/or to have fewer defects when compared to conventional composite structures that do not include the surface layer and the structural layer disclosed herein.

Initially heating at <NUM> may include initially heating the uncured composite structure to an initial temperature, such as to produce and/or generate the partially cured composite structure. As discussed, the uncured surface resin has the surface resin viscosity, the surface resin gel point temperature, and the surface resin gel time. The initially heating at <NUM> is sufficient to gel the uncured surface resin, such as to generate a cured surface resin, but is insufficient to gel the uncured structural resin. Stated another way, and subsequent to the initially heating at <NUM>, the surface resin may be cured, gelled, and/or hardened; however, the structural resin may be uncured, ungelled, and/or unhardened.

In some examples, the initially heating at <NUM> includes initially heating for an initial heating time. In some such examples, the initial heating time includes and/or is a dwell time during which the uncured composite structure is maintained at the initial temperature. Stated another way, the initially heating at <NUM> may include dwelling at the initial temperature for the initial heating time. In some such examples, the initial heating time also may be referred to herein as an initial dwell time at the initial temperature.

In some examples, the initial heating time includes and/or is a ramp time during which the uncured composite structure is ramped to the initial temperature. Stated another way, the initially heating at <NUM> may include ramping the uncured composite structure to the initial temperature during the initial heating time. In some such examples, the initial heating time also may be referred to herein as an initial ramp time to the initial temperature.

The initial temperature may have any suitable magnitude and/or value, such as may be selected based, at least in part, on one or more desired properties of the surface layer, the surface resin viscosity, the surface resin gel point temperature, and/or the surface resin gel time. Examples of the initial temperature include initial temperatures of at least <NUM> degrees Celsius (ºC), at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, and/or at most <NUM>ºC. Stated differently, the initial temperature may be bounded by any of the above temperatures, as appropriate.

Similarly, the initial heating time may have any suitable magnitude and/or value, such as may be selected based, at least in part, on one or more desired properties of the surface layer, the surface resin viscosity, the surface resin gel point temperature, and/or the surface resin gel time. Examples of the initial heating time include initial heating times of at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, and/or at most <NUM> minutes. Stated differently, the initial heating time may be bounded by any of the above times, as appropriate.

Subsequently heating at <NUM> may include subsequently heating the partially cured composite structure to a final cure temperature, which is greater than the initial temperature, to produce and/or generate a cured composite structure. The subsequently heating at <NUM> may be performed subsequent to, or after, the initially heating at <NUM>. As discussed, the uncured structural resin has a structural resin viscosity, which is less than the surface resin viscosity, a structural resin gel point temperature, which is greater than the surface resin gel point temperature, and a structural resin gel time, which is greater than the surface resin gel time. Subsequent to the subsequently heating at <NUM>, the uncured surface resin and the uncured structural resin are fully cured. Stated another way, a combination of the initially heating at <NUM> and the subsequently heating at <NUM> is sufficient to fully cure both the uncured surface resin and the uncured structural resin, such as to produce and/or generate the cured surface resin an a cured structural resin, respectively.

An example of the cured composite structure is illustrated in <FIG>. More specifically, <FIG> illustrates a composite structure <NUM> in the form of a cured composite structure <NUM>. Cured composite structure <NUM> may be formed from uncured composite structure <NUM> of <FIG>, such as by performing methods <NUM>. Cured composite structure <NUM> includes structural layer <NUM>, in the form of a cured structural layer <NUM>, that includes structural resin <NUM>, in the form of cured structural resin <NUM>. Cured composite structure <NUM> also includes surface layer <NUM>, in the form of a cured surface layer <NUM>, that includes surface resin <NUM>, in the form of cured surface resin <NUM>.

In some examples, the subsequently heating at <NUM> includes subsequently heating for a subsequent heating time. In some such examples, the subsequent heating time includes and/or is a dwell time during which the partially cured composite structure is maintained at the final cure temperature. Stated another way, the subsequently heating at <NUM> may include dwelling at the final cure temperature for the subsequent heating time. In some such examples, the subsequent heating time also may be referred to herein as a subsequent dwell time at the final cure temperature.

In some examples, the subsequent heating time includes and/or is a ramp time during which the uncured composite structure is ramped from the initial temperature and/or to the final cure temperature. Stated another way, the subsequently heating at <NUM> may include ramping the partially cured composite structure to the final cure temperature during the subsequent heating time. In some such examples, the subsequent heating time also may be referred to herein as a subsequent ramp time to the final cure temperature.

The final cure temperature may have any suitable magnitude and/or value, such as may be selected based, at least in part, on one or more desired properties of the structural layer, the structural resin viscosity, the structural resin gel point temperature, and/or the structural resin gel time. Examples of the final cure temperature include final cure temperatures of at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, and/or at most <NUM>ºC. Stated differently, the final cure temperature may be bounded by any of the above temperatures, as appropriate.

In some examples, the final cure temperature differs from, or is greater than, the initial temperature by a threshold temperature difference. As examples, the difference between the final cure temperature and the initial temperature may be at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, and/or at most <NUM>ºC. Stated differently, the threshold temperature difference may be bounded by any of the above temperature differences, as appropriate.

Similarly, the subsequent heating time may have any suitable magnitude and/or value, such as may be selected based, at least in part, on one or more desired properties of the structural layer, the structural resin viscosity, the structural resin gel point temperature, and/or the structural resin gel time. Examples of the subsequent heating time include subsequent heating times of at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, and/or at most <NUM> minutes. Stated differently, the subsequent heating time may be bounded by any of the above heating times, as appropriate.

In some examples, the subsequent heating time differs from, or is greater than, the initial heating time by a threshold time difference. Examples of the difference between the subsequent heating time and the initial heating time include times of at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, and/or at most <NUM> minutes. Stated differently, the difference between the subsequent heating time and the initial heating time may be bounded by any of the above time differences, as appropriate.

Structural layer <NUM> may, may be designed to, and/or may be selected to provide structural integrity to composite structure <NUM>. Stated another way, a primary function of structural layer <NUM>, within composite structure <NUM>, may be to provide structural integrity, such as via providing a desired level of rigidity, stiffness, and/or flexibility to composite structure <NUM>.

As illustrated in dashed lines in <FIG>, structural layer <NUM> may include a plurality of structural fibers <NUM>, which may be at least partially, or even completely, encapsulated within structural resin <NUM>, within uncured structural resin <NUM>, and/or within cured structural resin <NUM>. Examples of the plurality of structural fibers <NUM> include a plurality of structural carbon fibers, a plurality of structural fiberglass fibers, and/or a plurality of structural aramid fibers. In some examples, structural layer <NUM> and/or structural fibers <NUM> include and/or are arranged in a plurality of layered structural plies of composite material, which may include structural resin <NUM>, uncured structural resin <NUM>, and/or cured structural resin <NUM>.

Structural resin <NUM> may include and/or be any suitable resin that may have, define, and/or exhibit the structural resin viscosity, the structural resin gel point temperature, and/or the structural resin gel time. Examples of structural resin <NUM> include a structural epoxy resin, a structural thermoset resin, a structural phenolic resin, and/or a structural polybismaleimide resin.

Surface layer <NUM> may, may be designed to, and/or may be selected to provide a desired surface property and/or characteristic to composite structure <NUM>. In some examples, surface layer <NUM> may define an ultraviolet light-resistant layer of cured composite structure <NUM>. The ultraviolet light-resistant layer may be configured to resist degradation of cured composite structure <NUM> when exposed to ultraviolet light and/or to protect structural layer <NUM> from ultraviolet light. In other examples, surface layer <NUM> may define a sealing layer of cured composite structure <NUM>. The sealing layer may be configured to seal a remainder of cured composite structure <NUM>, such as structural layer <NUM>, from contact with an ambient environment that surrounds cured composite structure <NUM>. In yet other examples, surface layer <NUM> may define an adhesion layer, a primer layer, and/or an in-mold primer layer of cured composite structure <NUM>. The adhesion layer, the primer layer, and/or the in-mold primer layer may be configured to facilitate adhesion of another layer and/or material, such as paint, to cured composite structure <NUM>.

As illustrated in dashed lines in <FIG>, surface layer <NUM> may include a plurality of surface fibers <NUM>, which may be at least partially, or even completely, encapsulated within surface resin <NUM>, within uncured surface resin <NUM>, and/or within cured surface resin <NUM>. Examples of the plurality of surface fibers <NUM> include a plurality of fiberglass surface fibers, a plurality of polyester surface fibers, and/or a plurality of nylon surface fibers. The plurality of surface fibers <NUM> may be arranged, within surface layer <NUM>, in any suitable manner. As examples, the plurality of surface fibers may be arranged in a surface weave, a surface mat, and/or a surface scrim mat.

Surface resin <NUM> may include and/or be any suitable resin that may have, define, and/or exhibit the surface resin viscosity, the surface resin gel point temperature, and/or the surface resin gel time. Additionally or alternatively, surface resin <NUM> may include and/or be any suitable resin that may restrict ultraviolet light from being incident upon structural layer <NUM>, such as via reflection and/or absorption of the ultraviolet light. Additionally or alternatively, surface resin <NUM> may include and/or be any suitable resin that may seal structural layer <NUM>. Additionally or alternatively, surface resin <NUM> may include and/or be any suitable resin that may improve adhesion, may function as a primer, and/or may function as an in-mold primer for attachment of another layer and/or material to cured composite structure <NUM>. Examples of surface resin <NUM>, of uncured surface resin <NUM>, and/or of cured surface resin <NUM> include an ultraviolet light-resistant surface resin, an aliphatic epoxy surface resin, and/or a fire-retardant surface resin.

In some examples, and as illustrated in dashed lines in <FIG>, surface layer <NUM> includes an additive <NUM>. Additive <NUM>, when present, may be adapted, configured, formulated, synthesized, and/or selected to modify and/or change at least one property of surface layer <NUM>. Some examples of additive <NUM> include a flow-control additive. The flow-control additive may be selected such that the surface resin viscosity has a desired surface resin viscosity magnitude. Some examples of the flow-control additive include, or are, titanium dioxide.

As discussed, the surface resin viscosity is greater than the structural resin viscosity. Such a configuration may decrease a potential for the uncured surface resin to diffuse into and/or mix with the uncured structural resin, such as prior to and/or during the initially heating at <NUM>. Subsequent to the initially heating at <NUM>, and as discussed, the uncured surface resin has gelled, has crosslinked, and/or has been converted to the cured surface resin. The gelled uncured surface resin, the crosslinked uncured surface resin, and/or the cured surface resin then defines a unitary and/or cohesive surface layer, which resists mixing and/or diffusion with the uncured structural resin. Such a configuration permits cured composite structures formed via methods <NUM>, according to the present disclosure, to maintain at least partial separation between the cured surface resin and the cured structural resin within the cured composite structure, thereby permitting the cured surface resin to protect the structural layer from environmental factors, as discussed herein.

The surface resin viscosity may be greater than the structural resin viscosity by any suitable magnitude and/or value. As an example, the surface resin viscosity may be a threshold viscosity multiple of the structural resin viscosity. Stated differently, the structural resin viscosity may be a threshold viscosity fraction of the surface resin viscosity, with the threshold viscosity fraction being defined as an inverse of the threshold viscosity multiple. Examples of the threshold viscosity multiple include multiples of at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>,<NUM>, at most <NUM>,<NUM>, at most <NUM>,<NUM>, at most <NUM>,<NUM>, at most <NUM>,<NUM>, at most <NUM>,<NUM>, at most <NUM>,<NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, and/or at most <NUM>. Stated differently, the threshold viscosity multiple may be bounded by any of the above multiples, as appropriate.

As also discussed, the surface resin gel point temperature is less than the structural resin gel point temperature. As such, and as discussed, the initial temperature, which is attained during the initially heating at <NUM>, is sufficient to gel the uncured surface resin but insufficient to gel the uncured structural resin. Stated another way, by specifying that the surface resin gel point temperature is less than the structural resin gel point temperature, the composite structures and methods, disclosed herein, may gel the uncured surface resin earlier in the curing process when compared to the uncured structural resin. This earlier gelling of the uncured surface resin may permit and/or facilitate the at least partial separation between the cured surface resin and the cured structural resin within the cured composite structure, thereby permitting the cured surface resin to protect the structural layer from environmental factors, as discussed herein.

The surface resin gel point temperature may differ from the structural resin gel point temperature by any suitable amount. As an example, the surface resin gel point temperature may be a threshold gel temperature difference less than the structural resin gel point temperature. Stated differently, the structural resin gel point temperature may be the threshold gel temperature difference greater than the surface resin gel point temperature. Examples of the threshold gel temperature difference include temperature differences of at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at least <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, at most <NUM>ºC, and/or at most <NUM>ºC. Stated differently, the threshold gel temperature difference may be bounded by any of the above temperature differences, as appropriate.

As also discussed, the surface resin gel time may be less than the structural resin gel time. As such, and as discussed, the uncured surface resin may gel more quickly relative to the uncured structural resin at a given temperature. Stated differently, the cure kinetics of the uncured surface resin may be relatively faster when compared to the cure kinetics of the uncured structural resin. Stated another way, by specifying that the surface resin gel time is less than the structural resin gel time, the composite structures and methods, disclosed herein, may gel the uncured surface resin earlier in the curing process when compared to the uncured structural resin. This earlier gelling of the uncured surface resin may permit and/or facilitate the at least partial separation between the cured surface resin and the cured structural resin within the cured composite structure, thereby permitting the cured surface resin to protect the structural layer from environmental factors, as discussed herein.

The surface resin gel time may differ from the structural resin gel time by any suitable amount. As an example, the surface resin gel time may be a threshold gel time difference less than the structural resin gel time. Stated differently, the structural resin gel time may be the threshold gel time difference greater than the surface resin gel time. Examples of the threshold gel time difference include time differences of at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at least <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, at most <NUM> minutes, and/or at most <NUM> minutes. Stated differently, the threshold gel time difference may be bounded by any of the above time differences, as appropriate.

<FIG> is a plot illustrating viscosity as a function of cure temperature at a constant ramp rate for two resins that may be utilized with the uncured composite structures, composite structures, and methods, according to the present disclosure. <FIG> graphically illustrates the above-described differences between the surface resin, for which cure kinetics are illustrated in solid lines, and the structural resin, for which cure kinetics are illustrated in dashed lines. As illustrated in <FIG>, and as the cure temperature is ramped at the constant ramp rate, both the surface resin viscosity and the structural resin viscosity initially will decrease. However, and as also illustrated in <FIG>, the surface resin and the structural resin are selected such that the surface resin viscosity always remains greater than the structural resin viscosity.

<FIG> further illustrates that both the surface resin and the structural resin exhibit a corresponding viscosity minimum, which corresponds generally to the surface resin gel point temperature and the structural resin gel point temperature, respectively. Both the surface resin and the structural resin then exhibit a pronounced increase in viscosity due to gelling, or crosslinking, of the respective resins. As discussed herein, this increase in viscosity occurs at a lower temperature for the surface resin relative to the structural resin.

As discussed, uncured composite structures <NUM>, cured composite structures <NUM>, and methods <NUM>, which are disclosed herein, limit mixing, mingling, and/or diffusion between surface layer <NUM> and structural layer <NUM>. As also discussed, this limiting of the mixing, mingling, and/or diffusion permits the disclosed uncured composite structures <NUM>, cured composite structures <NUM>, and methods <NUM> to exhibit improved and/or desired characteristics when compared to conventional composite structures formed via conventional methods. More specifically, uncured composite structures <NUM>, cured composite structures <NUM>, and methods <NUM>, according to the present disclosure, permit surface layer <NUM> to be more uniform relative to conventional surface layers and/or permit surface layer <NUM> to better protect structural layer <NUM> from environmental factors when compared to the conventional surface layers.

Turning to <FIG>, it is expected that methods <NUM> and/or cured composite structures <NUM> formed therefrom may exhibit limited mixing, mingling, and/or diffusion between surface layer <NUM> and structural layer <NUM>. This limited mixing, mingling, and/or diffusion may be desirable in that it may increase adhesion between surface layer <NUM> and structural layer <NUM>. However, this mixing, mingling, and/or diffusion between cured surface layer <NUM> and cured structural layer <NUM>, within cured composite structure <NUM>, may be confined to an interface region <NUM> between cured surface layer <NUM> and cured structural layer <NUM>. Stated another way, and as illustrated in <FIG>, interface region <NUM> may include both cured surface resin <NUM> and cured structural resin <NUM> and/or may be a mixture of cured surface resin <NUM> and cured structural resin <NUM>. However, interface region <NUM> may be covered and/or encapsulated by cured surface layer <NUM>, which includes cured surface resin <NUM> but may not include cured structural resin <NUM>. Stated another way, cured surface layer <NUM> may define an exposed surface <NUM>, and at least exposed surface <NUM> may be free of and/or spaced-apart from cured structural resin <NUM>. Additionally or alternatively, cured structural layer <NUM> may be free of and/or spaced-apart from cured surface resin <NUM>. This is in contrast to conventional cured composite structures formed via conventional methods, within which the conventional structural resin generally extends to the exposed surface of the conventional surface layer, thereby decreasing the ability of the conventional surface layer to protect the conventional structural layer from environmental factors.

In some examples, and as illustrated in <FIG>, cured surface layer <NUM> defines a surface layer thickness <NUM>, or an average surface layer thickness <NUM>. Similarly, and in some such examples, interface region <NUM> defines an interface region thickness <NUM>, or an average interface region thickness <NUM>. In some such examples, a ratio of average surface layer thickness <NUM> to average interface region thickness <NUM> may be at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, at most <NUM>, and/or at most <NUM>. Such a configuration may provide a desired amount, or distance, of separation between exposed surface <NUM> and cured structural resin <NUM> contained within interface region <NUM>. Stated differently, the ratio may be bounded by any of the above ratios, as appropriate.

In another example according to the present disclosure, an uncured composite structure, comprises: a structural layer that includes an uncured structural resin, wherein the uncured structural resin has a structural resin viscosity, a structural resin gel point temperature, and a structural resin gel time; and a surface layer that includes an uncured surface resin, wherein the uncured surface resin includes a surface resin viscosity that is greater than the structural resin viscosity, a surface resin gel point temperature that is less than the structural resin gel point temperature, and a surface resin gel time that is less than the structural resin gel time.

Claim 1:
A method (<NUM>) of curing an uncured composite structure (<NUM>) that includes a structural layer (<NUM>), which includes an uncured structural resin (<NUM>) with a structural resin viscosity, a structural resin gel point temperature, and a structural resin gel time, and a surface layer (<NUM>), which includes an uncured surface resin (<NUM>) with a surface resin viscosity that is greater than the structural resin viscosity, a surface resin gel point temperature that is less than the structural resin gel point temperature, and a surface resin gel time that is less than the structural resin gel time, the method (<NUM>) comprising:
initially heating (<NUM>), for an initial heating time, the uncured composite structure (<NUM>) to an initial temperature to generate a partially cured composite structure, wherein the initially heating (<NUM>) is sufficient to gel the uncured surface resin (<NUM>) but insufficient to gel the uncured structural resin (<NUM>); and
subsequently heating (<NUM>), for a subsequent heating time, the partially cured composite structure to a final cure temperature, which is greater than the initial temperature, to generate a cured composite structure (<NUM>), wherein a combination of the initially heating (<NUM>) and the subsequently heating (<NUM>) is sufficient to fully cure both the uncured surface resin (<NUM>) and the uncured structural resin (<NUM>).